int fflush(FILE *stream);
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.
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:
- The O_NONBLOCK flag is set for the file descriptor underlying stream and the process would be delayed in the write operation.
- The file descriptor underlying stream is not valid.
- An attempt was made to write a file that exceeds the maximum file size.
- An attempt was made to write a file that exceeds the process' file size limit.
- The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream.
- The fflush() function was interrupted by a signal.
- 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.
- There was no free space remaining on the device containing the file.
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
The fflush() function may fail if:
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 gets() calls read strings from standard input and place the results in variables, for use later in the program.
#include <stdio.h> ... char user; char oldpasswd; char newpasswd; ... printf("User name: "); fflush(stdout); gets(user); printf("Old password: "); fflush(stdout); gets(oldpasswd); printf("New password: "); fflush(stdout); gets(newpasswd); ...
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.
COPYRIGHTPortions 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 .