C in a Nutshell

Name

fflush

Synopsis

Clears a file buffer

#include <stdio.h>
intfflush( FILE *fp );

The fflush() function empties the I/O buffer of the open file specified by the FILE pointer argument. If the file was opened for writing, fflush() writes the contents of the file. If the file is only opened for reading, the behavior of fflush() is not specified by the standard. Most implementations simply clear the input buffer. The function returns 0 if successful, or EOF if an error occurs in writing to the file.

The argument passed to fflush() may be a null pointer. In this case, fflush() flushes the output buffers of all the program’s open streams. The fflush() function does not close the file, and has no effect at all on unbuffered files (see “Files” in Chapter 13 for more information on unbuffered input and output).

Example

In the following example, the program fflush.c writes two lines of text to a file. If the macro FLUSH is defined, the program flushes the file output buffer to disk after each line. If not, only the first output line is explicitly flushed. Then the program raises a signal to simulate a fatal error, so that we can observe the effect with and without the second fflush() call.

/* fflush.c: Tests the effect of flushing output file buffers. */ FILE *fp; #ifdef FLUSH char filename[ ] = "twice.txt"; #else char filename[ ] = "once.txt"; #endif /* FLUSH */ fp = fopen( filename, "w" ); if ( fp == NULL) fprintf( stderr "Failed to open file '%s' to write.\n", filename ); fputs( "Going once ...