fflush - flush a stream
int fflush(FILE *stream);
[CX] 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.
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] 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, [CX] and set errno to indicate the error.
The fflush() function shall fail if:
- [CX] The O_NONBLOCK flag is set for the file descriptor underlying stream and the process would be delayed in the write operation.
- [CX] The file descriptor underlying stream is not valid.
- [CX] An attempt was made to write a file that exceeds the maximum file size.
- [XSI] An attempt was made to write a file that exceeds the process' file size limit.
- [CX] The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream.
- [CX] The fflush() function was interrupted by a signal.
- [CX] 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.
- [CX] There was no free space remaining on the device containing the file.
- [CX] 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:
- [CX] A request was made of a nonexistent device, or the request was outside the capabilities of the device.
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.
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.
Large File Summit extensions are added.
Extensions beyond the ISO C standard are marked.
The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:
The [EFBIG] error is added as part of the large file support extensions.
The [ENXIO] optional error condition is added.
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.