Question

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determine size of dynamically allocated memory in c

I have a struct in my c code, but this question is also valid for all other types so I will use int instead of struct I created.

int *a = realloc(NULL, 10*sizeof(int) );
printf("%d\n",sizeof(a)); //prints 4, needed to be 40

int b[10];
printf("%d\n",sizeof(b)); //prints 40

My question is this: I am using realloc in my code and I don’t know how I can find the total size of my array. What is the easiest way to do that? Thank you.

Answer 1

Remember that sizeof is evaluated at compile-time (except with VLA, but it’s not your case). Imagine that you take the size of realloc argument as user’s input : it’s impossible to the compiler to know the real allocated size. So it returns the size of the pointer.