glibc/sysdeps/posix/flock.c

/* Copyright (C) 1992-2021 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/* This file implements the `flock' function in terms of the POSIX.1 `fcntl'
   locking mechanism.  In 4BSD, these are two incompatible locking mechanisms,
   perhaps with different semantics?  */

#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/file.h>

/* Apply or remove an advisory lock, according to OPERATION,
   on the file FD refers to.  */
int
__flock (int fd, int operation)
{
  struct flock lbuf;

  switch (operation & ~LOCK_NB)
    {
    case LOCK_SH:
      lbuf.l_type = F_RDLCK;
      break;
    case LOCK_EX:
      lbuf.l_type = F_WRLCK;
      break;
    case LOCK_UN:
      lbuf.l_type = F_UNLCK;
      break;
    default:
      __set_errno (EINVAL);
      return -1;
    }

  lbuf.l_whence = SEEK_SET;
  lbuf.l_start = lbuf.l_len = 0L; /* Lock the whole file.  */

  return __fcntl (fd, (operation & LOCK_NB) ? F_SETLK : F_SETLKW, &lbuf);
}

weak_alias (__flock, flock)