flockfile(3) - NetBSD Manual Pages

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FLOCKFILE(3)            NetBSD Library Functions Manual           FLOCKFILE(3)


NAME

     flockfile, ftrylockfile, funlockfile -- stdio stream locking functions


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdio.h>

     void
     flockfile(FILE *file);

     int
     ftrylockfile(FILE *file);

     void
     funlockfile(FILE *file);


DESCRIPTION

     The flockfile(), ftrylockfile(), and funlockfile() functions provide
     applications with explicit control of locking of stdio stream objects.
     They can be used by a thread to execute a sequence of I/O operations as a
     unit, without interference from another thread.

     Locks on stdio streams are recursive, and a lock count is maintained.
     stdio streams are created unlocked, with a lock count of zero.  After
     successful acquisition of the lock, its count is incremented to one,
     indicating locked state of the stdio stream.  Each subsequent relock
     operation performed by the owner thread increments the lock count by one,
     and each subsequent unlock operation performed by the owner thread decre-
     ments the lock count by one, allowing matching lock and unlock operations
     to be nested.  After its lock count is decremented to zero, the stdio
     stream returns to unlocked state, and ownership of the stdio stream is
     relinquished.

     The flockfile() function acquires the ownership of file for the calling
     thread.  If file is already owned by another thread, the calling thread
     is suspended until the acquisition is possible (i.e., file is relin-
     quished again and the calling thread is scheduled to acquire it).

     The ftrylockfile() function acquires the ownership of file for the call-
     ing thread only if file is available.

     The funlockfile() function relinquishes the ownership of file previously
     granted to the calling thread.  Only the current owner of file may
     funlockfile() it.


RETURN VALUES

     If successful, the ftrylockfile() function returns 0.  Otherwise, it
     returns non-zero to indicate that the lock cannot be acquired.


SEE ALSO

     getc_unlocked(3), getchar_unlocked(3), putc_unlocked(3),
     putchar_unlocked(3)


STANDARDS

     The flockfile(), ftrylockfile() and funlockfile() functions conform to
     IEEE Std 1003.1-2001 (``POSIX.1'').


BUGS

     The design of these interfaces does not allow for addressing the problem
     of priority inversion.

NetBSD 5.1.2                   January 28, 2003                   NetBSD 5.1.2