fflush(3) — Linux manual page

fflush(3)               Library Functions Manual               fflush(3)

NAME

       fflush - flush a stream

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdio.h>

       int fflush(FILE *_Nullable stream);

DESCRIPTION

       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).

RETURN VALUE

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

ERRORS

       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).

ATTRIBUTES

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌─────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                           │ Attribute     │ Value   │
       ├─────────────────────────────────────┼───────────────┼─────────┤
       │ fflush()                            │ Thread safety │ MT-Safe │
       └─────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       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.

NOTES

       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).

SEE ALSO

       fsync(2), sync(2), write(2), fclose(3), fileno(3), fopen(3),
       fpurge(3), setbuf(3), unlocked_stdio(3)

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                      fflush(3)

Pages that refer to this page: fsync(2), fclose(3), fcloseall(3), fmemopen(3), fopen(3), fpurge(3), fseek(3), open_memstream(3), popen(3), setbuf(3), stdin(3), stdio(3), xdr(3)

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