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简介
记录上锁是读写锁的一种扩展类型，它可用于有亲缘关系或无亲缘关系的进程间共享某个文件的读与写。
fcntl记录上锁
被锁住的文件通过其描述字访问，执行上锁操作的函数是fcntl。这种类型的锁在内核中维护，其宿主标识为fcntl调用进程的进程ID。这意味着这些锁用于不同进程间的上锁，而不是同一进程内不同线程间的上锁。fcntl记录上锁即可用于读也可用于写，对于文件的任意字节，最多只能存在一种类型的锁（读锁或写锁）。而且，一个给定字节可以有多个读写锁，但只能有一个写入锁。
NAMEfcntl - manipulate file descriptor
SYNOPSIS#include <unistd.h>#include <fcntl.h>int fcntl(int fd, int cmd, ... /* arg */ );Linux implements traditional ("process-associated") UNIX record locks, as standardized by POSIX. For a Linux-specific alter‐native with better semantics, see the discussion of open file description locks below.F_SETLK, F_SETLKW, and F_GETLK are used to acquire, release, and test for the existence of record locks (also known as byte-range, file-segment, or file-region locks). The third argument, lock, is a pointer to a structure that has at least the fol‐lowing fields (in unspecified order).struct flock {...short l_type; /* Type of lock: F_RDLCK,F_WRLCK, F_UNLCK */short l_whence; /* How to interpret l_start:SEEK_SET, SEEK_CUR, SEEK_END */off_t l_start; /* Starting offset for lock */off_t l_len; /* Number of bytes to lock */pid_t l_pid; /* PID of process blocking our lock(set by F_GETLK and F_OFD_GETLK) */...};锁不能通过fork由子进程继承。对于一个打开着某个文件的给定进程来说，当它关闭该文件的任何一个描述字或者终止时，与该文件关联的所有锁都被删除。这个是非常有用的特性，当程序出现异常时，不需要用户去=清除锁状态。
典型应用
记录上锁的一个常见用途是确保某个程序（例如守护程序）任何时刻只有一个拷贝在运行。
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>#define PATH_PIDFILE "pidfile"
#define MAXLINE 4096 /* max text line length */
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) /* default permissions for new files */int
lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len)
{struct flock lock;lock.l_type = type; /* F_RDLCK, F_WRLCK, F_UNLCK */lock.l_start = offset; /* byte offset, relative to l_whence */lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */lock.l_len = len; /* #bytes (0 means to EOF) */return( fcntl(fd, cmd, &lock) ); /* -1 upon error */
}#define write_lock(fd, offset, whence, len) \lock_reg(fd, F_SETLK, F_WRLCK, offset, whence, len)int
main(int argc, char **argv) {int pidfd;char line[MAXLINE];/* 4open the PID file, create if nonexistent */pidfd = open(PATH_PIDFILE, O_RDWR | O_CREAT, FILE_MODE);if (pidfd == -1) {printf("Open error for %s", PATH_PIDFILE);exit(1);}/* 4try to write lock the entire file */if (write_lock(pidfd, 0, SEEK_SET, 0) < 0) {if (errno == EACCES || errno == EAGAIN)printf("unable to lock %s, is %s already running?",PATH_PIDFILE, argv[0]);elseprintf("unable to lock %s", PATH_PIDFILE);exit(1);}/* 4write my PID, leave file open to hold the write lock */snprintf(line, sizeof(line), "%ld\n", (long) getpid());if (ftruncate(pidfd, 0) == -1) {printf("unable to truncated %s", PATH_PIDFILE);exit(1);}if (write(pidfd, line, strlen(line)) == -1) {printf("unable to truncated %s", PATH_PIDFILE);exit(1);}/* then do whatever the daemon does ... */pause();
}bob@ubuntu:~$ ./daemon &
[1] 6555
bob@ubuntu:~$ ./daemon &
[2] 6556
unable to lock pidfile, is ./daemon already running?
[2]+ Exit 1 ./daemon
bob@ubuntu:~$ kill 6555
[1] Terminated ./daemon
bob@ubuntu:~$ ./daemon &
[2] 6557一个守护进程防止自身另外一个拷贝的启动还有其他方法，譬如使用信号量。记录上锁方法的优势在于，许多守护程序都编写某个文件写入本进程ID，而且如果某个守护进程过早崩溃了，那么内核会自动释放它的记录锁。
小结
fcntl记录上锁提供了一个文件的劝告性或强制性上锁功能，而该文件是通过它的打开着的描述符来访问。这些锁用于不同进程间上锁，而不是同一进程内不同线程间上锁。这种类型记录上锁的几乎所有用途都是作为协作进程间的劝告性锁，即使是强制性上锁也会导致不一致的数据。使用fcntl记录上锁时，等待着的读出者优先还是等待着的写入者优先没有保证。
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