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Flushes a stream.


int fflush(   
   FILE *stream   


Pointer to FILE structure.

Return Value

fflush returns 0 if the buffer was successfully flushed. The value 0 is also returned in cases in which the specified stream has no buffer or is open for reading only. A return value of EOF indicates an error.


If fflush returns EOF, data may have been lost due to a write failure. When setting up a critical error handler, it is safest to turn buffering off with the setvbuf function or to use low-level I/O routines such as _open, _close, and _write instead of the stream I/O functions.


The fflush function flushes the stream stream. If the stream was opened in write mode, or it was opened in update mode and the last operation was a write, the contents of the stream buffer are written to the underlying file or device and the buffer is discarded. If the stream was opened in read mode, or if the stream has no buffer, the call to fflush has no effect, and any buffer is retained. A call to fflush negates the effect of any prior call to ungetc for the stream. The stream remains open after the call.

If stream is NULL, the behavior is the same as a call to fflush on each open stream. All streams opened in write mode and all streams opened in update mode where the last operation was a write are flushed. The call has no effect on other streams.

Buffers are normally maintained by the operating system, which determines the optimal time to write the data automatically to disk: when a buffer is full, when a stream is closed, or when a program terminates normally without closing the stream. The commit-to-disk feature of the run-time library lets you ensure that critical data is written directly to disk rather than to the operating-system buffers. Without rewriting an existing program, you can enable this feature by linking the program's object files with COMMODE.OBJ. In the resulting executable file, calls to _flushall write the contents of all buffers to disk. Only _flushall and fflush are affected by COMMODE.OBJ.

For information about controlling the commit-to-disk feature, see Stream I/O, fopen, and _fdopen.

This function locks the calling thread and is therefore thread-safe. For a non-locking version, see _fflush_nolock.


Function Required header
fflush <stdio.h>

For additional compatibility information, see Compatibility in the Introduction.


// crt_fflush.c  
#include <stdio.h>  
#include <conio.h>  
int main( void )  
   int integer;  
   char string[81];  
   // Read each word as a string.  
   printf( "Enter a sentence of four words with scanf: " );  
   for( integer = 0; integer < 4; integer++ )  
      scanf_s( "%s", string, sizeof(string) );        
      printf( "%s\n", string );  
   // You must flush the input buffer before using gets.   
   // fflush on input stream is an extension to the C standard   
   fflush( stdin );     
   printf( "Enter the same sentence with gets: " );  
   gets_s( string, sizeof(string) );  
   printf( "%s\n", string );  
      This is a test  
This is a test  
      This is a test  
This is a testEnter a sentence of four words with scanf: This is a test  
Enter the same sentence with gets: This is a test  
This is a test  

.NET Framework Equivalent


See Also

Stream I/O
fclose, _fcloseall