Re: ISO opposes variable-length arrays???

[Paul Eggert]

On 10/14/24 22:02, Richard Stallman wrote:
>    > ! Second, because C11 no longer requires support for variable length
>    > ! arrays,
>
> Can you tell us what's behind this?  Is there some practical problem
> caused by supporting variable-length arrays?

I don't know why C11 made VLAs optional. However, I can guess:

A. VLAs make code less safe, due to problems with stack overflow and 
related issues. There is no portable way for a program to detect and 
recover from memory exhaustion, and indeed in many implementations 
(including default GNU, if I'm not mistaken) the system does not even 
detect memory exhaustion reliably - the program merely has undefined 
behavior instead. This dangerous behavior is particularly important in 
multithreaded apps where each thread has a relatively small stack.

B. They make code a bit slower.

C. I vaguely recall there would be some technical issues in making C 
code with VLAs interoperate with C++. I don't use C++, though, so am not 
the best person to explain this.

If I had to guess, I would guess (A) was the most important of these issues.

The (A) problems afflict only block-scope VLAs. Many GNU programs avoid 
block-scope VLAs, at least partly for this reason. Other VLA usage 
(e.g., as array declarations in function prototype scope) don't have the 
(A) problems and should be OK to use in code not intended to be portable 
to non-GCC compilers.