flockfile(3) Library Functions Manual flockfile(3)
NAME
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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 <= 2.23: */ _POSIX_C_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTION
The stdio functions are thread-safe. This is achieved by assigning to
each FILE object a lockcount and (if the lockcount is nonzero) an own-
ing 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 inter-
rupted 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 *filehan-
dle, 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 ob-
tains ownership and increments the lockcount otherwise.
RETURN VALUE
The ftrylockfile() function returns zero for success (the lock was ob-
tained), and nonzero for failure.
ERRORS
None.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│flockfile(), ftrylockfile(), funlockfile() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008.
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is de-
fined.
SEE ALSO
unlocked_stdio(3)
Linux man-pages 6.03 2023-02-05 flockfile(3)
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