#include <stdio.h>

int fflush(FILE *stream);


Partially implemented


IEEE Std 1003.1-2017


If stream points to an output stream or an update stream in which the most recent operation was not input, fflush() shall cause any unwritten data for that stream to be written to the file, and the last data modification and last file status change timestamps of the underlying file shall be marked for update.

For a stream open for reading with an underlying file description, if the file is not already at EOF, and the file is one capable of seeking, the file offset of the underlying open file description shall be set to the file position of the stream, and any characters pushed back onto the stream by ungetc() or ungetwc() that have not subsequently been read from the stream shall be discarded (without further changing the file offset).

If stream is a null pointer, fflush() shall perform this flushing action on all streams for which the behavior is defined above.

Return value#

Upon successful completion, fflush() shall return 0. Otherwise, it shall set the error indicator for the stream, return EOF, and set errno to indicate the error.


The fflush() function shall fail if:

  • EAGAIN - The O_NONBLOCK flag is set for the file descriptor underlying stream and the thread would be delayed in the write operation.

  • EBADF - The file descriptor underlying stream is not valid.

  • EFBIG - An attempt was made to write a file that exceeds the maximum file size.

  • EFBIG - An attempt was made to write a file that exceeds the file size limit of the process.

  • EFBIG - The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream.

  • EINTR - The fflush() function was interrupted by a signal.

  • EIO - The process is a member of a background process group attempting to write to its controlling terminal, TOSTOP is set, the calling thread is not blocking SIGTTOU, the process is not ignoring SIGTTOU, and the process group of the process is orphaned. This error may also be returned under implementation-defined conditions.

  • ENOMEM - The underlying stream was created by open_memstream() or open_wmemstream() and insufficient memory is available.

  • ENOSPC - There was no free space remaining on the device containing the file or in the buffer used by the fmemopen() function.

  • EPIPE - An attempt is made to write to a pipe or FIFO that is not open for reading by any process. A SIGPIPE signal shall also be sent to the thread.

The fflush() function may fail if:

  • ENXIO - A request was made of a nonexistent device, or the request was outside the capabilities of the device.



Known bugs#


See Also#

  1. Standard library functions

  2. Table of Contents