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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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fread(3) Library Functions Manual fread(3)
fread, fwrite - binary stream input/output
Standard C library (libc, -lc)
#include <stdio.h>
size_t fread(size_t size, size_t n;
void ptr[restrict size * n],
size_t size, size_t n,
FILE *restrict stream);
size_t fwrite(size_t size, size_t n;
const void ptr[restrict size * n],
size_t size, size_t n,
FILE *restrict stream);
The function fread() reads n 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 n 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).
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.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ fread(), fwrite() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
POSIX.1-2001, C89.
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);
}
read(2), write(2), feof(3), ferror(3), unlocked_stdio(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 fread(3)
Pages that refer to this page: pmlogger(1), read(2), write(2), fgetc(3), FILE(3type), getline(3), gets(3), getw(3), puts(3), setbuf(3), size_t(3type), stdin(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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