|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
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.1‐2017 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.
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.1‐2017, stdio.h(0p)
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(3P)
Pages that refer to this page: stdio.h(0p), freopen(3p), open_memstream(3p), popen(3p)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|