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 (), funlockfile () |
Thread safety |
MT-Safe |
POSIX.1-2001, POSIX.1-2008.
These functions are available when
_POSIX_THREAD_SAFE_FUNCTIONS is
defined.
unlocked_stdio(3)