{
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。
本文转自Phinecos(洞庭散人)博客园博客,原文链接:http://www.cnblogs.com/phinecos/archive/2008/05/29/1210311.html,如需转载请自行联系原作者