Problems Using memset and memcpy

Question

So I am trying to create a Memory Management System. In order to do this I have a set amount of space (allocated by malloc) and then I have a function myMalloc which will essentially return a pointer to the space allocated. Since we will then try and free it, we are trying to set a header of the allocated space to be the size of the allocated space, using memset.

memset(memPtr,sizeBytes,sizeof(int));

We then need to be able to read this so we can see the size of it. We are attempting to do this by using memcpy and getting the first sizeof(int) bytes into a variable. For testing purposes we are just trying to do memset and then immediately get the size back. I've included the entire method below so that you can see all declarations. Any help would be greatly appreciated! Thanks!

void* FirstFit::memMalloc(int sizeBytes){

node* listPtr = freelist;
void* memPtr;   

// Cycle through each node in freelist
while(listPtr != NULL)
{

    if(listPtr->size >= sizeBytes) 
    {
        // We found our space
        // This is where the new memory allocation begins
        memPtr = listPtr->head;
        memset(memPtr,sizeBytes,sizeof(int));

        void *size;
        memcpy(size, memPtr, sizeof(int));
        // Now let's shrink freelist
        listPtr->size = listPtr->size - sizeBytes;

        int *temp = (int*)listPtr->head + (sizeBytes*sizeof(int));  
        listPtr->head = (int*) temp;
        return memPtr;
    }

    listPtr = listPtr->next;
}

::Edit:: Sorry! When running this, we keep getting a seg fault when attempting to run the memcpy line. We have been playing with different ideas for the past hour or so and honestly just have no idea where the error is occurring.

::Edit2:: I also posted this as a comment, but figured I'd put it here as well, so it was easier to find...

Our problem is that we have an allocated space that we are allowed to work with, specified by one malloc call for 128MB. We can only use this, so we can't initialize size to anything using malloc. I guess, is there a way to do this without initializing size. If not, is there anyway to get the int that the header is set to without using memcpy.

Answer 1

The prototype for memcpy is

void * memcpy ( void * destination, const void * source, size_t num );

The problem lies here:

void *size; /* you haven't initialized this variable, and then you're writing to what it points to*/
memcpy(size, memPtr, sizeof(memPtr)); /* because size points to uninitialized memory it seg faults*/

EDIT1: Please review this tutorial on pointers in C and C++ Unless you understand pointers you will not understand why those two lines of code, back to back, are a bad pair.