#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>

#define PID_BUF_LEN  (20)
#define RUN_PID_FILE "/var/run/myserver.pid"

//服务进程单实例运行
//返回值: 1--正在运行，0--未运行，-1--出错
int server_is_running()
{
  int fd = open(RUN_PID_FILE, O_WRONLY|O_CREAT);
  if(fd < 0)
  {
    printf("open run pid err(%d)! %s\n", errno, RUN_PID_FILE);
    return -1;
  }
   
  // 加锁
  // LOCK_SH 建立共享锁定。多个进程可同时对同一个文件作共享锁定。
  // LOCK_EX 建立互斥锁定。一个文件同时只有一个互斥锁定。
  if(flock(fd, LOCK_EX|LOCK_NB) == -1)
  {
    //加不上锁，则是服务正在运行，已上锁了
    printf("server is runing now! errno=%d\n", errno);
    close(fd);
    return 1;
  }

  // 加锁成功，证明服务没有运行
  // 文件句柄不要关，也不要解锁
  // 进程退出，自动就解锁了
  printf("myserver is not running! begin to run..... pid=%ld\n", (long)getpid());

  char pid_buf[PID_BUF_LEN] = {0};
  snprintf(pid_buf, sizeof(pid_buf)-1, "%ld\n", (long)getpid());

  // 把进程pid写入到/var/run/myserver.pid文件
  write(fd, pid_buf, strlen(pid_buf));

  return 0;
}

int main(void)
{

  //进程单实例运行检测
  if(0 != server_is_running())
  {
    printf("myserver process is running!!!!! Current process will exit !\n");
    return -1;
  }

  while(1)
  {
    printf("myserver doing ... \n");
    sleep(2);
  }

  return 0;
}

[root@lincoding singleprocess]# ./myserver 
server is not running! begin to run..... pid=6965
myserver doing ... 
myserver doing ... 
myserver doing ... 
myserver doing ... 
myserver doing ... 
myserver doing ... 
myserver doing ... 
myserver doing ... 

[root@lincoding singleprocess]# cat /var/run/myserver.pid 
6965
[root@lincoding singleprocess]# 
[root@lincoding singleprocess]# ps auxf | grep myserver
root   6965 0.0 0.0  3924  460 pts/0  S+  00:32  0:00 |    \_ ./myserver
root   9976 0.0 0.0 103256  856 pts/1  S+  00:35  0:00     \_ grep myserver
[root@lincoding singleprocess]# 

[root@lincoding singleprocess]# ./myserver 
server is runing now! errno=11
myserver process is running!!!!! Current process will exit !