Question

Is memory allocation a system call? For example, malloc and new. Is the heap shared by different processes and managed by the OS. What about private heap? If memory allocation in the heap is managed by the OS, how expensive is this?

I would also like to have some link to places where I can read more about this topic.

Answer 1

In general, malloc and new do not perform a system call at each invocation. However, they use a lower-level mechanism to allocate large pages of memory. On Windows, the lower mechanism is VirtualAlloc(). I believe on POSIX systems, this is somewhat equivalent to mmap(). Both of these perform a system call to allocate memory to the process at the OS level. Subsequent allocations will use smaller parts of those large pages without incurring a system call.

The heap is normally inner-process and is not shared between processes. If you need this, most OSes have an API for allocating shared memory. A portable wrapper for these APIs is available in the Boost.Interprocess library.

If you would like to learn more about memory allocation and relationship with the OS, you should take a look at a good book on operating systems. I always suggest Modern Operating Systems by Andrew S. Tanenbaum as it is very easy to read.