FFLUSH(3) Linux Programmer's Manual FFLUSH(3)
fflush - flush a stream
int fflush(FILE *stream);
For output streams, fflush() forces a write of all user-space buffered
data for the given output or update stream via the stream's underlying
For input streams associated with seekable files (e.g., disk files, but
not pipes or terminals), fflush() discards any buffered data that has
been fetched from the underlying file, but has not been consumed by the
The open status of the stream is unaffected.
If the stream argument is NULL, fflush() flushes all open output
For a nonlocking counterpart, see unlocked_stdio(3).
Upon successful completion 0 is returned. Otherwise, EOF is returned
and errno is set to indicate the error.
EBADF stream is not an open stream, or is not open for writing.
The function fflush() may also fail and set errno for any of the errors
specified for write(2).
For an explanation of the terms used in this section, see
|Interface | Attribute | Value |
|fflush() | Thread safety | MT-Safe |
C89, C99, POSIX.1-2001, POSIX.1-2008.
POSIX.1-2001 did not specify the behavior for flushing of input
streams, but the behavior is specified in POSIX.1-2008.
Note that fflush() flushes only the user-space buffers provided by the
C library. To ensure that the data is physically stored on disk the
kernel buffers must be flushed too, for example, with sync(2) or
fsync(2), sync(2), write(2), fclose(3), fopen(3), setbuf(3),
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