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 IEEE Std 1003.1-2001 defers to
the ISO C standard.
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
st_ctime and st_mtime fields of the underlying file shall
be marked for update.
If stream is a null pointer, fflush() shall perform this
flushing action on all streams for which the behavior is
Upon successful completion, fflush() shall return 0; otherwise,
it shall set the error indicator for the stream, return
and set errno to indicate the error.
The fflush() function shall 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 valid.
attempt was made to write a file that exceeds the maximum file size.
An attempt was made to write a file that exceeds the process' file
The file is a regular file and an attempt was made to write at or
beyond the offset maximum associated with the corresponding
The fflush() function was interrupted by a signal.
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. This error may also be returned under
There was no free space remaining on the device containing the file.
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
The fflush() function may fail if:
request was made of a nonexistent device, or the request was outside
the capabilities of the device.
The following sections are informative.
Sending Prompts to Standard Output
The following example uses printf() calls to print a series
of prompts for
information the user must enter from standard input. The fflush()
calls force the output to standard output. The
fflush() function is used because standard output is usually
buffered and the prompt may not immediately be printed on the
output or terminal. The gets() calls read strings from standard
input and place the
results in variables, for use later in the program.
Data buffered by the system may make determining the validity of the
position of the current file descriptor impractical. Thus,
enforcing the repositioning of the file descriptor after fflush()
on streams open for read() is not mandated by IEEE Std 1003.1-2001.
getrlimit() , ulimit() , the Base Definitions volume of
IEEE Std 1003.1-2001, <stdio.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. 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