flockfile
NAME
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
SYNOPSIS
#include <stdio.h> I void flockfile(FILE * filehandle );
I int ftrylockfile(FILE * filehandle );
I void funlockfile(FILE * filehandle );
DESCRIPTION
The stdio functions are thread-safe.
This is achieved by assigning
to each
FILE
object a lockcount and (if the lockcount is non-zero)
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 non-locking versions of the stdio functions: with
getc_unlocked(3)
and
putc_unlocked(3)
instead of
getc(3)
and
putc(3).
The
R 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
R funlockfile ()
function decrements the lock count.
The
R ftrylockfile ()
function is a non-blocking version
of
R flockfile ().
It does nothing in case some other thread
owns *filehandle, and it obtains ownership and increments
the lockcount otherwise.
RETURN VALUE
The
R ftrylockfile ()
function returns zero for success
(the lock was obtained), and non-zero for failure.
ERRORS
None.
CONFORMING TO
POSIX.1-2001.
AVAILABILITY
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS
is defined.
They are in libc since libc 5.1.1 and in glibc
since glibc 2.0.
SEE ALSO
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