cosmopolitan/libc/stdio/flockfile.c

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/calls/calls.h"
#include "libc/intrin/cmpxchg.h"
#include "libc/intrin/lockcmpxchgp.h"
#include "libc/nexgen32e/threaded.h"
#include "libc/stdio/stdio.h"

/**
 * Acquires reentrant lock on stdio object, blocking if needed.
 */
void flockfile(FILE *f) {
  int me, owner;
  unsigned tries;
  if (__threaded) {
    for (tries = 0, me = gettid();;) {
      owner = 0;
      if (_lockcmpxchgp(&f->lock, &owner, me) || owner == me) {
        break;
      }
      if (++tries & 7) {
        __builtin_ia32_pause();
      } else {
        sched_yield();
      }
    }
  }
  ++f->reent;
}