SAS/C Library Reference, Vol. 1

fflush -- Flush Output Buffer

SYNOPSIS

 #include <stdio.h>

 int fflush(FILE *f);

DESCRIPTION

fflush flushes the output buffer for the stream associated with the FILE object addressed by f. The exact effect of fflush depends on the file type and stream type, as follows:

For a terminal file, the current output buffer is immediately written to the terminal. The carriage is not returned if this is supported by the operating system and the terminal device.
For a nonterminal file accessed as a binary stream, the current buffer is passed to the operating system for output and a record break is forced. Note that the record may not be physically written at this time. See SAS/C I/O Questions and Answers for further discussion of this point.
For a nonterminal file accessed as a text stream, fflush has no effect. Passing the output buffer to the operating system also forces a record break, which should occur only when a new-line character is written.

If you do not use fflush, output buffers are flushed as follows:

For a terminal file, the output buffer is flushed when it is full, when a new-line character is written, when terminal input is requested, or when a character is written to the terminal using a different stream.
For a nonterminal file accessed as text, the output buffer is flushed when it is full or when a new-line character (or any other control character that forces a new record) is written.
For a nonterminal file accessed as binary, the output buffer is flushed when it is full.

The effect of fflush on a read-only file is undefined.

RETURN VALUE

fflush returns 0, or EOF if an error occurs.

PORTABILITY

fflush is portable when used to ensure that output to an interactive device is written out. The use of fflush on other files, such as disk files, does not guarantee immediate I/O, and using fflush to force record breaks is completely nonportable.

EXAMPLE

  #include <stdio.h>
  #include <stdlib.h>

  main()
  {
     long partno;

     fputs("Enter part number:", stdout);
     fflush(stdout);     /* Force prompt to terminal. */
     scanf("%ld", &partno);  /* Read the part number. */
     ptintf("Request for part # %ld received.", partno);
  }

RELATED FUNCTIONS

afflush, fsync