Standard C library (libc, -lc)
#include <stdio.h> int fflush(FILE *_Nullable 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 write function.
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 application.
The open status of the stream is unaffected.
If the stream argument is NULL, fflush() flushes all open output streams.
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.
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 attributes(7).
C89, 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).
fsync(2), sync(2), write(2), fclose(3), fileno(3), fopen(3), fpurge(3), setbuf(3), unlocked_stdio(3)
fclose(3), fcloseall(3), fgetln.3bsd(3), fmemopen(3), fopen(3), fpurge(3), fseek(3), fsync(2), open_memstream(3), popen(3), setbuf(3), stdin(3), stdio(3), xdr(3).