int fflush(FILE *stream);
If stream points to an output stream or an update stream in which the
most recent operation was not input, fflush() causes any unwritten data
for that stream to be written to the file, and the st_ctime and
st_mtime fields of the underlying file are marked for update.
If stream points to an input stream or an update stream
into which the most recent operation was input, that stream is flushed if it
is seekable and is not already at end-of-file. Flushing an input stream
discards any buffered input and adjusts the file pointer such that the next
input operation accesses the byte after the last one read. A stream is
seekable if the underlying file is not a pipe, FIFO, socket, or TTY
If stream is a null pointer, fflush() performs this
flushing action on all streams for which the behavior is defined above.
An input stream, seekable or non-seekable, can be flushed by
explicitly calling fflush() with a non-null argument specifying that
Upon successful completion, fflush() returns 0. Otherwise, it
returns EOF and sets errno to indicate the error.
The fflush() function will fail if:
The O_NONBLOCK flag is set for the file descriptor
underlying stream and the process would be delayed in the write
The file descriptor underlying stream is not
An attempt was made to write a file that exceeds the
maximum file size or the process's file size limit; or the file is a regular
file and an attempt was made to write at or beyond the offset maximum
associated with the corresponding stream.
The fflush() function was interrupted by a
The process is a member of a background process group
attempting to write to its controlling terminal, TOSTOP
is set, the
process is neither ignoring nor blocking SIGTTOU
, and the process group
of the process is orphaned. stream
was created by
and a wide-character or byte sequence buffered is
invalid in the current locale.
There was no free space remaining on the device
containing the file.
An attempt is made to write to a pipe or FIFO that
is not open for reading by any process. A SIGPIPE signal will also be
sent to the calling process.
The fflush() function may fail if:
A request was made of a non-existent device, or the
request was beyond the limits of the device.
See attributes(7) for descriptions of the following attributes: