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The UNIX System Interface: File Descriptors

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The UNIX System Interface

The UNIX operating system provides its services through a set of system calls, which are in effect functions within the operating system that may be called by user programs. This chapter describes how to use some of the most important system calls from C programs. If you use UNIX, this should be directly helpful, for it is sometimes necessary to employ system calls for maximum efficiency, or to access some facility that is not in the library. Even if you use C on a different operating system, however, you should be able to glean insight into C programming from studying these examples; although details vary, similar code will be found on any system. Since the ANSI C library is in many cases modeled on UNIX facilities, this code may help your understanding of the library as well.




This chapter is divided into three major parts: input/output, file system, and storage allocation. The first two parts assume a modest familiarity with the external characteristics of UNIX systems.

Chapter 7 was concerned with an input/output interface that is uniform across operating systems. On any particular system the routines of the standard library have to be written in terms of the facilities provided by the host system. In the next few sections we will describe the UNIX system calls for input and output, and show how parts of the standard library can be implemented with them.

1 File Descriptors

In the UNIX operating system, all input and output is done by reading or writing files, because all peripheral devices, even keyboard and screen, are files in the file system. This means that a single homogeneous interface handles all communication between a program and peripheral devices.

In the most general case, before you read and write a file, you must inform the system of your intent to do so, a process called opening the file. If you are going to write on a file it may also be necessary to create it or to discard its previous contents. The system checks your right to do so (Does the file exist? Do you have permission to access it?) and if all is well, returns to the program a small non-negative integer called a file descriptor. Whenever input or output is to be done on the file, the file descriptor is used instead of the name to identify the file. (A file descriptor is analogous to the file pointer used by the standard library, or to the file handle of MS-DOS.) All information about an open file is maintained by the system; the user program refers to the file only by the file descriptor.

Since input and output involving keyboard and screen is so common, special arrangements exist to make this convenient. When the command interpreter (the ``shell'') runs a program, three files are open, with file descriptors 0, 1, and 2, called the standard input, the standard output, and the standard error. If a program reads 0 and writes 1 and 2, it can do input and output without worrying about opening files.

The user of a program can redirect I/O to and from files with < and >:


prog <infile >outfile

In this case, the shell changes the default assignments for the file descriptors 0 and 1 to the named files. Normally file descriptor 2 remains attached to the screen, so error messages can go there. Similar observations hold for input or output associated with a pipe. In all cases, the file assignments are changed by the shell, not by the program. The program does not know where its input comes from nor where its output goes, so long as it uses file 0 for input and 1 and 2 for output.

2 Low Level I/O - Read and Write

Input and output uses the read and write system calls, which are accessed from C programs through two functions called read and write. For both, the first argument is a file descriptor. The second argument is a character array in your program where the data is to go to or to come from. The third argument is the number is the number of bytes to be transferred.


int n_read = read(int fd, char *buf, int n);
int n_written = write(int fd, char *buf, int n);

Each call returns a count of the number of bytes transferred. On reading, the number of bytes returned may be less than the number requested. A return value of zero bytes implies end of file, and indicates an error of some sort. For writing, the return value is the number of bytes written; an error has occurred if this isn't equal to the number requested.

Any number of bytes can be read or written in one call. The most common values are 1, which means one character at a time (``unbuffered''), and a number like 1024 or 4096 that corresponds to a physical block size on a peripheral device. Larger sizes will be more efficient because fewer system calls will be made.

Putting these facts together, we can write a simple program to copy its input to its output, the equivalent of the file copying program written for Chapter 1. This program will copy anything to anything, since the input and output can be redirected to any file or device.


#include 'syscalls.h'

main() /* copy input to output */

We have collected function prototypes for the system calls into a file called syscalls.h so we can include it in the programs of this chapter. This name is not standard, however.

The parameter BUFSIZ is also defined in syscalls.h; its value is a good size for the local system. If the file size is not a multiple of BUFSIZ, some read will return a smaller number of bytes to be written by write; the next call to read after that will return zero.

It is instructive to see how read and write can be used to construct higher-level routines like getchar putchar, etc. For example, here is a version of getchar that does unbuffered input, by reading the standard input one character at a time.


#include 'syscalls.h'

/* getchar: unbuffered single character input */
int getchar(void)

c must be a char, because read needs a character pointer. Casting c to unsigned char in the return statement eliminates any problem of sign extension.

The second version of getchar does input in big chunks, and hands out the characters one at a time.


#include 'syscalls.h'

/* getchar: simple buffered version */
int getchar(void)

return (--n >= 0) ? (unsigned char) *bufp++ : EOF;
}

If these versions of getchar were to be compiled with <stdio.h> included, it would be necessary to #undef the name getchar in case it is implemented as a macro.

3 Open, Creat, Close, Unlink

Other than the default standard input, output and error, you must explicitly open files in order to read or write them. There are two system calls for this, open and creat [sic].

open is rather like the fopen discussed in Chapter 7, except that instead of returning a file pointer, it returns a file descriptor, which is just an int open returns if any error occurs.


#include <fcntl.h>

int fd;
int open(char *name, int flags, int perms);

fd = open(name, flags, perms);

As with fopen, the name argument is a character string containing the filename. The second argument, flags, is an int that specifies how the file is to be opened; the main values are

O_RDONLY

open for reading only

O_WRONLY

open for writing only

O_RDWR

open for both reading and writing

These constants are defined in <fcntl.h> on System V UNIX systems, and in <sys/file.h> on Berkeley (BSD) versions.

To open an existing file for reading,


fd = open(name, O_RDONLY,0);

The perms argument is always zero for the uses of open that we will discuss.

It is an error to try to open a file that does not exist. The system call creat is provided to create new files, or to re-write old ones.


int creat(char *name, int perms);

fd = creat(name, perms);

returns a file descriptor if it was able to create the file, and if not. If the file already exists, creat will truncate it to zero length, thereby discarding its previous contents; it is not an error to creat a file that already exists.

If the file does not already exist, creat creates it with the permissions specified by the perms argument. In the UNIX file system, there are nine bits of permission information associated with a file that control read, write and execute access for the owner of the file, for the owner's group, and for all others. Thus a three-digit octal number is convenient for specifying the permissions. For example, specifies read, write and execute permission for the owner, and read and execute permission for the group and everyone else.

To illustrate, here is a simplified version of the UNIX program cp, which copies one file to another. Our version copies only one file, it does not permit the second argument to be a directory, and it invents permissions instead of copying them.


#include <stdio.h>
#include <fcntl.h>
#include 'syscalls.h'
#define PERMS 0666 /* RW for owner, group, others */

void error(char *, );

/* cp: copy f1 to f2 */
main(int argc, char *argv[])

This program creates the output file with fixed permissions of . With the stat system call, described in Section 6, we can determine the mode of an existing file and thus give the same mode to the copy.

Notice that the function error is called with variable argument lists much like printf. The implementation of error illustrates how to use another member of the printf family. The standard library function vprintf is like printf except that the variable argument list is replaced by a single argument that has been initialized by calling the va_start macro. Similarly, vfprintf and vsprintf match fprintf and sprintf


#include <stdio.h>
#include <stdarg.h>

/* error: print an error message and die */
void error(char *fmt, )

There is a limit (often about 20) on the number of files that a program may open simultaneously. Accordingly, any program that intends to process many files must be prepared to re-use file descriptors. The function close(int fd) breaks the connection between a file descriptor and an open file, and frees the file descriptor for use with some other file; it corresponds to fclose in the standard library except that there is no buffer to flush. Termination of a program via exit or return from the main program closes all open files.

The function unlink(char *name) removes the file name from the file system. It corresponds to the standard library function remove

Exercise 8-1. Rewrite the program cat from Chapter 7 using read write open, and close instead of their standard library equivalents. Perform experiments to determine the relative speeds of the two versions.

4 Random Access - Lseek

Input and output are normally sequential: each read or write takes place at a position in the file right after the previous one. When necessary, however, a file can be read or written in any arbitrary order. The system call lseek provides a way to move around in a file without reading or writing any data:


long lseek(int fd, long offset, int origin);

sets the current position in the file whose descriptor is fd to offset, which is taken relative to the location specified by origin. Subsequent reading or writing will begin at that position. origin can be 0, 1, or 2 to specify that offset is to be measured from the beginning, from the current position, or from the end of the file respectively. For example, to append to a file (the redirection >> in the UNIX shell, or 'a' for fopen), seek to the end before writing:


lseek(fd, 0L, 2);

To get back to the beginning (``rewind''),


lseek(fd, 0L, 0);

Notice the 0L argument; it could also be written as (long) 0 or just as if lseek is properly declared.

With lseek, it is possible to treat files more or less like arrays, at the price of slower access. For example, the following function reads any number of bytes from any arbitrary place in a file. It returns the number read, or on error.


#include 'syscalls.h'

/*get: read n bytes from position pos */
int get(int fd, long pos, char *buf, int n)

The return value from lseek is a long that gives the new position in the file, or if an error occurs. The standard library function fseek is similar to lseek except that the first argument is a FILE * and the return is non-zero if an error occurred.

5 Example - An implementation of Fopen and Getc

Let us illustrate how some of these pieces fit together by showing an implementation of the standard library routines fopen and getc

Recall that files in the standard library are described by file pointers rather than file descriptors. A file pointer is a pointer to a structure that contains several pieces of information about the file: a pointer to a buffer, so the file can be read in large chunks; a count of the number of characters left in the buffer; a pointer to the next character position in the buffer; the file descriptor; and flags describing read/write mode, error status, etc.

The data structure that describes a file is contained in <stdio.h>, which must be included (by #include) in any source file that uses routines from the standard input/output library. It is also included by functions in that library. In the following excerpt from a typical <stdio.h>, names that are intended for use only by functions of the library begin with an underscore so they are less likely to collide with names in a user's program. This convention is used by all standard library routines.


#define NULL 0
#define EOF (-1)
#define BUFSIZ 1024
#define OPEN_MAX 20 /* max #files open at once */

typedef struct _iobuf FILE;
extern FILE _iob[OPEN_MAX];

#define stdin (&_iob[0])
#define stdout (&_iob[1])
#define stderr (&_iob[2])

enum _flags ;

int _fillbuf(FILE *);
int _flushbuf(int, FILE *);

#define feof(p) ((p)->flag & _EOF) != 0)
#define ferror(p) ((p)->flag & _ERR) != 0)
#define fileno(p) ((p)->fd)

#define getc(p) (--(p)->cnt >= 0
? (unsigned char) *(p)->ptr++ : _fillbuf(p))
#define putc(x,p) (--(p)->cnt >= 0
? *(p)->ptr++ = (x) : _flushbuf((x),p))

#define getchar() getc(stdin)
#define putcher(x) putc((x), stdout)

The getc macro normally decrements the count, advances the pointer, and returns the character. (Recall that a long #define is continued with a backslash.) If the count goes negative, however, getc calls the function _fillbuf to replenish the buffer, re-initialize the structure contents, and return a character. The characters are returned unsigned, which ensures that all characters will be positive.

Although we will not discuss any details, we have included the definition of putc to show that it operates in much the same way as getc, calling a function _flushbuf when its buffer is full. We have also included macros for accessing the error and end-of-file status and the file descriptor.

The function fopen can now be written. Most of fopen is concerned with getting the file opened and positioned at the right place, and setting the flag bits to indicate the proper state. fopen does not allocate any buffer space; this is done by _fillbuf when the file is first read.


#include <fcntl.h>
#include 'syscalls.h'
#define PERMS 0666 /* RW for owner, group, others */

FILE *fopen(char *name, char *mode)
else
fd = open(name, O_RDONLY, 0);
if (fd == -1) /* couldn't access name */
return NULL;
fp->fd = fd;
fp->cnt = 0;
fp->base = NULL;
fp->flag = (*mode == 'r') ? _READ : _WRITE;
return fp;
}

This version of fopen does not handle all of the access mode possibilities of the standard, though adding them would not take much code. In particular, our fopen does not recognize the ``b'' that signals binary access, since that is meaningless on UNIX systems, nor the `` '' that permits both reading and writing.

The first call to getc for a particular file finds a count of zero, which forces a call of _fillbuf. If _fillbuf finds that the file is not open for reading, it returns EOF immediately. Otherwise, it tries to allocate a buffer (if reading is to be buffered).

Once the buffer is established, _fillbuf calls read to fill it, sets the count and pointers, and returns the character at the beginning of the buffer. Subsequent calls to _fillbuf will find a buffer allocated.


#include 'syscalls.h'

/* _fillbuf: allocate and fill input buffer */
int _fillbuf(FILE *fp)

return (unsigned char) *fp->ptr++;
}

The only remaining loose end is how everything gets started. The array _iob must be defined and initialized for stdin stdout and stderr


FILE _iob[OPEN_MAX] = ,
,

};

The initialization of the flag part of the structure shows that stdin is to be read, stdout is to be written, and stderr is to be written unbuffered.

Exercise 8-2. Rewrite fopen and _fillbuf with fields instead of explicit bit operations. Compare code size and execution speed.

Exercise 8-3. Design and write _flushbuf fflush, and fclose

Exercise 8-4. The standard library function


int fseek(FILE *fp, long offset, int origin)

is identical to lseek except that fp is a file pointer instead of a file descriptor and return value is an int status, not a position. Write fseek. Make sure that your fseek coordinates properly with the buffering done for the other functions of the library.

6 Example - Listing Directories

A different kind of file system interaction is sometimes called for - determining information about a file, not what it contains. A directory-listing program such as the UNIX command ls is an example - it prints the names of files in a directory, and, optionally, other information, such as sizes, permissions, and so on. The MS-DOS dir command is analogous.

Since a UNIX directory is just a file, ls need only read it to retrieve the filenames. But is is necessary to use a system call to access other information about a file, such as its size. On other systems, a system call may be needed even to access filenames; this is the case on MS-DOS for instance. What we want is provide access to the information in a relatively system-independent way, even though the implementation may be highly system-dependent.

We will illustrate some of this by writing a program called fsize fsize is a special form of ls that prints the sizes of all files named in its commandline argument list. If one of the files is a directory, fsize applies itself recursively to that directory. If there are no arguments at all, it processes the current directory.

Let us begin with a short review of UNIX file system structure. A directory is a file that contains a list of filenames and some indication of where they are located. The ``location'' is an index into another table called the ``inode list.'' The inode for a file is where all information about the file except its name is kept. A directory entry generally consists of only two items, the filename and an inode number.

Regrettably, the format and precise contents of a directory are not the same on all versions of the system. So we will divide the task into two pieces to try to isolate the non-portable parts. The outer level defines a structure called a Dirent and three routines opendir readdir, and closedir to provide system-independent access to the name and inode number in a directory entry. We will write fsize with this interface. Then we will show how to implement these on systems that use the same directory structure as Version 7 and System V UNIX; variants are left as exercises.

The Dirent structure contains the inode number and the name. The maximum length of a filename component is NAME_MAX, which is a system-dependent value. opendir returns a pointer to a structure called DIR, analogous to FILE, which is used by readdir and closedir. This information is collected into a file called dirent.h


#define NAME_MAX 14 /* longest filename component; */
/* system-dependent */

typedef struct Dirent;

typedef struct DIR;

DIR *opendir(char *dirname);
Dirent *readdir(DIR *dfd);
void closedir(DIR *dfd);

The system call stat takes a filename and returns all of the information in the inode for that file, or if there is an error. That is,


char *name;
struct stat stbuf;
int stat(char *, struct stat *);

stat(name, &stbuf);

fills the structure stbuf with the inode information for the file name. The structure describing the value returned by stat is in <sys/stat.h>, and typically looks like this:


struct stat /* inode information returned by stat */
;

Most of these values are explained by the comment fields. The types like dev_t and ino_t are defined in <sys/types.h>, which must be included too.

The st_mode entry contains a set of flags describing the file. The flag definitions are also included in <sys/types.h>; we need only the part that deals with file type:


#define S_IFMT 0160000 /* type of file: */
#define S_IFDIR 0040000 /* directory */
#define S_IFCHR 0020000 /* character special */
#define S_IFBLK 0060000 /* block special */
#define S_IFREG 0010000 /* regular */
/* */

Now we are ready to write the program fsize. If the mode obtained from stat indicates that a file is not a directory, then the size is at hand and can be printed directly. If the name is a directory, however, then we have to process that directory one file at a time; it may in turn contain sub-directories, so the process is recursive.

The main routine deals with command-line arguments; it hands each argument to the function fsize


#include <stdio.h>
#include <string.h>
#include 'syscalls.h'
#include <fcntl.h> /* flags for read and write */
#include <sys/types.h> /* typedefs */
#include <sys/stat.h> /* structure returned by stat */
#include 'dirent.h'

void fsize(char *)

/* print file name */
main(int argc, char **argv)

The function fsize prints the size of the file. If the file is a directory, however, fsize first calls dirwalk to handle all the files in it. Note how the flag names S_IFMT and S_IFDIR are used to decide if the file is a directory. Parenthesization matters, because the precedence of & is lower than that of


int stat(char *, struct stat *);
void dirwalk(char *, void (*fcn)(char *));

/* fsize: print the name of file 'name' */
void fsize(char *name)

if ((stbuf.st_mode & S_IFMT) == S_IFDIR)
dirwalk(name, fsize);
printf('%8ld %sn', stbuf.st_size, name);
}

The function dirwalk is a general routine that applies a function to each file in a directory. It opens the directory, loops through the files in it, calling the function on each, then closes the directory and returns. Since fsize calls dirwalk on each directory, the two functions call each other recursively.


#define MAX_PATH 1024

/* dirwalk: apply fcn to all files in dir */
void dirwalk(char *dir, void (*fcn)(char *))

while ((dp = readdir(dfd)) != NULL)
}
closedir(dfd);
}

Each call to readdir returns a pointer to information for the next file, or NULL when there are no files left. Each directory always contains entries for itself, called , and its parent, ; these must be skipped, or the program will loop forever.

Down to this last level, the code is independent of how directories are formatted. The next step is to present minimal versions of opendir readdir, and closedir for a specific system. The following routines are for Version 7 and System V UNIX systems; they use the directory information in the header <sys/dir.h>, which looks like this:


#ifndef DIRSIZ
#define DIRSIZ 14
#endif
struct direct ;

Some versions of the system permit much longer names and have a more complicated directory structure.

The type ino_t is a typedef that describes the index into the inode list. It happens to be unsigned short on the systems we use regularly, but this is not the sort of information to embed in a program; it might be different on a different system, so the typedef is better. A complete set of ``system'' types is found in <sys/types.h>

opendir opens the directory, verifies that the file is a directory (this time by the system call fstat, which is like stat except that it applies to a file descriptor), allocates a directory structure, and records the information:


int fstat(int fd, struct stat *);

/* opendir: open a directory for readdir calls */
DIR *opendir(char *dirname)

closedir closes the directory file and frees the space:


/* closedir: close directory opened by opendir */
void closedir(DIR *dp)

}

Finally, readdir uses read to read each directory entry. If a directory slot is not currently in use (because a file has been removed), the inode number is zero, and this position is skipped. Otherwise, the inode number and name are placed in a static structure and a pointer to that is returned to the user. Each call overwrites the information from the previous one.


#include <sys/dir.h> /* local directory structure */

/* readdir: read directory entries in sequence */
Dirent *readdir(DIR *dp)

return NULL;
}

Although the fsize program is rather specialized, it does illustrate a couple of important ideas. First, many programs are not ``system programs''; they merely use information that is maintained by the operating system. For such programs, it is crucial that the representation of the information appear only in standard headers, and that programs include those headers instead of embedding the declarations in themselves. The second observation is that with care it is possible to create an interface to system-dependent objects that is itself relatively system-independent. The functions of the standard library are good examples.

Exercise 8-5. Modify the fsize program to print the other information contained in the inode entry.

7 Example - A Storage Allocator

In Chapter 5, we presented a vary limited stack-oriented storage allocator. The version that we will now write is unrestricted. Calls to malloc and free may occur in any order; malloc calls upon the operating system to obtain more memory as necessary. These routines illustrate some of the considerations involved in writing machine-dependent code in a relatively machine-independent way, and also show a real-life application of structures, unions and typedef

Rather than allocating from a compiled-in fixed-size array, malloc will request space from the operating system as needed. Since other activities in the program may also request space without calling this allocator, the space that malloc manages may not be contiguous. Thus its free storage is kept as a list of free blocks. Each block contains a size, a pointer to the next block, and the space itself. The blocks are kept in order of increasing storage address, and the last block (highest address) points to the first.

When a request is made, the free list is scanned until a big-enough block is found. This algorithm is called ``first fit,'' by contrast with ``best fit,'' which looks for the smallest block that will satisfy the request. If the block is exactly the size requested it is unlinked from the list and returned to the user. If the block is too big, it is split, and the proper amount is returned to the user while the residue remains on the free list. If no big-enough block is found, another large chunk is obtained by the operating system and linked into the free list.

Freeing also causes a search of the free list, to find the proper place to insert the block being freed. If the block being freed is adjacent to a free block on either side, it is coalesced with it into a single bigger block, so storage does not become too fragmented. Determining the adjacency is easy because the free list is maintained in order of decreasing address.



One problem, which we alluded to in Chapter 5, is to ensure that the storage returned by malloc is aligned properly for the objects that will be stored in it. Although machines vary, for each machine there is a most restrictive type: if the most restrictive type can be stored at a particular address, all other types may be also. On some machines, the most restrictive type is a double; on others, int or long suffices.

A free block contains a pointer to the next block in the chain, a record of the size of the block, and then the free space itself; the control information at the beginning is called the ``header.'' To simplify alignment, all blocks are multiples of the header size, and the header is aligned properly. This is achieved by a union that contains the desired header structure and an instance of the most restrictive alignment type, which we have arbitrarily made a long


typedef long Align; /* for alignment to long boundary */

union header s;
Align x; /* force alignment of blocks */
};

typedef union header Header;

The Align field is never used; it just forces each header to be aligned on a worst-case boundary.

In malloc, the requested size in characters is rounded up to the proper number of header-sized units; the block that will be allocated contains one more unit, for the header itself, and this is the value recorded in the size field of the header. The pointer returned by malloc points at the free space, not at the header itself. The user can do anything with the space requested, but if anything is written outside of the allocated space the list is likely to be scrambled.

The size field is necessary because the blocks controlled by malloc need not be contiguous - it is not possible to compute sizes by pointer arithmetic.

The variable base is used to get started. If freep is NULL, as it is at the first call of malloc, then a degenerate free list is created; it contains one block of size zero, and points to itself. In any case, the free list is then searched. The search for a free block of adequate size begins at the point (freep) where the last block was found; this strategy helps keep the list homogeneous. If a too-big block is found, the tail end is returned to the user; in this way the header of the original needs only to have its size adjusted. In all cases, the pointer returned to the user points to the free space within the block, which begins one unit beyond the header.


static Header base; /* empty list to get started */
static Header *freep = NULL; /* start of free list */

/* malloc: general-purpose storage allocator */
void *malloc(unsigned nbytes)

for (p = prevp->s.ptr; ; prevp = p, p = p->s.ptr)
freep = prevp;
return (void *)(p+1);
}
if (p == freep) /* wrapped around free list */
if ((p = morecore(nunits)) == NULL)
return NULL; /* none left */
}
}

The function morecore obtains storage from the operating system. The details of how it does this vary from system to system. Since asking the system for memory is a comparatively expensive operation. we don't want to do that on every call to malloc, so morecore requests al least NALLOC units; this larger block will be chopped up as needed. After setting the size field, morecore inserts the additional memory into the arena by calling free

The UNIX system call sbrk(n) returns a pointer to n more bytes of storage. sbrk returns if there was no space, even though NULL could have been a better design. The must be cast to char * so it can be compared with the return value. Again, casts make the function relatively immune to the details of pointer representation on different machines. There is still one assumption, however, that pointers to different blocks returned by sbrk can be meaningfully compared. This is not guaranteed by the standard, which permits pointer comparisons only within an array. Thus this version of malloc is portable only among machines for which general pointer comparison is meaningful.


#define NALLOC 1024 /* minimum #units to request */

/* morecore: ask system for more memory */
static Header *morecore(unsigned nu)

free itself is the last thing. It scans the free list, starting at freep, looking for the place to insert the free block. This is either between two existing blocks or at the end of the list. In any case, if the block being freed is adjacent to either neighbor, the adjacent blocks are combined. The only troubles are keeping the pointers pointing to the right things and the sizes correct.


/* free: put block ap in free list */
void free(void *ap)
else
bp->s.ptr = p->s.ptr;
if (p + p->size == bp) else
p->s.ptr = bp;
freep = p;
}

Although storage allocation is intrinsically machine-dependent, the code above illustrates how the machine dependencies can be controlled and confined to a very small part of the program. The use of typedef and union handles alignment (given that sbrk supplies an appropriate pointer). Casts arrange that pointer conversions are made explicit, and even cope with a badly-designed system interface. Even though the details here are related to storage allocation, the general approach is applicable to other situations as well.

Exercise 8-6. The standard library function calloc(n,size) returns a pointer to n objects of size size, with the storage initialized to zero. Write calloc, by calling malloc or by modifying it.

Exercise 8-7. malloc accepts a size request without checking its plausibility; free believes that the block it is asked to free contains a valid size field. Improve these routines so they make more pains with error checking.

Exercise 8- Write a routine bfree(p,n) that will free any arbitrary block p of n characters into the free list maintained by malloc and free. By using bfree, a user can add a static or external array to the free list at any time.

Appendix A - Reference Manual

A.1 Introduction

This manual describes the C language specified by the draft submitted to ANSI on 31 October, 1988, for approval as ``American Standard for Information Systems - programming Language C, X3.159-1989.'' The manual is an interpretation of the proposed standard, not the standard itself, although care has been taken to make it a reliable guide to the language.

For the most part, this document follows the broad outline of the standard, which in turn follows that of the first edition of this book, although the organization differs in detail. Except for renaming a few productions, and not formalizing the definitions of the lexical tokens or the preprocessor, the grammar given here for the language proper is equivalent to that of the standard.

Throughout this manual, commentary material is indented and written in smaller type, as this is. Most often these comments highlight ways in which ANSI Standard C differs from the language defined by the first edition of this book, or from refinements subsequently introduced in various compilers.

A.2 Lexical Conventions

A program consists of one or more translation units stored in files. It is translated in several phases, which are described in Par.A.12. The first phases do low-level lexical transformations, carry out directives introduced by the lines beginning with the # character, and perform macro definition and expansion. When the preprocessing of Par.A.12 is complete, the program has been reduced to a sequence of tokens.

A.2.1 Tokens

There are six classes of tokens: identifiers, keywords, constants, string literals, operators, and other separators. Blanks, horizontal and vertical tabs, newlines, formfeeds and comments as described below (collectively, ``white space'') are ignored except as they separate tokens. Some white space is required to separate otherwise adjacent identifiers, keywords, and constants.

If the input stream has been separated into tokens up to a given character, the next token is the longest string of characters that could constitute a token.

A.2.2 Comments

The characters introduce a comment, which terminates with the characters . Comments do not nest, and they do not occur within a string or character literals.

A.2.3 Identifiers

An identifier is a sequence of letters and digits. The first character must be a letter; the underscore counts as a letter. Upper and lower case letters are different. Identifiers may have any length, and for internal identifiers, at least the first 31 characters are significant; some implementations may take more characters significant. Internal identifiers include preprocessor macro names and all other names that do not have external linkage (Par.A.11.2). Identifiers with external linkage are more restricted: implementations may make as few as the first six characters significant, and may ignore case distinctions.

A.2.4 Keywords

The following identifiers are reserved for the use as keywords, and may not be used otherwise:


auto double int struct
break else long switch
case enum register typedef
char extern return union
const float short unsigned
continue for signed void
default goto sizeof volatile
do if static while

Some implementations also reserve the words fortran and asm

The keywords const signed, and volatile are new with the ANSI standard; enum and void are new since the first edition, but in common use; entry, formerly reserved but never used, is no longer reserved.

A.2.5 Constants

There are several kinds of constants. Each has a data type; Par.A.4.2 discusses the basic types:

    constant:
      integer-constant
      character-constant
      floating-constant
      enumeration-constant

A.2.5.1 Integer Constants

An integer constant consisting of a sequence of digits is taken to be octal if it begins with 0 (digit zero), decimal otherwise. Octal constants do not contain the digits or . A sequence of digits preceded by 0x or 0X (digit zero) is taken to be a hexadecimal integer. The hexadecimal digits include a or A through f or F with values through

An integer constant may be suffixed by the letter u or U, to specify that it is unsigned. It may also be suffixed by the letter l or L to specify that it is long.

The type of an integer constant depends on its form, value and suffix. (See Par.A.4 for a discussion of types). If it is unsuffixed and decimal, it has the first of these types in which its value can be represented: int long int unsigned long int. If it is unsuffixed, octal or hexadecimal, it has the first possible of these types: int unsigned int long int unsigned long int. If it is suffixed by u or U, then unsigned int unsigned long int. If it is suffixed by l or L, then long int unsigned long int. If an integer constant is suffixed by UL, it is unsigned long

The elaboration of the types of integer constants goes considerably beyond the first edition, which merely caused large integer constants to be long. The U suffixes are new.

A.2.5.2 Character Constants

A character constant is a sequence of one or more characters enclosed in single quotes as in 'x'. The value of a character constant with only one character is the numeric value of the character in the machine's character set at execution time. The value of a multi-character constant is implementation-defined.

Character constants do not contain the character or newlines; in order to represent them, and certain other characters, the following escape sequences may be used:

newline

NL (LF) 

n

backslash

horizontal tab

HT

t

question mark

vertical tab

VT

v

single quote

backspace

BS

b

double quote

carriage return

CR

r

octal number

ooo

ooo

formfeed

FF

f

hex number

hh

xhh

audible alert

BEL

a

The escape ooo consists of the backslash followed by 1, 2, or 3 octal digits, which are taken to specify the value of the desired character. A common example of this construction is (not followed by a digit), which specifies the character NUL. The escape xhh consists of the backslash, followed by x, followed by hexadecimal digits, which are taken to specify the value of the desired character. There is no limit on the number of digits, but the behavior is undefined if the resulting character value exceeds that of the largest character. For either octal or hexadecimal escape characters, if the implementation treats the char type as signed, the value is sign-extended as if cast to char type. If the character following the is not one of those specified, the behavior is undefined.

In some implementations, there is an extended set of characters that cannot be represented in the char type. A constant in this extended set is written with a preceding L, for example L'x', and is called a wide character constant. Such a constant has type wchar_t, an integral type defined in the standard header <stddef.h>. As with ordinary character constants, hexadecimal escapes may be used; the effect is undefined if the specified value exceeds that representable with wchar_t

Some of these escape sequences are new, in particular the hexadecimal character representation. Extended characters are also new. The character sets commonly used in the Americas and western Europe can be encoded to fit in the char type; the main intent in adding wchar_t was to accommodate Asian languages.

A.2.5.3 Floating Constants

A floating constant consists of an integer part, a decimal part, a fraction part, an e or E, an optionally signed integer exponent and an optional type suffix, one of f F l, or L. The integer and fraction parts both consist of a sequence of digits. Either the integer part, or the fraction part (not both) may be missing; either the decimal point or the e and the exponent (not both) may be missing. The type is determined by the suffix; F or f makes it float L or l makes it long double, otherwise it is double

A2.5.4 Enumeration Constants

Identifiers declared as enumerators (see Par.A.4) are constants of type int

A.2.6 String Literals

A string literal, also called a string constant, is a sequence of characters surrounded by double quotes as in . A string has type ``array of characters'' and storage class static (see Par.A.3 below) and is initialized with the given characters. Whether identical string literals are distinct is implementation-defined, and the behavior of a program that attempts to alter a string literal is undefined.

Adjacent string literals are concatenated into a single string. After any concatenation, a null byte is appended to the string so that programs that scan the string can find its end. String literals do not contain newline or double-quote characters; in order to represent them, the same escape sequences as for character constants are available.

As with character constants, string literals in an extended character set are written with a preceding L, as in L''. Wide-character string literals have type ``array of wchar_t.'' Concatenation of ordinary and wide string literals is undefined.

The specification that string literals need not be distinct, and the prohibition against modifying them, are new in the ANSI standard, as is the concatenation of adjacent string literals. Wide-character string literals are new.

A.3 Syntax Notation

In the syntax notation used in this manual, syntactic categories are indicated by italic type, and literal words and characters in typewriter style. Alternative categories are usually listed on separate lines; in a few cases, a long set of narrow alternatives is presented on one line, marked by the phrase ``one of.'' An optional terminal or nonterminal symbol carries the subscript ``opt,'' so that, for example,

means an optional expression, enclosed in braces. The syntax is summarized in Par.A.13.

Unlike the grammar given in the first edition of this book, the one given here makes precedence and associativity of expression operators explicit.

A.4 Meaning of Identifiers

Identifiers, or names, refer to a variety of things: functions; tags of structures, unions, and enumerations; members of structures or unions; enumeration constants; typedef names; and objects. An object, sometimes called a variable, is a location in storage, and its interpretation depends on two main attributes: its storage class and its type. The storage class determines the lifetime of the storage associated with the identified object; the type determines the meaning of the values found in the identified object. A name also has a scope, which is the region of the program in which it is known, and a linkage, which determines whether the same name in another scope refers to the same object or function. Scope and linkage are discussed in Par.A.11.

A.4.1 Storage Class

There are two storage classes: automatic and static. Several keywords, together with the context of an object's declaration, specify its storage class. Automatic objects are local to a block (Par.9.3), and are discarded on exit from the block. Declarations within a block create automatic objects if no storage class specification is mentioned, or if the auto specifier is used. Objects declared register are automatic, and are (if possible) stored in fast registers of the machine.

Static objects may be local to a block or external to all blocks, but in either case retain their values across exit from and reentry to functions and blocks. Within a block, including a block that provides the code for a function, static objects are declared with the keyword static. The objects declared outside all blocks, at the same level as function definitions, are always static. They may be made local to a particular translation unit by use of the static keyword; this gives them internal linkage. They become global to an entire program by omitting an explicit storage class, or by using the keyword extern; this gives them external linkage.

A.4.2 Basic Types

There are several fundamental types. The standard header <limits.h> described in Appendix B defines the largest and smallest values of each type in the local implementation. The numbers given in Appendix B show the smallest acceptable magnitudes.

Objects declared as characters (char) are large enough to store any member of the execution character set. If a genuine character from that set is stored in a char object, its value is equivalent to the integer code for the character, and is non-negative. Other quantities may be stored into char variables, but the available range of values, and especially whether the value is signed, is implementation-dependent.

Unsigned characters declared unsigned char consume the same amount of space as plain characters, but always appear non-negative; explicitly signed characters declared signed char likewise take the same space as plain characters.

unsigned char type does not appear in the first edition of this book, but is in common use. signed char is new.

Besides the char types, up to three sizes of integer, declared short int int, and long int, are available. Plain int objects have the natural size suggested by the host machine architecture; the other sizes are provided to meet special needs. Longer integers provide at least as much storage as shorter ones, but the implementation may make plain integers equivalent to either short integers, or long integers. The int types all represent signed values unless specified otherwise.

Unsigned integers, declared using the keyword unsigned, obey the laws of arithmetic modulo 2n where n is the number of bits in the representation, and thus arithmetic on unsigned quantities can never overflow. The set of non-negative values that can be stored in a signed object is a subset of the values that can be stored in the corresponding unsigned object, and the representation for the overlapping values is the same.

Any of single precision floating point (float), double precision floating point (double), and extra precision floating point (long double) may be synonymous, but the ones later in the list are at least as precise as those before.

long double is new. The first edition made long float equivalent to double; the locution has been withdrawn.

Enumerations are unique types that have integral values; associated with each enumeration is a set of named constants (Par.A.4). Enumerations behave like integers, but it is common for a compiler to issue a warning when an object of a particular enumeration is assigned something other than one of its constants, or an expression of its type.

Because objects of these types can be interpreted as numbers, they will be referred to as arithmetic types. Types char, and int of all sizes, each with or without sign, and also enumeration types, will collectively be called integral types. The types float double, and long double will be called floating types.

The void type specifies an empty set of values. It is used as the type returned by functions that generate no value.

A.4.3 Derived types

Beside the basic types, there is a conceptually infinite class of derived types constructed from the fundamental types in the following ways:

  arrays of objects of a given type;
  functions returning objects of a given type;
  pointers to objects of a given type;
  structures containing a sequence of objects of various types;
  unions capable of containing any of one of several objects of various types.

In general these methods of constructing objects can be applied recursively.

A.4.4 Type Qualifiers

An object's type may have additional qualifiers. Declaring an object const announces that its value will not be changed; declaring it volatile announces that it has special properties relevant to optimization. Neither qualifier affects the range of values or arithmetic properties of the object. Qualifiers are discussed in Par.A.2.

A.5 Objects and Lvalues

An Object is a named region of storage; an lvalue is an expression referring to an object. An obvious example of an lvalue expression is an identifier with suitable type and storage class. There are operators that yield lvalues, if E is an expression of pointer type, then *E is an lvalue expression referring to the object to which E points. The name ``lvalue'' comes from the assignment expression E1 = E2 in which the left operand E1 must be an lvalue expression. The discussion of each operator specifies whether it expects lvalue operands and whether it yields an lvalue.

A.6 Conversions

Some operators may, depending on their operands, cause conversion of the value of an operand from one type to another. This section explains the result to be expected from such conversions. Par.6.5 summarizes the conversions demanded by most ordinary operators; it will be supplemented as required by the discussion of each operator.

A.6.1 Integral Promotion

A character, a short integer, or an integer bit-field, all either signed or not, or an object of enumeration type, may be used in an expression wherever an integer may be used. If an int can represent all the values of the original type, then the value is converted to int; otherwise the value is converted to unsigned int. This process is called integral promotion.

A.6.2 Integral Conversions

Any integer is converted to a given unsigned type by finding the smallest non-negative value that is congruent to that integer, modulo one more than the largest value that can be represented in the unsigned type. In a two's complement representation, this is equivalent to left-truncation if the bit pattern of the unsigned type is narrower, and to zero-filling unsigned values and sign-extending signed values if the unsigned type is wider.

When any integer is converted to a signed type, the value is unchanged if it can be represented in the new type and is implementation-defined otherwise.

A.6.3 Integer and Floating

When a value of floating type is converted to integral type, the fractional part is discarded; if the resulting value cannot be represented in the integral type, the behavior is undefined. In particular, the result of converting negative floating values to unsigned integral types is not specified.

When a value of integral type is converted to floating, and the value is in the representable range but is not exactly representable, then the result may be either the next higher or next lower representable value. If the result is out of range, the behavior is undefined.

A.6.4 Floating Types

When a less precise floating value is converted to an equally or more precise floating type, the value is unchanged. When a more precise floating value is converted to a less precise floating type, and the value is within representable range, the result may be either the next higher or the next lower representable value. If the result is out of range, the behavior is undefined.

A.6.5 Arithmetic Conversions

Many operators cause conversions and yield result types in a similar way. The effect is to bring operands into a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions.

  • First, if either operand is long double, the other is converted to long double
  • Otherwise, if either operand is double, the other is converted to double
  • Otherwise, if either operand is float, the other is converted to float
  • Otherwise, the integral promotions are performed on both operands; then, if either operand is unsigned long int, the other is converted to unsigned long int
  • Otherwise, if one operand is long int and the other is unsigned int, the effect depends on whether a long int can represent all values of an unsigned int; if so, the unsigned int operand is converted to long int; if not, both are converted to unsigned long int
  • Otherwise, if one operand is long int, the other is converted to long int
  • Otherwise, if either operand is unsigned int, the other is converted to unsigned int
  • Otherwise, both operands have type int

There are two changes here. First, arithmetic on float operands may be done in single precision, rather than double; the first edition specified that all floating arithmetic was double precision. Second, shorter unsigned types, when combined with a larger signed type, do not propagate the unsigned property to the result type; in the first edition, the unsigned always dominated. The new rules are slightly more complicated, but reduce somewhat the surprises that may occur when an unsigned quantity meets signed. Unexpected results may still occur when an unsigned expression is compared to a signed expression of the same size.

A.6.6 Pointers and Integers

An expression of integral type may be added to or subtracted from a pointer; in such a case the integral expression is converted as specified in the discussion of the addition operator (Par.A.7.7).

Two pointers to objects of the same type, in the same array, may be subtracted; the result is converted to an integer as specified in the discussion of the subtraction operator (Par.A.7.7).

An integral constant expression with value 0, or such an expression cast to type void *, may be converted, by a cast, by assignment, or by comparison, to a pointer of any type. This produces a null pointer that is equal to another null pointer of the same type, but unequal to any pointer to a function or object.

Certain other conversions involving pointers are permitted, but have implementation-defined aspects. They must be specified by an explicit type-conversion operator, or cast (Pars.A.7.5 and A.8).

A pointer may be converted to an integral type large enough to hold it; the required size is implementation-dependent. The mapping function is also implementation-dependent.

A pointer to one type may be converted to a pointer to another type. The resulting pointer may cause addressing exceptions if the subject pointer does not refer to an object suitably aligned in storage. It is guaranteed that a pointer to an object may be converted to a pointer to an object whose type requires less or equally strict storage alignment and back again without change; the notion of ``alignment'' is implementation-dependent, but objects of the char types have least strict alignment requirements. As described in Par.A.6.8, a pointer may also be converted to type void * and back again without change.



A pointer may be converted to another pointer whose type is the same except for the addition or removal of qualifiers (Pars.A.4.4, A.2) of the object type to which the pointer refers. If qualifiers are added, the new pointer is equivalent to the old except for restrictions implied by the new qualifiers. If qualifiers are removed, operations on the underlying object remain subject to the qualifiers in its actual declaration.

Finally, a pointer to a function may be converted to a pointer to another function type. Calling the function specified by the converted pointer is implementation-dependent; however, if the converted pointer is reconverted to its original type, the result is identical to the original pointer.

A.6.7 Void

The (nonexistent) value of a void object may not be used in any way, and neither explicit nor implicit conversion to any non-void type may be applied. Because a void expression denotes a nonexistent value, such an expression may be used only where the value is not required, for example as an expression statement (Par.A.9.2) or as the left operand of a comma operator (Par.A.7.18).

An expression may be converted to type void by a cast. For example, a void cast documents the discarding of the value of a function call used as an expression statement.

void did not appear in the first edition of this book, but has become common since.

A.6.8 Pointers to Void

Any pointer to an object may be converted to type void * without loss of information. If the result is converted back to the original pointer type, the original pointer is recovered. Unlike the pointer-to-pointer conversions discussed in Par.A.6.6, which generally require an explicit cast, pointers may be assigned to and from pointers of type void *, and may be compared with them.

This interpretation of void * pointers is new; previously, char * pointers played the role of generic pointer. The ANSI standard specifically blesses the meeting of void * pointers with object pointers in assignments and relationals, while requiring explicit casts for other pointer mixtures.

A.7 Expressions

The precedence of expression operators is the same as the order of the major subsections of this section, highest precedence first. Thus, for example, the expressions referred to as the operands of (Par.A.7.7) are those expressions defined in Pars.A.7.1-A.7.6. Within each subsection, the operators have the same precedence. Left- or right-associativity is specified in each subsection for the operators discussed therein. The grammar given in Par.13 incorporates the precedence and associativity of the operators.

The precedence and associativity of operators is fully specified, but the order of evaluation of expressions is, with certain exceptions, undefined, even if the subexpressions involve side effects. That is, unless the definition of the operator guarantees that its operands are evaluated in a particular order, the implementation is free to evaluate operands in any order, or even to interleave their evaluation. However, each operator combines the values produced by its operands in a way compatible with the parsing of the expression in which it appears.

This rule revokes the previous freedom to reorder expressions with operators that are mathematically commutative and associative, but can fail to be computationally associative. The change affects only floating-point computations near the limits of their accuracy, and situations where overflow is possible.

The handling of overflow, divide check, and other exceptions in expression evaluation is not defined by the language. Most existing implementations of C ignore overflow in evaluation of signed integral expressions and assignments, but this behavior is not guaranteed. Treatment of division by 0, and all floating-point exceptions, varies among implementations; sometimes it is adjustable by a non-standard library function.

A.7.1 Pointer Conversion

If the type of an expression or subexpression is ``array of T,'' for some type T, then the value of the expression is a pointer to the first object in the array, and the type of the expression is altered to ``pointer to T.'' This conversion does not take place if the expression is in the operand of the unary & operator, or of sizeof, or as the left operand of an assignment operator or the operator. Similarly, an expression of type ``function returning T,'' except when used as the operand of the & operator, is converted to ``pointer to function returning T.''

A.7.2 Primary Expressions

Primary expressions are identifiers, constants, strings, or expressions in parentheses.

    primary-expression
      identifier
      constant
      string
      (expression)

An identifier is a primary expression, provided it has been suitably declared as discussed below. Its type is specified by its declaration. An identifier is an lvalue if it refers to an object (Par.A.5) and if its type is arithmetic, structure, union, or pointer.

A constant is a primary expression. Its type depends on its form as discussed in Par.A.2.5.

A string literal is a primary expression. Its type is originally ``array of char'' (for wide-char strings, ``array of wchar_t''), but following the rule given in Par.A.7.1, this is usually modified to ``pointer to char wchar_t) and the result is a pointer to the first character in the string. The conversion also does not occur in certain initializers; see Par.A.7.

A parenthesized expression is a primary expression whose type and value are identical to those of the unadorned expression. The precedence of parentheses does not affect whether the expression is an lvalue.

A.7.3 Postfix Expressions

The operators in postfix expressions group left to right.

    postfix-expression:
      primary-expression
      postfix-expression[expression]
      postfix-expression(argument-expression-listopt)
      postfix-expression.identifier
      postfix-expression
->identifier
      postfix-expression

      postfix-expression

    argument-expression-list:
      assignment-expression
      assignment-expression-list
assignment-expression

A.7.3.1 Array References

A postfix expression followed by an expression in square brackets is a postfix expression denoting a subscripted array reference. One of the two expressions must have type ``pointer to T'', where T is some type, and the other must have integral type; the type of the subscript expression is T. The expression E1[E2] is identical (by definition) to *((E1)+(E2)). See Par.A.6.2 for further discussion.

A.7.3.2 Function Calls

A function call is a postfix expression, called the function designator, followed by parentheses containing a possibly empty, comma-separated list of assignment expressions (Par.A7.17), which constitute the arguments to the function. If the postfix expression consists of an identifier for which no declaration exists in the current scope, the identifier is implicitly declared as if the declaration

extern int identifier

had been given in the innermost block containing the function call. The postfix expression (after possible explicit declaration and pointer generation, Par.A7.1) must be of type ``pointer to function returning T,'' for some type T, and the value of the function call has type T.

In the first edition, the type was restricted to ``function,'' and an explicit operator was required to call through pointers to functions. The ANSI standard blesses the practice of some existing compilers by permitting the same syntax for calls to functions and to functions specified by pointers. The older syntax is still usable.

The term argument is used for an expression passed by a function call; the term parameter is used for an input object (or its identifier) received by a function definition, or described in a function declaration. The terms ``actual argument (parameter)'' and ``formal argument (parameter)'' respectively are sometimes used for the same distinction.

In preparing for the call to a function, a copy is made of each argument; all argument-passing is strictly by value. A function may change the values of its parameter objects, which are copies of the argument expressions, but these changes cannot affect the values of the arguments. However, it is possible to pass a pointer on the understanding that the function may change the value of the object to which the pointer points.

There are two styles in which functions may be declared. In the new style, the types of parameters are explicit and are part of the type of the function; such a declaration os also called a function prototype. In the old style, parameter types are not specified. Function declaration is issued in Pars.A.6.3 and A.10.1.

If the function declaration in scope for a call is old-style, then default argument promotion is applied to each argument as follows: integral promotion (Par.A.6.1) is performed on each argument of integral type, and each float argument is converted to double. The effect of the call is undefined if the number of arguments disagrees with the number of parameters in the definition of the function, or if the type of an argument after promotion disagrees with that of the corresponding parameter. Type agreement depends on whether the function's definition is new-style or old-style. If it is old-style, then the comparison is between the promoted type of the arguments of the call, and the promoted type of the parameter, if the definition is new-style, the promoted type of the argument must be that of the parameter itself, without promotion.

If the function declaration in scope for a call is new-style, then the arguments are converted, as if by assignment, to the types of the corresponding parameters of the function's prototype. The number of arguments must be the same as the number of explicitly described parameters, unless the declaration's parameter list ends with the ellipsis notation . In that case, the number of arguments must equal or exceed the number of parameters; trailing arguments beyond the explicitly typed parameters suffer default argument promotion as described in the preceding paragraph. If the definition of the function is old-style, then the type of each parameter in the definition, after the definition parameter's type has undergone argument promotion.

These rules are especially complicated because they must cater to a mixture of old- and new-style functions. Mixtures are to be avoided if possible.

The order of evaluation of arguments is unspecified; take note that various compilers differ. However, the arguments and the function designator are completely evaluated, including all side effects, before the function is entered. Recursive calls to any function are permitted.

A.7.3.3 Structure References

A postfix expression followed by a dot followed by an identifier is a postfix expression. The first operand expression must be a structure or a union, and the identifier must name a member of the structure or union. The value is the named member of the structure or union, and its type is the type of the member. The expression is an lvalue if the first expression is an lvalue, and if the type of the second expression is not an array type.

A postfix expression followed by an arrow (built from and >) followed by an identifier is a postfix expression. The first operand expression must be a pointer to a structure or union, and the identifier must name a member of the structure or union. The result refers to the named member of the structure or union to which the pointer expression points, and the type is the type of the member; the result is an lvalue if the type is not an array type.

Thus the expression E1->MOS is the same as (*E1).MOS. Structures and unions are discussed in Par.A.3.

In the first edition of this book, it was already the rule that a member name in such an expression had to belong to the structure or union mentioned in the postfix expression; however, a note admitted that this rule was not firmly enforced. Recent compilers, and ANSI, do enforce it.

A.7.3.4 Postfix Incrementation

A postfix expression followed by a or operator is a postfix expression. The value of the expression is the value of the operand. After the value is noted, the operand is incremented or decremented by 1. The operand must be an lvalue; see the discussion of additive operators (Par.A.7.7) and assignment (Par.A.7.17) for further constraints on the operand and details of the operation. The result is not an lvalue.

A.7.4 Unary Operators

Expressions with unary operators group right-to-left.

    unary-expression:
      postfix expression
      
unary expression unary expression
      unary-operator cast-expression
      
sizeof unary-expression
      
sizeof(type-name)

    unary operator: one of
      
& * + - ~ !

A.7.4.1 Prefix Incrementation Operators

A unary expression followed by a or operator is a unary expression. The operand is incremented or decremented by 1. The value of the expression is the value after the incrementation (decrementation). The operand must be an lvalue; see the discussion of additive operators (Par.A.7.7) and assignment (Par.A.7.17) for further constraints on the operands and details of the operation. The result is not an lvalue.

A.7.4.2 Address Operator

The unary operator & takes the address of its operand. The operand must be an lvalue referring neither to a bit-field nor to an object declared as register, or must be of function type. The result is a pointer to the object or function referred to by the lvalue. If the type of the operand is T, the type of the result is ``pointer to T.''

A.7.4.3 Indirection Operator

The unary operator denotes indirection, and returns the object or function to which its operand points. It is an lvalue if the operand is a pointer to an object of arithmetic, structure, union, or pointer type. If the type of the expression is ``pointer to T,'' the type of the result is T.

A.7.4.4 Unary Plus Operator

The operand of the unary operator must have arithmetic type, and the result is the value of the operand. An integral operand undergoes integral promotion. The type of the result is the type of the promoted operand.

The unary is new with the ANSI standard. It was added for symmetry with the unary

A.7.4.5 Unary Minus Operator

The operand of the unary operator must have arithmetic type, and the result is the negative of its operand. An integral operand undergoes integral promotion. The negative of an unsigned quantity is computed by subtracting the promoted value from the largest value of the promoted type and adding one; but negative zero is zero. The type of the result is the type of the promoted operand.

A.7.4.6 One's Complement Operator

The operand of the operator must have integral type, and the result is the one's complement of its operand. The integral promotions are performed. If the operand is unsigned, the result is computed by subtracting the value from the largest value of the promoted type. If the operand is signed, the result is computed by converting the promoted operand to the corresponding unsigned type, applying , and converting back to the signed type. The type of the result is the type of the promoted operand.

A.7.4.7 Logical Negation Operator

The operand of the operator must have arithmetic type or be a pointer, and the result is 1 if the value of its operand compares equal to 0, and 0 otherwise. The type of the result is int

A.7.4.8 Sizeof Operator

The sizeof operator yields the number of bytes required to store an object of the type of its operand. The operand is either an expression, which is not evaluated, or a parenthesized type name. When sizeof is applied to a char, the result is 1; when applied to an array, the result is the total number of bytes in the array. When applied to a structure or union, the result is the number of bytes in the object, including any padding required to make the object tile an array: the size of an array of n elements is n times the size of one element. The operator may not be applied to an operand of function type, or of incomplete type, or to a bit-field. The result is an unsigned integral constant; the particular type is implementation-defined. The standard header <stddef.h> (See appendix B) defines this type as size_t

A.7.5 Casts

A unary expression preceded by the parenthesized name of a type causes conversion of the value of the expression to the named type.

    cast-expression:
      unary expression
      (type-name) cast-expression

This construction is called a cast. The names are described in Par.A. The effects of conversions are described in Par.A.6. An expression with a cast is not an lvalue.

A.7.6 Multiplicative Operators

The multiplicative operators , and group left-to-right.

    multiplicative-expression:
      multiplicative-expression
cast-expression
      multiplicative-expression
cast-expression
      multiplicative-expression
cast-expression

The operands of and must have arithmetic type; the operands of must have integral type. The usual arithmetic conversions are performed on the operands, and predict the type of the result.

The binary operator denotes multiplication.

The binary operator yields the quotient, and the operator the remainder, of the division of the first operand by the second; if the second operand is 0, the result is undefined. Otherwise, it is always true that (a/b)*b + a%b is equal to a. If both operands are non-negative, then the remainder is non-negative and smaller than the divisor, if not, it is guaranteed only that the absolute value of the remainder is smaller than the absolute value of the divisor.

A.7.7 Additive Operators

The additive operators and group left-to-right. If the operands have arithmetic type, the usual arithmetic conversions are performed. There are some additional type possibilities for each operator.

    additive-expression:
      multiplicative-expression
      additive-expression
multiplicative-expression
      additive-expression
multiplicative-expression

The result of the operator is the sum of the operands. A pointer to an object in an array and a value of any integral type may be added. The latter is converted to an address offset by multiplying it by the size of the object to which the pointer points. The sum is a pointer of the same type as the original pointer, and points to another object in the same array, appropriately offset from the original object. Thus if P is a pointer to an object in an array, the expression P+1 is a pointer to the next object in the array. If the sum pointer points outside the bounds of the array, except at the first location beyond the high end, the result is undefined.

The provision for pointers just beyond the end of an array is new. It legitimizes a common idiom for looping over the elements of an array.

The result of the operator is the difference of the operands. A value of any integral type may be subtracted from a pointer, and then the same conversions and conditions as for addition apply.

If two pointers to objects of the same type are subtracted, the result is a signed integral value representing the displacement between the pointed-to objects; pointers to successive objects differ by 1. The type of the result is defined as ptrdiff_t in the standard header <stddef.h>. The value is undefined unless the pointers point to objects within the same array; however, if P points to the last member of an array, then (P+1)-P has value 1.

A.7.8 Shift Operators

The shift operators << and >> group left-to-right. For both operators, each operand must be integral, and is subject to integral the promotions. The type of the result is that of the promoted left operand. The result is undefined if the right operand is negative, or greater than or equal to the number of bits in the left expression's type.

    shift-expression:
      additive-expression
      shift-expression
<< additive-expression
      shift-expression
>> additive-expression

The value of E1<<E2 is E1 (interpreted as a bit pattern) left-shifted E2 bits; in the absence of overflow, this is equivalent to multiplication by 2E2. The value of E1>>E2 is E1 right-shifted E2 bit positions. The right shift is equivalent to division by 2E2 if E1 is unsigned or it has a non-negative value; otherwise the result is implementation-defined.

A.7.9 Relational Operators

The relational operators group left-to-right, but this fact is not useful; a<b<c is parsed as (a<b)<c, and evaluates to either 0 or 1.

    relational-expression:
      shift-expression
      relational-expression
< shift-expression
      relational-expression
> shift-expression
      relational-expression
<= shift-expression
      relational-expression
>= shift-expression

The operators < (less), > (greater), <= (less or equal) and >= (greater or equal) all yield 0 if the specified relation is false and 1 if it is true. The type of the result is int. The usual arithmetic conversions are performed on arithmetic operands. Pointers to objects of the same type (ignoring any qualifiers) may be compared; the result depends on the relative locations in the address space of the pointed-to objects. Pointer comparison is defined only for parts of the same object; if two pointers point to the same simple object, they compare equal; if the pointers are to members of the same structure, pointers to objects declared later in the structure compare higher; if the pointers refer to members of an array, the comparison is equivalent to comparison of the the corresponding subscripts. If P points to the last member of an array, then P+1 compares higher than P, even though P+1 points outside the array. Otherwise, pointer comparison is undefined.

These rules slightly liberalize the restrictions stated in the first edition, by permitting comparison of pointers to different members of a structure or union. They also legalize comparison with a pointer just off the end of an array.

A.7.10 Equality Operators

    equality-expression:
      relational-expression
      equality-expression
relational-expression
      equality-expression
relational-expression

The (equal to) and the (not equal to) operators are analogous to the relational operators except for their lower precedence. (Thus a<b == c<d is 1 whenever a<b and c<d have the same truth-value.)

The equality operators follow the same rules as the relational operators, but permit additional possibilities: a pointer may be compared to a constant integral expression with value 0, or to a pointer to void. See Par.A.6.6.

A.7.11 Bitwise AND Operator

    AND-expression:
      equality-expression
      AND-expression
& equality-expression

The usual arithmetic conversions are performed; the result is the bitwise AND function of the operands. The operator applies only to integral operands.

A.7.12 Bitwise Exclusive OR Operator

    exclusive-OR-expression:
      AND-expression
      exclusive-OR-expression
AND-expression

The usual arithmetic conversions are performed; the result is the bitwise exclusive OR function of the operands. The operator applies only to integral operands.

A.7.13 Bitwise Inclusive OR Operator

    inclusive-OR-expression:
      exclusive-OR-expression
      inclusive-OR-expression
exclusive-OR-expression

The usual arithmetic conversions are performed; the result is the bitwise inclusive OR function of the operands. The operator applies only to integral operands.

A.7.14 Logical AND Operator

    logical-AND-expression:
      inclusive-OR-expression
      logical-AND-expression
&& inclusive-OR-expression

The && operator groups left-to-right. It returns 1 if both its operands compare unequal to zero, 0 otherwise. Unlike & && guarantees left-to-right evaluation: the first operand is evaluated, including all side effects; if it is equal to 0, the value of the expression is 0. Otherwise, the right operand is evaluated, and if it is equal to 0, the expression's value is 0, otherwise 1.

The operands need not have the same type, but each must have arithmetic type or be a pointer. The result is int

A.7.15 Logical OR Operator

    logical-OR-expression:
      logical-AND-expression
      logical-OR-expression
logical-AND-expression

The operator groups left-to-right. It returns 1 if either of its operands compare unequal to zero, and 0 otherwise. Unlike guarantees left-to-right evaluation: the first operand is evaluated, including all side effects; if it is unequal to 0, the value of the expression is 1. Otherwise, the right operand is evaluated, and if it is unequal to 0, the expression's value is 1, otherwise 0.

The operands need not have the same type, but each must have arithmetic type or be a pointer. The result is int

A.7.16 Conditional Operator

    conditional-expression:
      logical-OR-expression
      logical-OR-expression
expression conditional-expression

The first expression is evaluated, including all side effects; if it compares unequal to 0, the result is the value of the second expression, otherwise that of the third expression. Only one of the second and third operands is evaluated. If the second and third operands are arithmetic, the usual arithmetic conversions are performed to bring them to a common type, and that type is the type of the result. If both are void, or structures or unions of the same type, or pointers to objects of the same type, the result has the common type. If one is a pointer and the other the constant 0, the 0 is converted to the pointer type, and the result has that type. If one is a pointer to void and the other is another pointer, the other pointer is converted to a pointer to void, and that is the type of the result.

In the type comparison for pointers, any type qualifiers (Par.A.2) in the type to which the pointer points are insignificant, but the result type inherits qualifiers from both arms of the conditional.

A.7.17 Assignment Expressions

There are several assignment operators; all group right-to-left.

    assignment-expression:
      conditional-expression
      unary-expression assignment-operator assignment-expression

    assignment-operator: one of
      
= *= /= %= += -= <<= >>= &= ^= |=

All require an lvalue as left operand, and the lvalue must be modifiable: it must not be an array, and must not have an incomplete type, or be a function. Also, its type must not be qualified with const; if it is a structure or union, it must not have any member or, recursively, submember qualified with const. The type of an assignment expression is that of its left operand, and the value is the value stored in the left operand after the assignment has taken place.

In the simple assignment with , the value of the expression replaces that of the object referred to by the lvalue. One of the following must be true: both operands have arithmetic type, in which case the right operand is converted to the type of the left by the assignment; or both operands are structures or unions of the same type; or one operand is a pointer and the other is a pointer to void, or the left operand is a pointer and the right operand is a constant expression with value 0; or both operands are pointers to functions or objects whose types are the same except for the possible absence of const or volatile in the right operand.

An expression of the form E1 op= E2 is equivalent to E1 = E1 op (E2) except that E1 is evaluated only once.

A.7.18 Comma Operator

    expression:
      assignment-expression
      expression
assignment-expression

A pair of expressions separated by a comma is evaluated left-to-right, and the value of the left expression is discarded. The type and value of the result are the type and value of the right operand. All side effects from the evaluation of the left-operand are completed before beginning the evaluation of the right operand. In contexts where comma is given a special meaning, for example in lists of function arguments (Par.A.7.3.2) and lists of initializers (Par.A.7), the required syntactic unit is an assignment expression, so the comma operator appears only in a parenthetical grouping, for example,


f(a, (t=3, t+2), c)

has three arguments, the second of which has the value 5.

A.7.19 Constant Expressions

Syntactically, a constant expression is an expression restricted to a subset of operators:

    constant-expression:
      conditional-expression

Expressions that evaluate to a constant are required in several contexts: after case, as array bounds and bit-field lengths, as the value of an enumeration constant, in initializers, and in certain preprocessor expressions.

Constant expressions may not contain assignments, increment or decrement operators, function calls, or comma operators; except in an operand of sizeof. If the constant expression is required to be integral, its operands must consist of integer, enumeration, character, and floating constants; casts must specify an integral type, and any floating constants must be cast to integer. This necessarily rules out arrays, indirection, address-of, and structure member operations. (However, any operand is permitted for sizeof

More latitude is permitted for the constant expressions of initializers; the operands may be any type of constant, and the unary & operator may be applied to external or static objects, and to external and static arrays subscripted with a constant expression. The unary & operator can also be applied implicitly by appearance of unsubscripted arrays and functions. Initializers must evaluate either to a constant or to the address of a previously declared external or static object plus or minus a constant.

Less latitude is allowed for the integral constant expressions after #if sizeof expressions, enumeration constants, and casts are not permitted. See Par.A.12.5.

A.8 Declarations

Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions. Declarations have the form

    declaration:
      declaration-specifiers init-declarator-listopt

The declarators in the init-declarator list contain the identifiers being declared; the declaration-specifiers consist of a sequence of type and storage class specifiers.

    declaration-specifiers:
      storage-class-specifier declaration-specifiersopt
      type-specifier declaration-specifiersopt
      type-qualifier declaration-specifiersopt

    init-declarator-list:
      init-declarator
      init-declarator-list
init-declarator

    init-declarator:
      declarator
      declarator
initializer

Declarators will be discussed later (Par.A.5); they contain the names being declared. A declaration must have at least one declarator, or its type specifier must declare a structure tag, a union tag, or the members of an enumeration; empty declarations are not permitted.

A.1 Storage Class Specifiers

The storage class specifiers are:

    storage-class specifier:
      
auto register static extern typedef

The meaning of the storage classes were discussed in Par.A.4.4.

The auto and register specifiers give the declared objects automatic storage class, and may be used only within functions. Such declarations also serve as definitions and cause storage to be reserved. A register declaration is equivalent to an auto declaration, but hints that the declared objects will be accessed frequently. Only a few objects are actually placed into registers, and only certain types are eligible; the restrictions are implementation-dependent. However, if an object is declared register, the unary & operator may not be applied to it, explicitly or implicitly.

The rule that it is illegal to calculate the address of an object declared register, but actually taken to be auto, is new.

The static specifier gives the declared objects static storage class, and may be used either inside or outside functions. Inside a function, this specifier causes storage to be allocated, and serves as a definition; for its effect outside a function, see Par.A.11.2.

A declaration with extern, used inside a function, specifies that the storage for the declared objects is defined elsewhere; for its effects outside a function, see Par.A.11.2.



The typedef specifier does not reserve storage and is called a storage class specifier only for syntactic convenience; it is discussed in Par.A.9.

At most one storage class specifier may be given in a declaration. If none is given, these rules are used: objects declared inside a function are taken to be auto; functions declared within a function are taken to be extern; objects and functions declared outside a function are taken to be static, with external linkage. See Pars. A.10-A.11.

A.2 Type Specifiers

The type-specifiers are

    type specifier:
      
void char short int long float double signed unsigned
      struct-or-union-specifier
      enum-specifier
      typedef-name

At most one of the words long or short may be specified together with int; the meaning is the same if int is not mentioned. The word long may be specified together with double. At most one of signed or unsigned may be specified together with int or any of its short or long varieties, or with char. Either may appear alone in which case int is understood. The signed specifier is useful for forcing char objects to carry a sign; it is permissible but redundant with other integral types.

Otherwise, at most one type-specifier may be given in a declaration. If the type-specifier is missing from a declaration, it is taken to be int

Types may also be qualified, to indicate special properties of the objects being declared.

    type-qualifier:
      
const volatile

Type qualifiers may appear with any type specifier. A const object may be initialized, but not thereafter assigned to. There are no implementation-dependent semantics for volatile objects.

The const and volatile properties are new with the ANSI standard. The purpose of const is to announce objects that may be placed in read-only memory, and perhaps to increase opportunities for optimization. The purpose of volatile is to force an implementation to suppress optimization that could otherwise occur. For example, for a machine with memory-mapped input/output, a pointer to a device register might be declared as a pointer to volatile, in order to prevent the compiler from removing apparently redundant references through the pointer. Except that it should diagnose explicit attempts to change const objects, a compiler may ignore these qualifiers.

A.3 Structure and Union Declarations

A structure is an object consisting of a sequence of named members of various types. A union is an object that contains, at different times, any of several members of various types. Structure and union specifiers have the same form.

    struct-or-union-specifier:
      struct-or-union identifieropt

      struct-or-union identifier

    struct-or-union:
      
struct union

A struct-declaration-list is a sequence of declarations for the members of the structure or union:

    struct-declaration-list:
      struct declaration
      struct-declaration-list struct declaration

    struct-declaration:       specifier-qualifier-list struct-declarator-list

    specifier-qualifier-list:
      type-specifier specifier-qualifier-listopt
      type-qualifier specifier-qualifier-listopt

    struct-declarator-list:
      struct-declarator
      struct-declarator-list
struct-declarator

Usually, a struct-declarator is just a declarator for a member of a structure or union. A structure member may also consist of a specified number of bits. Such a member is also called a bit-field; its length is set off from the declarator for the field name by a colon.

    struct-declarator:
      declarator       declaratoropt
constant-expression

A type specifier of the form

    struct-or-union identifier

declares the identifier to be the tag of the structure or union specified by the list. A subsequent declaration in the same or an inner scope may refer to the same type by using the tag in a specifier without the list:

    struct-or-union identifier

If a specifier with a tag but without a list appears when the tag is not declared, an incomplete type is specified. Objects with an incomplete structure or union type may be mentioned in contexts where their size is not needed, for example in declarations (not definitions), for specifying a pointer, or for creating a typedef, but not otherwise. The type becomes complete on occurrence of a subsequent specifier with that tag, and containing a declaration list. Even in specifiers with a list, the structure or union type being declared is incomplete within the list, and becomes complete only at the terminating the specifier.

A structure may not contain a member of incomplete type. Therefore, it is impossible to declare a structure or union containing an instance of itself. However, besides giving a name to the structure or union type, tags allow definition of self-referential structures; a structure or union may contain a pointer to an instance of itself, because pointers to incomplete types may be declared.

A very special rule applies to declarations of the form

    struct-or-union identifier

that declare a structure or union, but have no declaration list and no declarators. Even if the identifier is a structure or union tag already declared in an outer scope (Par.A.11.1), this declaration makes the identifier the tag of a new, incompletely-typed structure or union in the current scope.

This recondite is new with ANSI. It is intended to deal with mutually-recursive structures declared in an inner scope, but whose tags might already be declared in the outer scope.

A structure or union specifier with a list but no tag creates a unique type; it can be referred to directly only in the declaration of which it is a part.

The names of members and tags do not conflict with each other or with ordinary variables. A member name may not appear twice in the same structure or union, but the same member name may be used in different structures or unions.

In the first edition of this book, the names of structure and union members were not associated with their parent. However, this association became common in compilers well before the ANSI standard.

A non-field member of a structure or union may have any object type. A field member (which need not have a declarator and thus may be unnamed) has type int unsigned int, or signed int, and is interpreted as an object of integral type of the specified length in bits; whether an int field is treated as signed is implementation-dependent. Adjacent field members of structures are packed into implementation-dependent storage units in an implementation-dependent direction. When a field following another field will not fit into a partially-filled storage unit, it may be split between units, or the unit may be padded. An unnamed field with width 0 forces this padding, so that the next field will begin at the edge of the next allocation unit.

The ANSI standard makes fields even more implementation-dependent than did the first edition. It is advisable to read the language rules for storing bit-fields as ``implementation-dependent'' without qualification. Structures with bit-fields may be used as a portable way of attempting to reduce the storage required for a structure (with the probable cost of increasing the instruction space, and time, needed to access the fields), or as a non-portable way to describe a storage layout known at the bit-level. In the second case, it is necessary to understand the rules of the local implementation.

The members of a structure have addresses increasing in the order of their declarations. A non-field member of a structure is aligned at an addressing boundary depending on its type; therefore, there may be unnamed holes in a structure. If a pointer to a structure is cast to the type of a pointer to its first member, the result refers to the first member.

A union may be thought of as a structure all of whose members begin at offset 0 and whose size is sufficient to contain any of its members. At most one of the members can be stored in a union at any time. If a pointr to a union is cast to the type of a pointer to a member, the result refers to that member.

A simple example of a structure declaration is


struct tnode

which contains an array of 20 characters, an integer, and two pointers to similar structures. Once this declaration has bene given, the declaration


struct tnode s, *sp;

declares s to be a structure of the given sort, and sp to be a pointer to a structure of the given sort. With these declarations, the expression


sp->count

refers to the count field of the structure to which sp points;


s.left

refers to the left subtree pointer of the structure s, and


s.right->tword[0]

refers to the first character of the tword member of the right subtree of s

In general, a member of a union may not be inspected unless the value of the union has been assigned using the same member. However, one special guarantee simplifies the use of unions: if a union contains several structures that share a common initial sequence, and the union currently contains one of these structures, it is permitted to refer to the common initial part of any of the contained structures. For example, the following is a legal fragment:


union n;
struct ni;
struct nf;
} u;

u.nf.type = FLOAT;
u.nf.floatnode = 3.14;

if (u.n.type == FLOAT)
sin(u.nf.floatnode)

A.4 Enumerations

Enumerations are unique types with values ranging over a set of named constants called enumerators. The form of an enumeration specifier borrows from that of structures and unions.

    enum-specifier:
      
enum identifieropt enum identifier

    enumerator-list:
      enumerator
      enumerator-list
enumerator

    enumerator:
      identifier
      identifier
constant-expression

The identifiers in an enumerator list are declared as constants of type int, and may appear wherever constants are required. If no enumerations with appear, then the values of the corresponding constants begin at 0 and increase by 1 as the declaration is read from left to right. An enumerator with gives the associated identifier the value specified; subsequent identifiers continue the progression from the assigned value.

Enumerator names in the same scope must all be distinct from each other and from ordinary variable names, but the values need not be distinct.

The role of the identifier in the enum-specifier is analogous to that of the structure tag in a struct-specifier; it names a particular enumeration. The rules for enum-specifiers with and without tags and lists are the same as those for structure or union specifiers, except that incomplete enumeration types do not exist; the tag of an enum-specifier without an enumerator list must refer to an in-scope specifier with a list.

Enumerations are new since the first edition of this book, but have been part of the language for some years.

A.5 Declarators

Declarators have the syntax:

    declarator:
      pointeropt direct-declarator

    direct-declarator:
      identifier
      
declarator
      direct-declarator
constant-expressionopt
      direct-declarator
parameter-type-list
      direct-declarator
identifier-listopt

    pointer:
      
type-qualifier-listopt
      
type-qualifier-listopt pointer

    type-qualifier-list:
      type-qualifier
      type-qualifier-list type-qualifier

The structure of declarators resembles that of indirection, function, and array expressions; the grouping is the same.

A.6 Meaning of Declarators

A list of declarators appears after a sequence of type and storage class specifiers. Each declarator declares a unique main identifier, the one that appears as the first alternative of the production for direct-declarator. The storage class specifiers apply directly to this identifier, but its type depends on the form of its declarator. A declarator is read as an assertion that when its identifier appears in an expression of the same form as the declarator, it yields an object of the specified type.

Considering only the type parts of the declaration specifiers (Par. A.2) and a particular declarator, a declaration has the form ``T D,'' where T is a type and D is a declarator. The type attributed to the identifier in the various forms of declarator is described inductively using this notation.

In a declaration T D where D is an unadored identifier, the type of the identifier is T

In a declaration T D where D has the form


( D1 )

then the type of the identifier in D1 is the same as that of D. The parentheses do not alter the type, but may change the binding of complex declarators.

A.6.1 Pointer Declarators

In a declaration T D where D has the form

type-qualifier-listopt D1

and the type of the identifier in the declaration T D1 is ``type-modifier T,'' the type of the identifier of D is ``type-modifier type-qualifier-list pointer to T.'' Qualifiers following apply to pointer itself, rather than to the object to which the pointer points.

For example, consider the declaration


int *ap[];

Here, ap[] plays the role of D1; a declaration ``int ap[]'' (below) would give ap the type ``array of int,'' the type-qualifier list is empty, and the type-modifier is ``array of.'' Hence the actual declaration gives ap the type ``array to pointers to int

As other examples, the declarations


int i, *pi, *const cpi = &i;
const int ci = 3, *pci;

declare an integer i and a pointer to an integer pi. The value of the constant pointer cpi may not be changed; it will always point to the same location, although the value to which it refers may be altered. The integer ci is constant, and may not be changed (though it may be initialized, as here.) The type of pci is ``pointer to const int,'' and pci itself may be changed to point to another place, but the value to which it points may not be altered by assigning through pci

A.6.2 Array Declarators

In a declaration T D where D has the form

D1 [constant-expressionopt

and the type of the identifier in the declaration T D1 is ``type-modifier T,'' the type of the identifier of D is ``type-modifier array of T.'' If the constant-expression is present, it must have integral type, and value greater than 0. If the constant expression specifying the bound is missing, the array has an incomplete type.

An array may be constructed from an arithmetic type, from a pointer, from a structure or union, or from another array (to generate a multi-dimensional array). Any type from which an array is constructed must be complete; it must not be an array of structure of incomplete type. This implies that for a multi-dimensional array, only the first dimension may be missing. The type of an object of incomplete aray type is completed by another, complete, declaration for the object (Par.A.10.2), or by initializing it (Par.A.7). For example,


float fa[17], *afp[17];

declares an array of float numbers and an array of pointers to float numbers. Also,


static int x3d[3][5][7];

declares a static three-dimensional array of integers, with rank 3 X X 7. In complete detail, x3d is an array of three items: each item is an array of five arrays; each of the latter arrays is an array of seven integers. Any of the expressions x3d x3d[i] x3d[i][j] x3d[i][j][k] may reasonably appear in an expression. The first three have type ``array,'', the last has type int. More specifically, x3d[i][j] is an array of 7 integers, and x3d[i] is an array of 5 arrays of 7 integers.

The array subscripting operation is defined so that E1[E2] is identical to *(E1+E2). Therefore, despite its asymmetric appearance, subscripting is a commutative operation. Because of the conversion rules that apply to and to arrays (Pars.A6.6, A.7.1, A.7.7), if E1 is an array and E2 an integer, then E1[E2] refers to the E2-th member of E1

In the example, x3d[i][j][k] is equivalent to *(x3d[i][j] + k). The first subexpression x3d[i][j] is converted by Par.A.7.1 to type ``pointer to array of integers,'' by Par.A.7.7, the addition involves multiplication by the size of an integer. It follows from the rules that arrays are stored by rows (last subscript varies fastest) and that the first subscript in the declaration helps determine the amount of storage consumed by an array, but plays no other part in subscript calculations.

A.6.3 Function Declarators

In a new-style function declaration T D where D has the form

D1 (parameter-type-list)

and the type of the identifier in the declaration T D1 is ``type-modifier T,'' the type of the identifier of D is ``type-modifier function with arguments parameter-type-list returning T

The syntax of the parameters is

    parameter-type-list:
      parameter-list
      parameter-list

    parameter-list:
      parameter-declaration
      parameter-list
parameter-declaration

    parameter-declaration:
      declaration-specifiers declarator
      declaration-specifiers abstract-declaratoropt

In the new-style declaration, the parameter list specifies the types of the parameters. As a special case, the declarator for a new-style function with no parameters has a parameter list consisting soley of the keyword void. If the parameter list ends with an ellipsis `` '', then the function may accept more arguments than the number of parameters explicitly described, see Par.A.7.3.2.

The types of parameters that are arrays or functions are altered to pointers, in accordance with the rules for parameter conversions; see Par.A.10.1. The only storage class specifier permitted in a parameter's declaration is register, and this specifier is ignored unless the function declarator heads a function definition. Similarly, if the declarators in the parameter declarations contain identifiers and the function declarator does not head a function definition, the identifiers go out of scope immediately. Abstract declarators, which do not mention the identifiers, are discussed in Par.A.

In an old-style function declaration T D where D has the form

D1(identifier-listopt)

and the type of the identifier in the declaration T D1 is ``type-modifier T,'' the type of the identifier of D is ``type-modifier function of unspecified arguments returning T.'' The parameters (if present) have the form

    identifier-list:
      identifier
      identifier-list
identifier

In the old-style declarator, the identifier list must be absent unless the declarator is used in the head of a function definition (Par.A.10.1). No information about the types of the parameters is supplied by the declaration.

For example, the declaration


int f(), *fpi(), (*pfi)();

declares a function f returning an integer, a function fpi returning a pointer to an integer, and a pointer pfi to a function returning an integer. In none of these are the parameter types specified; they are old-style.

In the new-style declaration


int strcpy(char *dest, const char *source), rand(void);

strcpy is a function returning int, with two arguments, the first a character pointer, and the second a pointer to constant characters. The parameter names are effectively comments. The second function rand takes no arguments and returns int

Function declarators with parameter prototypes are, by far, the most important language change introduced by the ANSI standard. They offer an advantage over the ``old-style'' declarators of the first edition by providing error-detection and coercion of arguments across function calls, but at a cost: turmoil and confusion during their introduction, and the necessity of accomodating both forms. Some syntactic ugliness was required for the sake of compatibility, namely void as an explicit marker of new-style functions without parameters.

The ellipsis notation `` '' for variadic functions is also new, and, together with the macros in the standard header <stdarg.h>, formalizes a mechanism that was officially forbidden but unofficially condoned in the first edition.

These notations were adapted from the C++ language.

A.7 Initialization

When an object is declared, its init-declarator may specify an initial value for the identifier being declared. The initializer is preceded by , and is either an expression, or a list of initializers nested in braces. A list may end with a comma, a nicety for neat formatting.

    initializer:
      assignment-expression
      

    initializer-list:
      initializer
      initializer-list
initializer

All the expressions in the initializer for a static object or array must be constant expressions as described in Par.A.7.19. The expressions in the initializer for an auto or register object or array must likewise be constant expressions if the initializer is a brace-enclosed list. However, if the initializer for an automatic object is a single expression, it need not be a constant expression, but must merely have appropriate type for assignment to the object.

The first edition did not countenance initialization of automatic structures, unions, or arrays. The ANSI standard allows it, but only by constant constructions unless the initializer can be expressed by a simple expression.

A static object not explicitly initialized is initialized as if it (or its members) were assigned the constant 0. The initial value of an automatic object not explicitly intialized is undefined.

The initializer for a pointer or an object of arithmetic type is a single expression, perhaps in braces. The expression is assigned to the object.

The initializer for a structure is either an expression of the same type, or a brace-enclosed list of initializers for its members in order. Unnamed bit-field members are ignored, and are not initialized. If there are fewer initializers in the list than members of the structure, the trailing members are initialized with 0. There may not be more initializers than members. Unnamed bit-field members are ignored,and are not initialized.

The initializer for an array is a brace-enclosed list of initializers for its members. If the array has unknown size, the number of initializers determines the size of the array, and its type becomes complete. If the array has fixed size, the number of initializers may not exceed the number of members of the array; if there are fewer, the trailing members are initialized with 0.

As a special case, a character array may be initialized by a string literal; successive characters of the string initialize successive members of the array. Similarly, a wide character literal (Par.A.2.6) may initialize an array of type wchar_t. If the array has unknown size, the number of characters in the string, including the terminating null character, determines its size; if its size is fixed, the number of characters in the string, not counting the terminating null character, must not exceed the size of the array.

The initializer for a union is either a single expression of the same type, or a brace-enclosed initializer for the first member of the union.

The first edition did not allow initialization of unions. The ``first-member'' rule is clumsy, but is hard to generalize without new syntax. Besides allowing unions to be explicitly initialized in at least a primitive way, this ANSI rule makes definite the semantics of static unions not explicitly initialized.

An aggregate is a structure or array. If an aggregate contains members of aggregate type, the initialization rules apply recursively. Braces may be elided in the initialization as follows: if the initializer for an aggregate's member that itself is an aggregate begins with a left brace, then the succeding comma-separated list of initializers initializes the members of the subaggregate; it is erroneous for there to be more initializers than members. If, however, the initializer for a subaggregate does not begin with a left brace, then only enough elements from the list are taken into account for the members of the subaggregate; any remaining members are left to initialize the next member of the aggregate of which the subaggregate is a part.

For example,


int x[] = ;

declares and initializes x as a 1-dimensional array with three members, since no size was specified and there are three initializers.


float y[4][3] = {
,
,
,
};

is a completely-bracketed initialization: 1, 3 and 5 initialize the first row of the array y[0], namely y[0][0] y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early, and therefore the elements of y[3] are initialized with 0. Precisely the same effect could have been achieved by


float y[4][3] = ;

The initializer for y begins with a left brace, but that for y[0] does not; therefore three elements from the list are used. Likewise the next three are taken successively for y[1] and for y[2]. Also,


float y[4][3] = , , ,
};

initializes the first column of y (regarded as a two-dimensional array) and leaves the rest 0.

Finally,


char msg[] = 'Syntax error on line %sn';

shows a character array whose members are initialized with a string; its size includes the terminating null character.

A.8 Type names

In several contexts (to specify type conversions explicitly with a cast, to declare parameter types in function declarators, and as argument of sizeof) it is necessary to supply the name of a data type. This is accomplished using a type name, which is syntactically a declaration for an object of that type omitting the name of the object.

    type-name:
      specifier-qualifier-list abstract-declaratoropt

    abstract-declarator:
      pointer
      pointeropt direct-abstract-declarator

    direct-abstract-declarator:
      ( abstract-declarator )
      direct-abstract-declaratoropt [constant-expressionopt]
      direct-abstract-declaratoropt (parameter-type-listopt)

It is possible to identify uniquely the location in the abstract-declarator where the identifier would appear if the construction were a declarator in a declaration. The named type is then the same as the type of the hypothetical identifier. For example,


int
int *
int *[3]
int (*)[]
int *()
int (*[])(void)

name respectively the types ``integer,'' ``pointer to integer,'' ``array of 3 pointers to integers,'' ``pointer to an unspecified number of integers,'' ``function of unspecified parameters returning pointer to integer,'' and ``array, of unspecified size, of pointers to functions with no parameters each returning an integer.''

A.9 Typedef

Declarations whose storage class specifier is typedef do not declare objects; instead they define identifiers that name types. These identifiers are called typedef names.

    typedef-name:
      identifier

A typedef declaration attributes a type to each name among its declarators in the usual way (see Par.A.6). Thereafter, each such typedef name is syntactically equivalent to a type specifier keyword for the associated type.

For example, after


typedef long Blockno, *Blockptr;
typedef struct Complex;

the constructions


Blockno b;
extern Blockptr bp;
Complex z, *zp;

are legal declarations. The type of b is long, that of bp is ``pointer to long,'' and that of z is the specified structure; zp is a pointer to such a structure.

typedef does not introduce new types, only synonyms for types that could be specified in another way. In the example, b has the same type as any long object.

Typedef names may be redeclared in an inner scope, but a non-empty set of type specifiers must be given. For example,


extern Blockno;

does not redeclare Blockno, but


extern int Blockno;

does.

A.10 Type Equivalence

Two type specifier lists are equivalent if they contain the same set of type specifiers, taking into account that some specifiers can be implied by others (for example, long alone implies long int). Structures, unions, and enumerations with different tags are distinct, and a tagless union, structure, or enumeration specifies a unique type.

Two types are the same if their abstract declarators (Par.A.8), after expanding any typedef types, and deleting any function parameter specifiers, are the same up to the equivalence of type specifier lists. Array sizes and function parameter types are significant.






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