FLOCKFILE
Section: Linux Programmer's Manual (3)
Updated: 2017-07-13
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NAME
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
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 versions <= 2.23: */ _POSIX_C_SOURCE
|| /* Glibc versions <= 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 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.
RETURN VALUE
The
ftrylockfile()
function returns zero for success
(the lock was obtained), and nonzero for failure.
ERRORS
None.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
flockfile(),
ftrylockfile(),
funlockfile()
| Thread safety | MT-Safe
|
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
AVAILABILITY
These functions are available when
_POSIX_THREAD_SAFE_FUNCTIONS
is defined.
SEE ALSO
unlocked_stdio(3)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- AVAILABILITY
-
- SEE ALSO
-
- COLOPHON
-
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