Allocating proper memory size

Question

I am having an issue with allocating the right size of memory in my program. I do the following:

void * ptr = sbrk(sizeof(void *)+sizeof(unsigned int));

When I do this, I think it is adding too much memory to the heap because it is allocating it in units of void* instead of bytes. How do I tell it that I want sizeof(whatever) to mean whatever bytes instead of whatever other units?

EDIT:

I have seen other people cast things as a char so that the compiler takes the size in bytes. If sizeof(unsigned int) is 4 bytes, but the type that I was using is void *, will the compiler break 4 times the size of a void * instead of 4 bytes?

Answer 1

Pass a number of bytes as the argument of sbrk.

In Linux, the prototype of sbrk is:

void *sbrk(intptr_t increment);

http://www.kernel.org/doc/man-pages/online/pages/man2/brk.2.html

sbrk() increments the program's data space by increment bytes.

But as some people in the comments added, if you want to dynamically allocate memory you are looking for the malloc function and not sbrk. brk and sbrk are syscalls that are usually used internally for the implementation of the malloc user function.

Answer 2

The kernel manages process memory in a page granularity. This means the process address space must grow (or shrink) by a whole number of pages.
So even though sbrk gets a number of bytes, it would add at least one page to the process.