File descriptors

I noticed that if I execute the following code, then the program is able to read the file even if the files' permissions are changed around the /mark/ section in such a way that the UID under which the program is running should not have any permission to read the file.

This is not a desirable behaviour.
How can I prevent this behaviour on my system?


#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
    if (argc != 2) {
            printf("Usage: %s filename\n", argv[0]);
            exit(EXIT_FAILURE);
    }

   FILE *fd;
   char *line = NULL;
   size_t len = 0;

   fd = fopen(argv[2], "r");

    /* mark */

    if (fd == NULL) {
            exit(EXIT_FAILURE);
    }

    while (getline(&line, &len, fd) != -1) {
       printf("%s", line);
   }
    fclose(fd);
   exit(EXIT_SUCCESS);
}

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