Support MSVC: remove use of VLAs

[Nathan Grenville-Hunt]

Hi Paul,

I'm using your (excellent) library in a project under Windows/MSVC. The library builds and works without issues except for two places in which variable length stack based arrays (VLAs) are present. Microsft have confirmed that no version of MSVC will ever support VLAs, so I submit a patch to remove their use.

Rather than replace the arrays with some other stack allocated memory (with alloca or similar) I opted to add two new dynamically allocated buffers to the VTermState object, both of which are allocated on first use and resized as required. I wasn't sure about the best naming to use (you'll be the judge), but I have tried to keep the code style in keeping with the rest of the project as far as I could (I'm no C programmer).

Please take a look at the patch (attached), which is based on r783. Also, please advise if you would prefer that I submit this patch some other way (I'm not particularly familiar with launchpad).

Thanks again,
Nathan

[Paul "LeoNerd" Evans (leonerd)]

Yes; it turns out the VLAs are problematic for a few reasons. The large unbounded one sometimes grows too big for the stack as well.

Performing a dynamic re-allocation during runtime is not possible; a main design point about libvterm is that it only allocates on creation or resize, and not during normal runtime.

Instead I've solved this by using the tmpbuffer for the main decoding buffer (and defaulting it a much larger 4096 byte size), and simply putting the recombine chars on the stack as a fixed-size array, because we only support up to 6 chars per cell anyway. That's small enough it can live on the stack.

That's now in source as of -r795