c - Does malloc reserve more space while allocating memory?

Question

I am observing the following behavior in my test program:

I am doing malloc() for 1 MB and then free() it after sleep(10). I am doing this five times. I am observing memory consumption in top while the program is running.

Once free()-d, I am expecting the program's virtual memory (VIRT) consumption to be down by 1 MB. But actually it isn't. It stays stable. What is the explanation for this behavior? Does malloc() do some reserve while allocating memory?

Answer 1

Once free()-d, I am expecting program's virtual memory (VIRT) consumption to be down by 1MB.

Well, this is not guaranteed by the C standard. It only says, once you free() the memory, you should not be accessing that any more.

Whether the memory block is actually returned to the available memory pool or kept aside for future allocations is decided by the memory manager.