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.
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 fwrite() function shall write, from the array pointed to by ptr,
up to nitems elements whose size is specified by size, to the stream
pointed to by stream. For each object, size calls shall be made to
the fputc() function, taking the values (in order) from 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 written. If an error occurs, the resulting
value of the file-position indicator for the stream is unspecified.
The last data modification and last file status change timestamps of
the file shall be marked for update between the successful execution
of fwrite() and the next successful completion of a call to fflush()
or fclose() on the same stream, or a call to exit() or abort().
The fwrite() function shall return the number of elements
successfully written, which may be less than nitems if a write error
is encountered. If size or nitems is 0, fwrite() shall return 0 and
the state of the stream remains unchanged. Otherwise, if a write
error occurs, the error indicator for the stream shall be set, and
errno shall be set to indicate the error.
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.
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
IEEE/The Open Group 2013 FWRITE(3P)