The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


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 POSIX.1-2017 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 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). [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 file size limit of the process. [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 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. [Option End]
[CX] [Option Start] The underlying stream was created by open_memstream() or open_wmemstream() and insufficient memory is available. [Option End]
[CX] [Option Start] There was no free space remaining on the device containing the file or in the buffer used by the fmemopen() function. [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 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);




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.1-2017.




Standard I/O Streams, fmemopen, getrlimit, open_memstream, ulimit

XBD <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''.

Issue 7

Austin Group Interpretation 1003.1-2001 #002 is applied, clarifying the interaction of file descriptors and streams.

The [ENOSPC] error condition is updated and the [ENOMEM] error is added from The Open Group Technical Standard, 2006, Extended API Set Part 1.

The EXAMPLES section is revised.

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0126 [87], XSH/TC1-2008/0127 [79], and XSH/TC1-2008/0128 [14] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0112 [816] and XSH/TC2-2008/0113 [626] are applied.

End of informative text.


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