Implementation of single process control of Linux C background service program

#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"

//Service process single instance running //Return value: 1--running, 0--not running, -1--error 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;
  }
   
  // Lock // LOCK_SH establishes a shared lock. Multiple processes can share a lock on the same file at the same time.
  // LOCK_EX establishes a mutual exclusion lock. There can be only one exclusive lock on a file at a time.
  if (flock(fd, LOCK_EX|LOCK_NB) == -1)
  {
    //If the lock cannot be added, the service is running and has been locked printf("server is running now! errno=%d\n", errno);
    close(fd);
    return 1;
  }

  // Locking is successful, proving that the service is not running // Do not close or unlock the file handle // The process exits and is automatically unlocked 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());

  // Write the process pid to the /var/run/myserver.pid file write(fd, pid_buf, strlen(pid_buf));

  return 0;
}

int main(void)
{

  //Process single instance running detection 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 running now! errno=11
myserver process is running!!!!! Current process will exit !