如何使用“fcntl()”锁定和解锁 pid 文件

Question

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

您能否指出一个使用fcntl() 锁定和解锁pid 文件的示例？

不应阻止锁（如果文件已被锁定）

Answer 1

当您标记 Linux 时，逐字记录 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 保留了 lockf() 和 fcntl( 2) 未指定的锁。一个可移植的应用程序可能应该避免混合调用这些接口。

因此查找当前的 glibc 源 (eglibc-2.11.3/io/lockf.c) 可能使用 fcntl() 来实现锁定，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);
}

首先需要一些模组来编译：

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

.. 其次让它变成 async-signal-save:

• 将对memset() 的调用替换为对struct fcntl 变量的适当分配。