Name

fread, fwrite — binary stream input/output

Synopsis

#include <stdio.h>
size_t fread( void *restrict ptr,
  size_t size,
  size_t nmemb,
  FILE *restrict stream);
 
size_t fwrite( const void *restrict ptr,
  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 attributes(7).

Interface Attribute Value
fread(), fwrite() Thread safety MT-Safe

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, C89.

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 = fopen("/bin/sh", "rb");
    if (!fp) {
        perror("fopen");
        return EXIT_FAILURE;
    }

    unsigned char buffer[4];

    size_t 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)

COLOPHON

This page is part of release 5.11 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright (c) 1990, 1991 The Regents of the University of California.
and Copyright (c) 2020 Arkadiusz Drabczyk <arkadiuszdrabczyk.org>
All rights reserved.

This code is derived from software contributed to Berkeley by
Chris Torek and the American National Standards Committee X3,
on Information Processing Systems.

%%%LICENSE_START(BSD_4_CLAUSE_UCB)
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modification, are permitted provided that the following conditions
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   notice, this list of conditions and the following disclaimer.
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   may be used to endorse or promote products derived from this software
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THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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    (#)fread.3 6.6 (Berkeley) 6/29/91

Converted for Linux, Mon Nov 29 15:37:33 1993, faithcs.unc.edu
Sun Feb 19 21:26:54 1995 by faith, return values
Modified Thu Apr 20 20:43:53 1995 by Jim Van Zandt <jrvvanzandt.mv.com>
Modified Fri May 17 10:21:51 1996 by Martin Schulze <joeyinfodrom.north.de>