#include <stdio.h>

#include <unistd.h>

#include <errno.h>

FILE *f; //file pointer

//tests return errno = 13 for permission denied

void read_test(char *name) 

{

    if ((f = fopen(name, "r")) == NULL) {

      printf("%s is not readable, errno = %d\n", name, errno);

    } else {

      printf("%s is readable\n", name); fclose(f);

    }

}

void write_test(char *name) 

{

    if ((f = fopen(name, "r+")) == NULL) {

      printf("%s is not writable, errno = %d\n", name, errno);

    } else {

      printf("%s is writable\n", name); fclose(f);

    }

}

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

{   

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

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

    read_test(argv[1]);

    write_test(argv[1]);

    //lowering the access rights to the caller

    if (setuid(getuid())) {  

      printf("could not reset setuid, errno = %d\n", errno); 

      return 1;

    }

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

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

    read_test(argv[1]);

    write_test(argv[1]);

    return 0;

}