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.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" and Copyright (c) 2020 Arkadiusz Drabczyk <arkadiusz@drabczyk.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)
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\" %%%LICENSE_END
.\"
.\" @(#)fread.3 6.6 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 15:37:33 1993, faith@cs.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 <jrv@vanzandt.mv.com>
.\" Modified Fri May 17 10:21:51 1996 by Martin Schulze <joey@infodrom.north.de>
.\"
.TH FREAD 3 2020-08-13 "GNU" "Linux Programmer's Manual"
.SH NAME
fread, fwrite \- binary stream input/output
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "size_t fread(void *" ptr ", size_t " size ", size_t " nmemb \
", FILE *" stream );
.PP
.BI "size_t fwrite(const void *" ptr ", size_t " size ", size_t " nmemb ,
.BI " FILE *" stream );
.fi
.SH DESCRIPTION
The function
.BR fread ()
reads
.I nmemb
items of data, each
.I size
bytes long, from the stream pointed to by
.IR stream ,
storing them at the location given by
.IR ptr .
.PP
The function
.BR fwrite ()
writes
.I nmemb
items of data, each
.I size
bytes long, to the stream pointed to by
.IR stream ,
obtaining them from the location given by
.IR ptr .
.PP
For nonlocking counterparts, see
.BR unlocked_stdio (3).
.SH RETURN VALUE
On success,
.BR fread ()
and
.BR fwrite ()
return the number of items read or written.
This number equals the number of bytes transferred only when
.I 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).
.PP
The file position indicator for the stream is advanced by the number
of bytes successfully read or written.
.PP
.BR fread ()
does not distinguish between end-of-file and error, and callers must use
.BR feof (3)
and
.BR ferror (3)
to determine which occurred.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw17 lb lb
l l l.
Interface Attribute Value
T{
.BR fread (),
.BR fwrite ()
T} Thread safety MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89.
.SH EXAMPLES
The program below demonstrates the use of
.BR fread ()
by parsing /bin/sh ELF executable in binary mode and printing its
magic and class:
.PP
.in +4n
.EX
$ \fB./a.out\fP
ELF magic: 0x7f454c46
Class: 0x02
.EE
.in
.SS Program source
\&
.EX
#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, ARRAY_SIZE(buffer), sizeof(*buffer), fp);
if (ret != sizeof(*buffer)) {
fprintf(stderr, "fread() failed: %zu\en", ret);
exit(EXIT_FAILURE);
}
printf("ELF magic: %#04x%02x%02x%02x\en", buffer[0], buffer[1],
buffer[2], buffer[3]);
ret = fread(buffer, 1, 1, fp);
if (ret != 1) {
fprintf(stderr, "fread() failed: %zu\en", ret);
exit(EXIT_FAILURE);
}
printf("Class: %#04x\en", buffer[0]);
fclose(fp);
exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR read (2),
.BR write (2),
.BR feof (3),
.BR ferror (3),
.BR unlocked_stdio (3)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I 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/.
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