NAME
fflush - flush a stream
SYNOPSIS
#include <stdio.h>
int fflush(FILE *stream);
DESCRIPTION
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 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 process 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 process’
file size limit.
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 process
is neither ignoring nor blocking SIGTTOU, and the process group
of the process is orphaned. This error may also be returned
under implementation-defined conditions.
ENOSPC There was no free space remaining on the device containing the
file.
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 gets() calls read
strings from standard input and place the results in variables, for use
later in the program.
#include <stdio.h>
...
char user[100];
char oldpasswd[100];
char newpasswd[100];
...
printf("User name: ");
fflush(stdout);
gets(user);
printf("Old password: ");
fflush(stdout);
gets(oldpasswd);
printf("New password: ");
fflush(stdout);
gets(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 IEEE Std 1003.1-2001.
FUTURE DIRECTIONS
None.
SEE ALSO
getrlimit() , ulimit() , the Base Definitions volume of
IEEE Std 1003.1-2001, <stdio.h>
COPYRIGHT
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 http://www.opengroup.org/unix/online.html .