malloc returns memory or virtual address space

Question

Does malloc allocate a block of memory on the heap or should it be called Virtual Address Space?

Am I being picky calling it Virtual Address Space or this just the legacy of DOS? How about Linux?

EDIT:

many answers with great details, none of them answer my question, though.

Answer 1

malloc allocates memory on the heap, period.

Your C library typically keeps a list (or some more intricate data structure) of available memory chunks, finding a suitable chunk to satisfy a malloc (possibly splitting a larger chunk into a number of smaller ones) and returning free'd memory to the list (possibly merging a few smaller chunks into a bigger one)

Only when the list doesn't contain a large enough chunk to satisfy your malloc, the library will ask the OS for more memory, e.g. using the sbrk syscall. The address returned by this syscall may be a virtual address, or a real one, depending on your hardware, but as a programmer you cannot (and don't need to) know this.

Saying that malloc allocates virtual adress space rather than a block on the heap is like saying that read reads from your hard disk rather than from a file: it is irrelevant from the caller's perspective, and not always true.

Answer 2

There are at least 3 ways of measuring memory consumption:

• virtual address space - the amount of your process's address space consumed by the allocation. this also affects fragmentation and the maximum contiguous future allocations you can make.
• commit charge - this is the operating system's accounting of the maximum possible physical storage required to maintain all of the writable, non-file/device-backed memory allocated to your process. if the OS allows it to exceed the total physical memory + swap, very bad things could happen the first time the excess is written to.
• physical memory - the amount of physical resources (potentially including swap, depending on your interpretation) your process is currently occupying. This could be less than commit charge due to virgin zero pages and virgin private writable maps of files, or more than commit charge due to non-writable or shared mappings the process is using (but these are usually swappable/discardable).

malloc generally affects them all.

Edit: So, the best way I can think to answer your question is to say:

malloc allocates virtual memory.

And virtual memory consumes:

• virtual address space,
• commit charge, and
• physical resources, if it's been written to.