| FLOCKFILE(3) | Library Functions Manual | FLOCKFILE(3) | 
flockfile, ftrylockfile,
  funlockfile —
#include <stdio.h>
void
  
  flockfile(FILE
    *file);
int
  
  ftrylockfile(FILE
    *file);
void
  
  funlockfile(FILE
    *file);
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 decrements 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 relinquished again and the calling thread is
    scheduled to acquire it).
The ftrylockfile() function acquires the
    ownership of file for the calling 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.
ftrylockfile() function returns 0.
  Otherwise, it returns non-zero to indicate that the lock cannot be acquired.
flockfile(), ftrylockfile()
  and funlockfile() functions conform to
  IEEE Std 1003.1-2001 (“POSIX.1”).
flockfile() function first appeared in
  FreeBSD 2.0.
| October 15, 2011 | NetBSD 10.0 |