通过文件锁来实现，在程序运行的一开始，检查某文件是否存在，如果存在则说明改程序已经在运行了，如果不存在则利用open语句创建该文件,程序退出时关闭并删除此文件。
static char file_lock[sizeof(ctl_addr.sun_path)] = /var/run/file.pid;
static bool file_lock_created = FALSE;
static int
create_lock(void)
{
int fd = open(file_lock, O_WRONLY O_CREAT O_EXCL O_TRUNC,
S_IRUSR S_IRGRP S_IROTH);
if (fd < 0)
{
if (errno == EEXIST)
{
fprintf(stderr, "file: lock file "%s" already existsn", file_lock);
exit_file(10);
}
else
{
fprintf(stderr, "file: unable to create lock file "%s" (%d %s)n"
, file_lock, errno, strerror(errno));
exit_file(1);
}
}
file_lock_created = TRUE;
return fd;
}
static bool
fill_lock(int lockfd)
{
char buf[30]; /* holds "n" */
pid_t pid;
int len;
pid = getpid();
len = snprintf(buf, sizeof(buf), "%un", (unsigned int) pid);
bool ok = len > 0 && write(lockfd, buf, len) == len;
close(lockfd);
return ok;
}
static void
delete_lock(void)
{
if (file_lock_created)
{
//delete_ctl_socket();
unlink(file_lock); /* is noting failure useful? */
}
}