Manpages - flockfile.3

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)):

All functions shown above:

      /* 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.

None.

For an explanation of the terms used in this section, see *attributes*(7).

POSIX.1-2001, POSIX.1-2008.

These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is defined.

*unlocked_stdio*(3)

This page is part of release 5.13 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/.