Getting error while debugging a C program on pointers

Question

Here is the c code:

#include<stdio.h>

int main()
{
    int a[]={0,1,2,3,4};
    int*p[]={a,a+1,a+2,a+3,a+4};
    int**ptr=p;
    ptr++;

    printf("%d , %d  ",*ptr-a , **ptr);
    *ptr++;
    printf("\n%d , %d  ",*ptr-a , **ptr);
    *++ptr;
    printf("\n%d , %d ",*ptr-a , **ptr);
    ++*ptr;
    printf("\n%d , %d ", *ptr-a , **ptr);

    return 0;
}

I am unable to understand the error and what should be amended in the code.

You didn't let debugger execute line 8. I guess that's the reason you are getting this error. — Gaurav Pathak
– Gaurav Pathak, Commented Aug 1, 2017 at 11:31
Yes that works. Thank you. I didn't know how break points work.. — user7015283
– user7015283, Commented Aug 1, 2017 at 11:36
You need to cast *ptr - a to int, because the resulting type is ptrdiff_t: ideone.com/CfjqzO Also the * in *ptr++; is unnecessary. — mch
– mch, Commented Aug 1, 2017 at 11:36
BTW, the message Warning: Source file is more recent than executable should be taken into account. You need to recompile your program — Basile Starynkevitch
– Basile Starynkevitch, Commented Aug 1, 2017 at 13:15

dlmeetei · Accepted Answer · 2017-08-01 18:05:32Z

1

You have not yet executed line 8 in gdb.

8    int **ptr = p;

So till now ptr is not defined yet in your program. Hence, can not access it. You can press n command in gdb prompt and then follow by print

(gdb) n 
(gdb) print *ptr

Also, your program should have %ld instead of %d in printf as format specifier to silence compiler warning as in

printf("%ld , %ld  ",*ptr-a , **ptr);

edited Aug 1, 2017 at 18:05

answered Aug 1, 2017 at 13:02

dlmeetei

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Comments

Ravi Tiwari · Accepted Answer · 2017-08-01 11:57:23Z

1

Replace %d with %ld then it should be working fine.

You can refer format specifiers details for further clarification on this.

answered Aug 1, 2017 at 11:57

Ravi Tiwari

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2 Comments

chux Over a year ago

*ptr - a results in a type of ptrdiff_t. "%ld" is for long. Since C99, should use "%td".

Toby Speight Over a year ago

It's also worth recommending to add -Wformat to the compilation command (or, better, -Wall -Wextra).

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