FREAD(3P) POSIX Programmer's Manual FREAD(3P)PROLOG
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
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 posi‐
tion 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 sup‐
plied 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 indica‐
tor for the stream shall be set, and errno shall be set to indicate the
Refer to fgetc().
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.
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
The following example reads multiple single-byte elements from the fp
stream into the array pointed to by buf.
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.
Section 2.5, Standard I/O Streams, feof(), ferror(), fgetc(), fopen(),
fscanf(), getc(), gets()
The Base Definitions volume of POSIX.1‐2008, <stdio.h>
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 Electri‐
cal 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.ker‐
IEEE/The Open Group 2013 FREAD(3P)