CODEWITHSUNDEEP
cpointers
Jake
Jake

Reputation: 16837

C - index applied to pointer to pointer

I want to ask what does C do when it sees an index on a pointer to pointer; for example:

struct X {
  int a;
  int b;
};

struct X ** ptr;

What will happen if a statement contains : 

ptr[i] // where i is an unsigned int

Upvotes: 2

Views: 141

Answers (3)

Some programmer dude
Some programmer dude

Reputation: 409404

Any pointer can be used with array indexing, so ptr[i] will be a pointer to struct X.

However, you have to allocate memory for ptr first of course, otherwise you will dereference an uninitialized pointer leading to undefined behavior. And if you dereference ptr[i] without initializing that pointer, then you're again have undefined behavior.

Upvotes: 3

phschoen
phschoen

Reputation: 2081

the pointer is only derefernced only once. This means the type of the expression is struct X *. You now have a 1D pointer array to all pointer with the a particuar row of your first 2D array.

Upvotes: 0

kotAPI
kotAPI

Reputation: 419

It returns a garbage value. Since "ptr" is a pointer to a pointer. You haven't declared what it's actually pointing to. For example..

#include<stdio.h>

struct X {
  int a;
  int b;
};
int main()
{
struct X ** ptr;

unsigned int i=1;
printf("%d",ptr[i]);
return 0;
}

I got the output.

1483736418

which is a garbage value of a pointer that I haven't defined.

Upvotes: 1

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