Hello,
I would like to know how a variable length array is managed (what extra variables or data structures are kept on the stack in order to have variable length arrays).
Thanks a lot.
+4
A:
It's just a dynamically sized array (implementation-dependent, but most commonly on the stack). It's pretty much like alloca in the old days, with the exception that sizeof will return the actual size of the array, which implies that the size of the array must also be stored somewhere (implementation-dependent as well, but probably on the stack too).
Paul R
2010-05-03 15:54:18
That's not entirely true, because there was no way to interrogate the size of an `alloca()` array after it had been created, but `sizeof` works on VLAs (so if you do call `sizeof` on the array, that size must be kept somewhere).
caf
2010-05-04 01:20:25
@caf: good point - I guess the size also needs to be stored somewhere (implementation-dependent) then - I'll update my answer.
Paul R
2010-05-04 05:59:25
A:
The size of variable length arrays is determined on run-time, instead of compilation time.
The way it's managed depends on the compiler.
GCC, for instance, allocates memory on the stack.
But there is no special structure. It's just a normal array, whose size is known at run-time.
Macmade
2010-05-03 15:56:52
A:
alternatively you could use some containers, e.g. ArrayList in java or vector in c/c++
poeschlorn
2010-05-04 06:03:31
-1 not anything to do with the question (how is a feature of C99 might be implemented) .
Pete Kirkham
2010-05-04 14:41:57