Linux C 一种实现单实例的方法

2021-10-31

利用fcntl、flock对文件加锁，来实现进程的单实例。

fcntl: 针对文件描述符提供控制.参数fd是被参数cmd操作的描述符

函数原型：     

int fcntl(int fd, int cmd);

int fcntl(int fd, int cmd, long arg);     

int fcntl(int fd, int cmd, struct flock *lock);

fcntl实现文件加锁前提是文件必须是打开的.

int lockfile(int fd)
{
    struct flock fl;

    fl.l_type   = F_WRLCK;
    fl.l_start  = 0;
    fl.l_whence = SEEK_SET;
    fl.l_len    = 0;

    return (fcntl(fd, F_SETLK, &fl));
}

bool proc_is_run(const char *procname)
{
    int  fd;
    char buf[16];
    char filename[64];
    sprintf(filename, "/var/run/%s.pid", procname);
	mode_t f_attrib;
	f_attrib = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH; /* 文件权限位 */
    fd = open(filename, O_RDWR | O_CREAT, f_attrib);
    if (fd < 0) {
        printf("open file \"%s\" failed!!!\n", filename);
        return true;
    }

    if (lockfile(fd) == -1) {                          /* 尝试对文件进行加锁 */
        printf("file \"%s\" locked. proc already running!!!\n", filename);
        close(fd);
        return true;
    } else {
        ftruncate(fd, 0);                              /* 写入运行实例的pid */
        sprintf(buf, "%ld", (long)getpid());
        write(fd, buf, strlen(buf) + 1);
        return false;
    }
}