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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | AVAILABILITY | SEE ALSO | COLOPHON |
FLOCKFILE(3) Linux Programmer's Manual FLOCKFILE(3)
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
#include <stdio.h>
void flockfile(FILE *filehandle);
int ftrylockfile(FILE *filehandle);
void funlockfile(FILE *filehandle);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
All functions shown above:
/* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
|| /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
The stdio functions are thread-safe. This is achieved by assigning
to each FILE object a lockcount and (if the lockcount is nonzero) an
owning thread. For each library call, these functions wait until the
FILE object is no longer locked by a different thread, then lock it,
do the requested I/O, and unlock the object again.
(Note: this locking has nothing to do with the file locking done by
functions like flock(2) and lockf(3).)
All this is invisible to the C-programmer, but there may be two
reasons to wish for more detailed control. On the one hand, maybe a
series of I/O actions by one thread belongs together, and should not
be interrupted by the I/O of some other thread. On the other hand,
maybe the locking overhead should be avoided for greater efficiency.
To this end, a thread can explicitly lock the FILE object, then do
its series of I/O actions, then unlock. This prevents other threads
from coming in between. If the reason for doing this was to achieve
greater efficiency, one does the I/O with the nonlocking versions of
the stdio functions: with getc_unlocked(3) and putc_unlocked(3)
instead of getc(3) and putc(3).
The flockfile() function waits for *filehandle to be no longer locked
by a different thread, then makes the current thread owner of
*filehandle, and increments the lockcount.
The funlockfile() function decrements the lock count.
The ftrylockfile() function is a nonblocking version of flockfile().
It does nothing in case some other thread owns *filehandle, and it
obtains ownership and increments the lockcount otherwise.
The ftrylockfile() function returns zero for success (the lock was
obtained), and nonzero for failure.
None.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────┼───────────────┼─────────┤
│flockfile(), ftrylockfile(), │ Thread safety │ MT-Safe │
│funlockfile() │ │ │
└──────────────────────────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008.
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is
defined.
unlocked_stdio(3)
This page is part of release 4.12 of the Linux man-pages project. A
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latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
2017-07-13 FLOCKFILE(3)
Pages that refer to this page: stdio_ext(3), unlocked_stdio(3)