#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
  FILE *fp;
  uid_t uid;

  if(argc<2){
    fprintf(srderr, "Usage: %s file_name\n", argv[0]);
    exit(1);
  }
  printf("uid=%ld,  euid=%ld", getuid(), geteuid());
  if((fp = fopen(argv[1], "r"))==NULL){
    perror(argv[0]);
    exit(2);
  }
  else{
    printf("first open successful\n");
    fclose(fp);
  }
  setuid(uid = getuid());
  printf("after setuid(%ld):\n uid=%ld and euid=%ld\n", uid, getuid(), geteuid());
  if((fp = fopen(argv[1], "r"))==NULL){
    perror(argv[0]);
    exit(2);
  }
  else{
    printf("second open successful\n");
    fclose(fp);
  }
}