LOCKFILE(3) Linux Programmer's Manual LOCKFILE(3)
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
void flockfile(FILE *filehandle);
int ftrylockfile(FILE *filehandle);
void funlockfile(FILE *filehandle);
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 *file-
handle, 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.
The ftrylockfile() function returns zero for success (the lock was obtained), and nonzero for failure.
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.