只允许一个程序运行一个实例 代码

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <errno.h>

int main()
{
int fd,val;
struct flock lock;

fd=open("/home/lwd/a.txt",O_RDWR | O_CREAT | O_TRUNC );
if(fd<0)
{
  printf("open file error,%s\n",strerror(errno));

  return 1;
}

if ( (val = fcntl(fd, F_GETFL, 0)) < 0)
{
  printf("fcntl F_GETFL error\n");
  return 2;
}
   
lock.l_type = F_WRLCK;     /* F_RDLCK, F_WRLCK, F_UNLCK */
    lock.l_start = 0;  /* byte offset, relative to l_whence */
    lock.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */
    lock.l_len = 0;       /* #bytes (0 means to EOF) */

if(fcntl(fd, F_SETLK, &lock)<0)
{
  printf("fcntl F_SETFL error\n");
  return 3;
}

sleep(30);

printf("it is over\n");

close(fd);

return 0;
}