#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

int main(int argc, char *argv[])

{   

    FILE *f; //file pointer

    printf("Real UID = %d\n", getuid());

    printf("Effective UID = %d\n", geteuid());

    //read test

    if ((f = fopen(argv[1], "r")) == NULL) {

      fprintf(stderr, "%s is not readable\n", argv[1]);

    } else {

      fprintf(stderr, "%s is readable\n", argv[1]); fclose(f);

    }

    //write test

    if ((f = fopen(argv[1], "w")) == NULL) {

      fprintf(stderr, "%s is not writable\n", argv[1]);

    } else {

      fprintf(stderr, "%s is writable\n", argv[1]); fclose(f);

    }

    //lowering the access rights to the caller

    if (setuid(getuid())) {  

      fprintf(stderr, "Could not reset setuid\n"); return 1;

    }

    printf("Real UID = %d\n", getuid());

    printf("Effective UID = %d\n", geteuid());      

    //read test

    if ((f = fopen(argv[1], "r")) == NULL) {

      fprintf(stderr, "%s is not readable\n", argv[1]);

    } else {

      fprintf(stderr, "%s is readable\n", argv[1]); fclose(f);

    }

    //write test

    if ((f = fopen(argv[1], "w")) == NULL) {

      fprintf(stderr, "%s is not writable\n", argv[1]);

    } else {

      fprintf(stderr, "%s is writable\n", argv[1]); fclose(f);

    }

    return 0;

}