zircon/third_party/ulib/musl/src/stdio/__lockfile.c

#include "futex_impl.h"
#include "stdio_impl.h"
#include "threads_impl.h"

int __lockfile(FILE* f) {
  int owner, tid = __thread_get_tid_for_filelock();
  if (atomic_load(&f->lock) == tid)
    return 0;
  while ((owner = a_cas_shim(&f->lock, 0, tid)))
    __wait(&f->lock, &f->waiters, owner);
  return 1;
}

void __unlockfile(FILE* f) {
  atomic_store(&f->lock, 0);

  /* The following read is technically invalid under situations
   * of self-synchronized destruction. Another thread may have
   * called fclose as soon as the above store has completed.
   * Nonetheless, since FILE objects always live in memory
   * obtained by malloc from the heap, it's safe to assume
   * the dereferences below will not fault. In the worst case,
   * a spurious syscall will be made. If the implementation of
   * malloc changes, this assumption needs revisiting. */

  if (atomic_load(&f->waiters))
    _zx_futex_wake(&f->lock, 1);
}