The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


fflush - flush a stream


#include <stdio.h>

int fflush(FILE *


[CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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, [CX] [Option Start]  and the st_ctime and st_mtime fields of the underlying file shall be marked for update. [Option End]

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, [CX] [Option Start]  and set errno to indicate the error. [Option End]


The fflush() function shall fail if:

[CX] [Option Start] The O_NONBLOCK flag is set for the file descriptor underlying stream and the thread would be delayed in the write operation. [Option End]
[CX] [Option Start] The file descriptor underlying stream is not valid. [Option End]
[CX] [Option Start] An attempt was made to write a file that exceeds the maximum file size. [Option End]
[XSI] [Option Start] An attempt was made to write a file that exceeds the process' file size limit. [Option End]
[CX] [Option Start] The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream. [Option End]
[CX] [Option Start] The fflush() function was interrupted by a signal. [Option End]
[CX] [Option Start] 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. [Option End]
[CX] [Option Start] There was no free space remaining on the device containing the file. [Option End]
[CX] [Option Start] 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. [Option End]

The fflush() function may fail if:

[CX] [Option Start] A request was made of a nonexistent device, or the request was outside the capabilities of the device. [Option End]

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[100];
char oldpasswd[100];
char newpasswd[100];
printf("User name: ");

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.




getrlimit(), ulimit(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Large File Summit extensions are added.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The RETURN VALUE section is updated to note that the error indicator shall be set for the stream. This is for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/31 is applied, updating the [EAGAIN] error in the ERRORS section from ``the process would be delayed'' to ``the thread would be delayed''.

End of informative text.

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