flockfile(3) -- Linux man page
NAME
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
SYNOPSIS
#include <stdio.h>
void flockfile(FILE *filehandle);
int ftrylockfile(FILE *filehandle);
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 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 non-locking versions of the stdio functions: with
getc_unlocked() and putc_unlocked() instead of
getc() and putc().
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 non-blocking 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.
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.
CONFORMING TO
POSIX.1
SEE ALSO
unlocked_stdio(3)
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