Question

Possible Duplicate:
sizeof array clarification

I have 2 arrays declared

GLfloat gCubeVertexData[216] = { list of numbers};

and an array declared:

GLfloat *resultArray = malloc(sizeof(GLfloat) * [arrayOfVerticies count]);

    for(int i = 0; i < [arrayOfVerticies count]; i++)
    {
        resultArray[i] = [[arrayOfVerticies objectAtIndex:i] floatValue];

    }

why is it when I do sizeof(gCubeVertexData) I get 864 ( a GLflot is 4 bits so divided by 4 and you get 216)

and when I do sizeof(resultArray) I get 4? event though if I were to print out resultArray[100] I get the correct number, and there is a lot more than 4 numbers stored?

Answer 1

Because gCubeVertexData is an array, and resultArray is a pointer. In the case of an array, the compiler knows how many bytes it is required to allocate for the array, so it explicitly knows a size (in the case of variable-length arrays in C99, it can also be computed easily at runtime, perhaps by messing with the stack pointer).

However, in the case of malloc(), the compiler has no knowledge about the size of the memory pointed by the pointer (that size can be obtained in a non-standard and platform-dependent way only anyways…), so it just returns the size of the variable itself, which is a pointer in this case, so you’ll get back sizeof(GLfloat *) in the end.