man3/fread.3

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.\"     @(#)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  2015-07-23 "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>

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) / sizeof(*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)