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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 .
IEEE/The Open Group 2017 FFLUSH(3POSIX)