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FFLUSH(3P) POSIX Programmer's Manual FFLUSH(3P)
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.
fflush — flush a stream
#include <stdio.h>
int fflush(FILE *stream);
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.1‐2008 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.
If stream is a null pointer, fflush() shall perform this flushing
action on all streams for which the behavior is defined above.
For a stream open for reading, 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).
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.
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
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);
None.
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.
None.
Section 2.5, Standard I/O Streams, fmemopen(3p), getrlimit(3p),
open_memstream(3p), ulimit(3p)
The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 FFLUSH(3P)
Pages that refer to this page: stdio.h(0p), freopen(3p), open_memstream(3p), popen(3p)