Character array and pointer

Question

The code snippet is given as:

char *s[] = {"program", "test", "load", "frame", "stack", NULL};
char **p = s + 2;

We need to find the output of following statement:

printf("%s", p[-2] + 3);

What does p[-2] refer to?

As @vz0 says, there's a curly brace { after the printf. It should be a open parenthesis (, instead. Don't worry - I've fixed it for you. — Mateen Ulhaq
– Mateen Ulhaq, Commented Sep 6, 2011 at 0:26

vz0 · Accepted Answer · 2011-02-14 12:39:04Z

2

char *s[] = {"program","test","load","frame","stack",NULL};
char **p = s + 2
printf("%s", p[-2] + 3);

The variable s is an array of char* pointers.
The variable p is a pointer to a pointer. Pointer arithmetics downgrades the array s to a char**, initializing p to a value of two times the size of a char**. On a 32bit machine, if s points to 1000, p will point to 1008.

The expression p[-2] is equivalent to *(p - 2), returning a simple pointer to a char*. In this case, a value pointing to the first element of the strings array: "program".

Finally, since *(p - 2) is the expression pointing to the first letter of the string "program", *(p - 2) + 3 points to the fourth letter of that word: "gram".

printf("%s", *(p - 2) + 3); /* prints: gram */

edited Feb 14, 2011 at 12:39

answered Feb 14, 2011 at 12:28

vz0

33k7 gold badges47 silver badges80 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Mateen Ulhaq Over a year ago

Aren't negative array indexes UB?

GrahamS · Accepted Answer · 2011-02-14 12:41:48Z

0

Did you try compiling your code? Once the syntax errors are fixed, the output is gram.

#include <stdio.h>

int main()
{
    char *s[] = {"program","test","load","frame","stack",NULL};
    char **p = s + 2;

    printf("%s",p[-2] + 3);

    return 0;
};

See http://ideone.com/eVAUv for the compilation and output.

answered Feb 14, 2011 at 12:41

GrahamS

10.4k10 gold badges51 silver badges64 bronze badges

Collectives™ on Stack Overflow

Character array and pointer

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related