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FREAD(3P) POSIX Programmer's Manual FREAD(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
fread — binary input
#include <stdio.h>
size_t fread(void *restrict ptr, size_t size, size_t nitems,
FILE *restrict stream);
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements described
here and the ISO C standard is unintentional. This volume of
POSIX.1‐2008 defers to the ISO C standard.
The fread() function shall read into the array pointed to by ptr up
to nitems elements whose size is specified by size in bytes, from the
stream pointed to by stream. For each object, size calls shall be
made to the fgetc() function and the results stored, in the order
read, in an array of unsigned char exactly overlaying the object. The
file position indicator for the stream (if defined) shall be advanced
by the number of bytes successfully read. If an error occurs, the
resulting value of the file position indicator for the stream is
unspecified. If a partial element is read, its value is unspecified.
The fread() function may mark the last data access timestamp of the
file associated with stream for update. The last data access
timestamp shall be marked for update by the first successful
execution of fgetc(), fgets(), fread(), fscanf(), getc(), getchar(),
getdelim(), getline(), gets(), or scanf() using stream that returns
data not supplied by a prior call to ungetc().
Upon successful completion, fread() shall return the number of
elements successfully read which is less than nitems only if a read
error or end-of-file is encountered. If size or nitems is 0, fread()
shall return 0 and the contents of the array and the state of the
stream remain unchanged. Otherwise, if a read error occurs, the error
indicator for the stream shall be set, and errno shall be set to
indicate the error.
Refer to fgetc(3p).
The following sections are informative.
Reading from a Stream
The following example reads a single element from the fp stream into
the array pointed to by buf.
#include <stdio.h>
...
size_t elements_read;
char buf[100];
FILE *fp;
...
elements_read = fread(buf, sizeof(buf), 1, fp);
...
If a read error occurs, elements_read will be zero but the number of
bytes read from the stream could be anything from zero to
sizeof(buf)−1.
The following example reads multiple single-byte elements from the fp
stream into the array pointed to by buf.
#include <stdio.h>
...
size_t bytes_read;
char buf[100];
FILE *fp;
...
bytes_read = fread(buf, 1, sizeof(buf), fp);
...
If a read error occurs, bytes_read will contain the number of bytes
read from the stream.
The ferror() or feof() functions must be used to distinguish between
an error condition and an end-of-file condition.
Because of possible differences in element length and byte ordering,
files written using fwrite() are application-dependent, and possibly
cannot be read using fread() by a different application or by the
same application on a different processor.
None.
None.
Section 2.5, Standard I/O Streams, feof(3p), ferror(3p), fgetc(3p),
fopen(3p), fscanf(3p), getc(3p), gets(3p)
The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FREAD(3P)
Pages that refer to this page: stdio.h(0p), fgetc(3p), fgets(3p), fgetws(3p), stdin(3p)