1 #include <stdio.h>

     2 #include <unistd.h>

     3 #include <errno.h>

     4 

     5 FILE *f; //file pointer

     6 

     7 //tests return errno = 13 for permission denied

     8 void read_test(char *name) 

     9 {

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

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

    12     } else {

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

    14     }

    15 }

    16 

    17 void write_test(char *name) 

    18 {

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

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

    21     } else {

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

    23     }

    24 }

    25 

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

    27 {   

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

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

    30     

    31     read_test(argv[1]);

    32     write_test(argv[1]);

    33 

    34     //lowering the access rights to the caller

    35     if (setuid(getuid())) {  

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

    37       return 1;

    38     }

    39         

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

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

    42 

    43     read_test(argv[1]);

    44     write_test(argv[1]);

    45 

    46     return 0;

    47 }