Provided by: manpages-posix-dev_2017a-2_all 
PROLOG
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.
NAME
fflush — flush a stream
SYNOPSIS
#include <stdio.h>
int fflush(FILE *stream);
DESCRIPTION
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.1‐2017 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 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.
ERRORS
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.
The following sections are informative.
EXAMPLES
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 getline() function
calls read strings from standard input and place the results in variables, for use later
in the program.
char *user;
char *oldpasswd;
char *newpasswd;
ssize_t llen;
size_t blen;
struct termios term;
tcflag_t saveflag;
printf("User name: ");
fflush(stdout);
blen = 0;
llen = getline(&user, &blen, stdin);
user[llen-1] = 0;
tcgetattr(fileno(stdin), &term);
saveflag = term.c_lflag;
term.c_lflag &= ~ECHO;
tcsetattr(fileno(stdin), TCSANOW, &term);
printf("Old password: ");
fflush(stdout);
blen = 0;
llen = getline(&oldpasswd, &blen, stdin);
oldpasswd[llen-1] = 0;
printf("\nNew password: ");
fflush(stdout);
blen = 0;
llen = getline(&newpasswd, &blen, stdin);
newpasswd[llen-1] = 0;
term.c_lflag = saveflag;
tcsetattr(fileno(stdin), TCSANOW, &term);
free(user);
free(oldpasswd);
free(newpasswd);
APPLICATION USAGE
None.
RATIONALE
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 POSIX.1‐2008.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.5, Standard I/O Streams, fmemopen(), getrlimit(), open_memstream(), ulimit()
The Base Definitions volume of POSIX.1‐2017, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .