c - 如何使用“fcntl()”锁定和解锁 pid 文件

Question

我在网上甚至在 stackoverflow 上进行了研究，以便找到一个fcntl()用于锁定和解锁 pid 文件的示例，"/var/run/myapp.pid"但我没有找到一个明确的示例。

你能指出一个fcntl()用于锁定和解锁 pid 文件的示例吗？

不应阻止锁（如果文件已被锁定）

Answer 1

正如您标记 Linux，逐字形式man lockf（我强调）：

在 Linux 上，lockf ()只是fcntl (2) 锁定之上的一个接口。许多其他系统以这种方式实现lockf ()，但请注意 POSIX.1-2001未指定lockf () 和fcntl (2) 锁之间的关系。一个可移植的应用程序应该避免混合调用这些接口。

因此，查找当前 glibc 源 ( eglibc-2.11.3/io/lockf.c) 实现锁定的可能用法fcntl()如下所示：

/* Copyright (C) 1994,1996,1997,1998,2000,2003 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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

/* lockf is a simplified interface to fcntl's locking facilities.  */

int
lockf (int fd, int cmd, off_t len)
{
  struct flock fl;

  memset ((char *) &fl, '\0', sizeof (fl));

  /* lockf is always relative to the current file position.  */
  fl.l_whence = SEEK_CUR;
  fl.l_start = 0;
  fl.l_len = len;

  switch (cmd)
    {
    case F_TEST:
      /* Test the lock: return 0 if FD is unlocked or locked by this process;
         return -1, set errno to EACCES, if another process holds the lock.  */
      fl.l_type = F_RDLCK;
      if (__fcntl (fd, F_GETLK, &fl) < 0)
        return -1;
      if (fl.l_type == F_UNLCK || fl.l_pid == __getpid ())
        return 0;
      __set_errno (EACCES);
      return -1;

    case F_ULOCK:
      fl.l_type = F_UNLCK;
      cmd = F_SETLK;
      break;
    case F_LOCK:
      fl.l_type = F_WRLCK;
      cmd = F_SETLKW;
      break;
    case F_TLOCK:
      fl.l_type = F_WRLCK;
      cmd = F_SETLK;
      break;

    default:
      __set_errno (EINVAL);
      return -1;
    }

  /* lockf() is a cancellation point but so is fcntl() if F_SETLKW is
     used.  Therefore we don't have to care about cancellation here,
     the fcntl() function will take care of it.  */
  return __fcntl (fd, cmd, &fl);
}

首先需要一些 mods 才能编译：

• 替换__fcntl为fcntl
• 替换__set_errno(<errno-define>)为errno = <errno-define>

..其次让它成为异步信号保存：

• 用对变量memset()的适当分配替换调用。struct fcntl