How to make the bytes of the block be initialized so that they contain all 0s

Question

I am writing the calloc function in a memory management assignment (I am using C). I have one question, I wrote the malloc function and thinking about using it for calloc as it says calloc will take num and size and return a block of memory that is (num * size) which I can use malloc to create, however, it says that I need to initialize all bytes to 0 and I am confused about how to do that in general? If you need more info please ask me :)

So malloc will return a pointer (Void pointer) to the first of the usable memory and i have to go through the bytes, initialize them to zero, and return the pointer to that front of the usable memory.

Answer 1

When you need to set a block of memory to the same value, use the memset function.

It looks like this: void * memset ( void * ptr, int value, size_t num );

You can find more information about the function at: http://www.cplusplus.com/reference/clibrary/cstring/memset/

Answer 2

Hints:

• there is already a posix library function for zeroing a block of memory

• consider casting the void * to some pointer type that you can dereference / assign to.

Answer 3

I am assuming you can't use memset because it's a homework assignment assignment, and deals with memory management. So, I would just go in a loop and set all bytes to 0. Pseudocode:

for i = 1 to n:
   data[i] = 0

Oh, if you're having trouble understanding how to dereference void *, remember you can do:

void *b;
/* now make b point to somewhere useful */
unsigned char *a = b;

Answer 4

If you can't use memset, then you'll need to resort to setting each byte individually.

Since you're calling malloc from your calloc function, I'm going to assume it looks something like this:

void *calloc (size_t count, size_t sz) {
    size_t realsz = count * sz;
    void *block = malloc (realsz);
    if (block != NULL) {
        // Zero memory here.
    }
    return block;
}

and you just need the code for "// Zero memory here.".

Here's what you need to know.

1. In order to process the block one byte at a time, you need to cast the pointer to a type that references bytes (char would be good). To cast your pointer to (for example) an int pointer, you would use int *block2 = (int*)block;.

2. Once you have the right type of pointer, you can use that to store the correct data value based on the type. You would do this by storing the desired value in a loop which increments the pointer and decrements the count until the count reaches zero.

Hopefully that's enough to start with without giving away every detail of the solution. If you still have problems, leave a comment and I'll flesh out the answer until you have it correct (since it's homework, I'll be trying to get you to do most of the thinking).

Update: Since an answer's already been accepted, I'll post my full solution. To write a basic calloc in terms of just malloc:

void *calloc (size_t count, size_t sz) {
    size_t realsz, i;
    char *cblock;

    // Get size to allocate (detect size_t overflow as well).

    realsz = count * sz;
    if (count != 0)
        if (realsz / count != sz)
            return NULL;

    // Allocate the block.

    cblock = malloc (realsz);

    // Initialize all elements to zero (if allocation worked).

    if (cblock != NULL) {
        for (i = 0; i < realsz; i++)
            cblock[i] = 0;
    }

    // Return allocated, cleared block.

    return cblock;
}

Note that you can work directly with char pointers within the function since they freely convert to and from void pointers.