Data Stack

This is pretty C specific, but may be useful for C++ as well in some cases.

Data stack makes it very easy to implement functions returning dynamic data but without having to worry much about memory management like freeing the result or having large enough buffers for result.

It's basically a stack with stack frames that you define. Like:

  t_push();
  // allocate memory here
  t_pop();
  // they're free'd now

Advantages over control stack (including alloca()):

• Functions can return a value allocated from data stack
• We can portably specify how much data we want to allocate at runtime

Advantages over malloc():

• FAST, most of the time allocating memory mean only updating a couple of pointers and integers. Freeing the memory all at once also is a fast operation.
• No need to free() each allocation resulting in prettier code
• No memory leaks
• No memory fragmentation

Disadvantages:

• Allocating memory inside loops can accidentally allocate a lot of memory if the loops are long and you forgot to place t_push() and t_pop() there.
• t_malloc()ed data could be accidentally stored into permanent location and accessed after it's already been free'd. const'ing the return values helps for most uses though (see the t_malloc() description).
• Debugging invalid memory usage requires recompilation with -DDISABLE_DATA_STACK which then uses malloc() and free() for all allocations and keeping track of them for stack frames making it much slower.

The code

data-stack.c, data-stack.h. For more complete sources and use cases see Dovecot (especially the strfuncs.h).

GNU obstack is also a data stack, but I find it's API a bit too bloated and difficult to use.