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)):

    /* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
        || /* glibc <= 2.23: */ _POSIX_C_SOURCE
        || /* glibc <= 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.

unlocked_stdio(3)