Memory is not reallocating

Question

I'm in the middle of a project and I'm trying to use `malloc()` and `realloc()`. I know when I malloc, it works, but when I use realloc, it doesn't change the amount of alloced memory at all. I've always though that realloc will re-allocate your already malloced memory. Here is what I have: This include: ``` #include <stdlib.h> ``` I have a struct: ``` struct student { int age; int numOfClasses; int gender; //0 male; 1 female } student; ``` When I want to make 7 of those structs using malloc, I will use this line of code: ``` student stud* = (structure*) malloc(7*sizeof(student)); ``` This line works. That line of code takes the size of the structure and multiples that by 7. In short, this will grab enough memory to make an array of 7 structures. Now, if I want to change that to 8, I would do this where `A` is the previous malloced memory, and `B` is the new malloced (or realloced) memory: Here is how I have it in code: ``` stud = (student*)realloc(stud, 8*sizeof(student)); ``` From w…

Accepted Answer

Your understanding of `realloc`'s behaviour is nearly correct. It doesn't have to return a different pointer; it may be that there was enough unused memory after the initial block, so the heap manager can just return the same pointer back to you (but adjust its own internal state such that it knows the block is now bigger).

You have made one small mistake, though.

```
stud = (student*)realloc(stud, 8*sizeof(student));
```

Here you are replacing your `stud` pointer with the return value from `realloc`. If it happens to return `NULL` due to memory starvation, then you have lost your original `stud` pointer and the memory is leaked. You should use a temporary pointer instead.

```
tmp = realloc(stud, 8*sizeof(student));
if (tmp)
    stud = tmp;
```

Also note that you still have to actually put something in the eighth slot. After the realloc the eighth slot is valid allocated memory, but contains garbage until you store something meaningful in it.

Memory is not reallocating

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