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### 4.7 `fflush`—flush buffered file output

Synopsis

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

Description
The `stdio` output functions can buffer output before delivering it to the host system, in order to minimize the overhead of system calls.

Use `fflush` to deliver any such pending output (for the file or stream identified by fp) to the host system.

If fp is `NULL`, `fflush` delivers pending output from all open files.

Returns
`fflush` returns `0` unless it encounters a write error; in that situation, it returns `EOF`.

Portability
ANSI C requires `fflush`.

No supporting OS subroutines are required.