NAME

fwrite — binary output

SYNOPSIS

#include <stdio.h>

size_t fwrite(const void *restrict ptr, size_t size, size_t nitems,
       FILE *restrict stream);


DESCRIPTION

^[CX] 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.1-2024 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.

^[CX] 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().

RETURN VALUE

The fwrite() function shall return the number of elements successfully written, which shall be less than nitems only 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, ^[CX]  and errno shall be set to indicate the error.

ERRORS

Refer to fputc.

EXAMPLES

None.

APPLICATION USAGE

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, ferror, fopen, fprintf, putc, puts, write

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

Extensions beyond the ISO C standard are marked.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

• The fwrite() prototype is updated.

• The DESCRIPTION is updated to clarify how the data is written out using fputc().

Issue 7

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0228 [14] is applied.

Issue 8

Austin Group Defect 1196 is applied, clarifying the RETURN VALUE section.