FFLUSH(P) POSIX Programmer's Manual FFLUSH(P)
fflush - flush a stream
int fflush(FILE *stream);
If stream points to an output stream or an update stream in which the most recent opera-
tion 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.
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
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 capabil-
ities 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 out-
put. 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 pro-
printf("User name: ");
printf("Old password: ");
printf("New password: ");
Data buffered by the system may make determining the validity of the position of the cur-
rent 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.
getrlimit() , ulimit() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 FFLUSH(P)