fread(3) Library Functions Manual fread(3)
NAME
fread, fwrite - binary stream input/output
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdio.h>
size_t fread(void ptr[restrict .size * .nmemb],
size_t size, size_t nmemb,
FILE *restrict stream);
size_t fwrite(const void ptr[restrict .size * .nmemb],
size_t size, size_t nmemb,
FILE *restrict stream);
DESCRIPTION
The function fread() reads nmemb items of data, each size bytes long,
from the stream pointed to by stream, storing them at the location
given by ptr.
The function fwrite() writes nmemb items of data, each size bytes long,
to the stream pointed to by stream, obtaining them from the location
given by ptr.
For nonlocking counterparts, see unlocked_stdio(3).
RETURN VALUE
On success, fread() and fwrite() return the number of items read or
written. This number equals the number of bytes transferred only when
size is 1. If an error occurs, or the end of the file is reached, the
return value is a short item count (or zero).
The file position indicator for the stream is advanced by the number of
bytes successfully read or written.
fread() does not distinguish between end-of-file and error, and callers
must use feof(3) and ferror(3) to determine which occurred.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│fread(), fwrite() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008, C99.
EXAMPLES
The program below demonstrates the use of fread() by parsing /bin/sh
ELF executable in binary mode and printing its magic and class:
$ ./a.out
ELF magic: 0x7f454c46
Class: 0x02
Program source
#include <stdio.h>
#include <stdlib.h>
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
int
main(void)
{
FILE *fp;
size_t ret;
unsigned char buffer[4];
fp = fopen("/bin/sh", "rb");
if (!fp) {
perror("fopen");
return EXIT_FAILURE;
}
ret = fread(buffer, sizeof(*buffer), ARRAY_SIZE(buffer), fp);
if (ret != ARRAY_SIZE(buffer)) {
fprintf(stderr, "fread() failed: %zu\n", ret);
exit(EXIT_FAILURE);
}
printf("ELF magic: %#04x%02x%02x%02x\n", buffer[0], buffer[1],
buffer[2], buffer[3]);
ret = fread(buffer, 1, 1, fp);
if (ret != 1) {
fprintf(stderr, "fread() failed: %zu\n", ret);
exit(EXIT_FAILURE);
}
printf("Class: %#04x\n", buffer[0]);
fclose(fp);
exit(EXIT_SUCCESS);
}
SEE ALSO
read(2), write(2), feof(3), ferror(3), unlocked_stdio(3)
Linux man-pages 6.03 2022-12-29 fread(3)
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