I've got the following code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char ** argv)
{
//just checking to see where the stack
printf("The stack is around %p\n", &argc); is, making sure arr isn't in it
char ** arr = malloc(8*sizeof(char*));
printf("arr is size %li, at %p\n", sizeof(arr), arr);
arr = realloc(arr, 100); //I picked a weird number to show it isn't doing anything. I've picked different numbers (like 200, 2*sizeof(char*)*sizeof(arr), and 16)
printf("arr is size %li, at %p\n", sizeof(arr), arr);
}
That's the entirety of the file (it's a unit test; I was noticing it elsewhere)
The output of the above is as follows:
The stack is around 0x7fff5b94d12c
arr is size 8, at 0x120f010
arr is size 8, at 0x120f010
Perhaps I'm misunderstanding what realloc should do. I'm expecting the following output.
The stack is around 0x7fff5b94d12c
arr is size 8, at 0x120f010
arr is size <size>, at <somewhere>
where <size> is... something odd like 12... at least not 8 and <somewhere> is most likely 0x120f010 but possibly anywhere reasonable.
Are my expectations wrong or am I using realloc incorrectly?
The output of your program is correct, because
malloc nor realloc have anything to do with the automatic storage (i.e. "the stack"). They allocate memory from the dynamic storage area (i.e. "the heap"). One should not expect the position of the top of the stack to change in response to calls to malloc, realloc, or for that matter, any other function.sizeof(arr) does not depend on what you have allocated to it. It is computed at compile time, and it is always equal to the size of a pointer. On your system, pointers use 8 bytes.malloc often gives you more memory that you ask, and stores the actual value in a special location that realloc can access at a later time. If you realloc down, or realloc within the bounds, the value returned by realloc does not change. That's the reason why it may perform better than simply calling malloc and memcpy.