fflush

Flushes a stream.

Syntax

int fflush(
   FILE *stream
);

Parameters

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.

Note

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.

Remarks

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.

Requirements

Function Required header
fflush <stdio.h>

For additional compatibility information, see Compatibility.

Example

// crt_fflush.c
// Compile with: cl /W4 crt_fflush.c
// This sample gets a number from the user, then writes it to a file.
// It ensures the write isn't lost on crash by calling fflush.
#include <stdio.h>

int * crash_the_program = 0;

int main(void)
{
    FILE * my_file;
    errno_t err = fopen_s(&my_file, "myfile.txt", "w");
    if (my_file && !err)
    {
        printf("Write a number: ");

        int my_number = 0;
        scanf_s("%d", &my_number);

        fprintf(my_file, "User selected %d\n", my_number);

        // Write data to a file immediately instead of buffering.
        fflush(my_file);
    
        if (my_number == 5)
        {
            // Without using fflush, no data was written to the file 
            // prior to the crash, so the data is lost.
            *crash_the_program = 5;
        }

        // Normally, fflush is not needed as closing the file will write the buffer.
        // Note that files are automatically closed and flushed during normal termination.
        fclose(my_file);
    }
    return 0;
}
5
User selected 5

See also

Stream I/O
fclose, _fcloseall
_flushall
setvbuf