fwrite man page


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.

fwrite — binary output


#include <stdio.h>

size_t fwrite(const 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 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().

Return Value

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.


Refer to fputc().

The following sections are informative.



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.



Future Directions


See Also

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

The Base Definitions volume of POSIX.1‐2008, <stdio.h>


2013 IEEE/The Open Group POSIX Programmer's Manual