void my_lock(int fd)
{
struct flock lock; lock.l_type = F_WRLCK; lock.l_whence = SEEK_SET; lock.l_start = 0;
lock.l_len = 0; /**//* write lock entire file */ Fcntl(fd, F_SETLKW, &lock);
}void my_unlock(int fd){ struct flock lock; lock.l_type = F_UNLCK; lock.l_whence = SEEK_SET; lock.l_start = 0; lock.l_len = 0; /**//* unlock entire file */ Fcntl(fd, F_SETLK, &lock);}
static void rwlock_cancelrdwait(void *arg){ pthread_rwlock_t *rw; rw = arg; rw->rw_nwaitreaders--; pthread_mutex_unlock(&rw->rw_mutex);}/**//* end rwlock_cancelrdwait */int pthread_rwlock_rdlock(pthread_rwlock_t *rw){ int result; if (rw->rw_magic != RW_MAGIC) return(EINVAL); if ( (result = pthread_mutex_lock(&rw->rw_mutex)) != 0) return(result); /**//* 4give preference to waiting writers */ while (rw->rw_refcount < 0 || rw->rw_nwaitwriters > 0) { rw->rw_nwaitreaders++; pthread_cleanup_push(rwlock_cancelrdwait, (void *) rw); result = pthread_cond_wait(&rw->rw_condreaders, &rw->rw_mutex); pthread_cleanup_pop(0); rw->rw_nwaitreaders--; if (result != 0) break;
} if (result == 0) rw->rw_refcount++; /**//* another reader has a read lock */ pthread_mutex_unlock(&rw->rw_mutex); return (0);}
static void rwlock_cancelwrwait(void *arg){ pthread_rwlock_t *rw; rw = arg; rw->rw_nwaitwriters--; pthread_mutex_unlock(&rw->rw_mutex);}/**//* end rwlock_cancelwrwait */int pthread_rwlock_wrlock(pthread_rwlock_t *rw){ int result; if (rw->rw_magic != RW_MAGIC) return(EINVAL); if ( (result = pthread_mutex_lock(&rw->rw_mutex)) == -1) return(result); while (rw->rw_refcount != 0) { rw->rw_nwaitwriters++; pthread_cleanup_push(rwlock_cancelwrwait, (void *) rw); result = pthread_cond_wait(&rw->rw_condwriters, &rw->rw_mutex); pthread_cleanup_pop(0); rw->rw_nwaitwriters--; if (result != 0) break; } if (result == 0) rw->rw_refcount = -1; pthread_mutex_unlock(&rw->rw_mutex); return(0);}
如果想让进程只有一个拷贝运行，可以维护一个文件，在进程运行时先对其记录上写锁，则其他拷贝就无法获取到写锁，也就无法运行其他拷贝。
#include "unpipc.h"#define PATH_PIDFILE "pidfile"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); /**//* 4try to write lock the entire file */ if (write_lock(pidfd, 0, SEEK_SET, 0) < 0) { if (errno == EACCES || errno == EAGAIN) err_quit("unable to lock %s, is %s already running?", PATH_PIDFILE, argv[0]); else err_sys("unable to lock %s", PATH_PIDFILE); } /**//* 4write my PID, leave file open to hold the write lock */ snprintf(line, sizeof(line), "%ld\n", (long) getpid()); Ftruncate(pidfd, 0); Write(pidfd, line, strlen(line)); /**//* then do whatever the daemon does */ pause();}当然一个守护进程防止自身另一个拷贝的启动还有其他方法，比如可以使用信号灯。
若以O_CREATE和O_EXCL来调用open函数，则若文件存在，就返回一个错误，我们可以用这个技巧来将文件作为锁用。
#include "unpipc.h"#define LOCKFILE "/tmp/seqno.lock"void my_lock(int fd){ int tempfd; while ( (tempfd = open(LOCKFILE, O_RDWR|O_CREAT|O_EXCL, FILE_MODE)) < 0) { if (errno != EEXIST) err_sys("open error for lock file"); /**//* someone else has the lock, loop around and try again */ } Close(tempfd); /**//* opened the file, we have the lock */}void my_unlock(int fd){ Unlink(LOCKFILE); /**//* release lock by removing file */}
这种技巧有三个问题：
1，若持有该锁的进程没有释放锁就终止了，则此文件名没有被删除，对于这个问有一些办法，比如检查文件的最近访问时间，若大于某个阀值，则认定它已经被遗忘了。另一个办法是把持有锁的进程ID写入锁文件中，则其他进程可以读出该进程ID，并去检查进程是否还在运行，但这也有问题，因为进程ID在过一段时间后是会重用的。但这些情形对于fcntl记录上锁来说都不成问题，因为进程终止时，它持有的记录锁都自动释放。
2，若已经有另外一个进程打开了锁文件，则当前进程只是在一个无限循环中不断调用open,也就是轮询。一个替代技巧是sleep一下，再尝试open.当然若使用fcntl进行记录上锁，这不成问题，只要想持有锁的进程指定FSETLKW命令，那么内核将把该进程投入睡眠，直到锁可用再唤醒它。
3，调用open和unlink创建和删除额外一个文件涉及文件系统的访问，这比调用fcntl两次（一个获取锁，一次释放锁）所花时间长得多。
互斥锁和信号量的区别是：互斥锁必须总是由锁住它的线程来解锁，但信号量的signal却不必由执行过它的wait操作的同一个线程来执行。
