程序单实例运行(简单的pid文件实现)

有时我们需要程序同时只能有一个实例在运行，或是受于资源限制或是受于锁或者其他原因。因此，需要一种机制在程序启动时候判断是否有别的实例已经在运行。

• 用bash脚本或者supervisor之类管理工具判断
  这种受于程序名字限制。
• 用文件锁
  程序启动时候往文件写入pid或其他信息，退出时释放文件锁。其实本质上是独占写，程序退出时候会释放写锁。

这里我用了第二种方式，以最简单的 c 程序给出示例。
代码

#define LOCKFILE "/var/run/xxx.pid"
// 带w的函数是阻塞调用
#define lockrd(fd, offset, whence, len)     setlock(fd, F_SETLK, F_RDLCK, offset, whence, len)
#define lockrdw(fd, offset, whence, len)    setlock(fd, F_SETLKW, F_RDLCK, offset, whence, len)
#define lockwr(fd, offset, whence, len)     setlock(fd, F_SETLK, F_WRLCK, offset, whence, len)
#define lockwrw(fd, offset, whence, len)    setlock(fd, F_SETLKW, F_WRLCK, offset, whence, len)
#define unlock(fd, offset, whence, len)     setlock(fd, F_SETLK, F_UNLCK, offset, whence, len)

int setlock(int fd, short cmd, short type, off_t offset, int whence, off_t len) {
    struct flock lock;
    lock.l_type = type; /* F_RDLCK, F_WRLCK, F_UNLCK */
    lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */
    lock.l_start = offset; /* byte offset, relative to l_whence */
    lock.l_len = len; /* #bytes (0 means to EOF) */
    lock.l_pid = getpid();
    return fcntl(fd, cmd, &lock);
}

/**
 * return value:
 *   -1 for error
 *   0  for successfully set lock
 *   1  for another lock has been set
 */
int isAnotherRunning(void) {
    int fd = open(LOCKFILE, O_RDWR|O_CREAT, 0644);
    if (fd < 0)
    {
        // printf("Open %s fail!\n", LOCKFILE);
        return -1;
    }

    // add lock
    if (lockwr(fd, 0, SEEK_SET, 0) < 0)
    {
        // printf("Add lock failed maybe another instance is running");
        close(gLockfd);
        return 1;
    }

    ftruncate(fd, 0);
    dprintf(fd, "%ld", (long)getpid());

    return 0;
}