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Manual Pages  — FFLUSH


fflush, fflush_unlocked, fpurge – flush a stream



Standard C Library (libc, -lc)


#include <stdio.h>

fflush(FILE *stream);

fflush_unlocked(FILE *stream);

fpurge(FILE *stream);


The function fflush() forces a write of all buffered data for the given output or update stream via the stream's underlying write function. The open status of the stream is unaffected.

If the stream argument is NULL, fflush() flushes all open output streams.

The fflush_unlocked() function is equivalent to fflush(), except that the caller is responsible for locking the stream with flockfile(3) before calling it. This function may be used to avoid the overhead of locking the stream and to prevent races when multiple threads are operating on the same stream.

The function fpurge() erases any input or output buffered in the given stream. For output streams this discards any unwritten output. For input streams this discards any input read from the underlying object but not yet obtained via getc(3); this includes any text pushed back via ungetc(3).


Upon successful completion 0 is returned. Otherwise, EOF is returned and the global variable errno is set to indicate the error.


  The stream argument is not an open stream.

The function fflush() may also fail and set errno for any of the errors specified for the routine write(2), except that in case of stream being a read-only descriptor, fflush() returns 0.


write(2), fclose(3), fopen(3), setbuf(3)


The fflush() function conforms to ISO/IEC 9899:1990 ("ISO C90").


The fflush() function first appeared in AT&T v4 . The fpurge() function first appeared in BSD 4.4 .

FFLUSH (3) May 1, 2020

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