Re: [Caml-list] Occasional malformed strings; OCaml -> OpenGL, via tgls/ctypes.

[Jeremy Yallop <yallop@gmail.com>]

On 8 May 2016 at 18:08, Anthony Tavener <anthony.tavener@gmail.com> wrote:
> TL;DR: Constant string like "fontColor" can (rarely) come out like
> "\220\552\663\1" after foreign call to OpenGL.
>
> I was seeing glitches in render output, and have tracked it down to "uniform
> variable names" which are given as constant strings, but on OpenGL side they
> can appear malformed.
>
> OCaml call:
>
>     let color = Gl.get_uniform_location sp "fontColor" in
>
> In a trace of the OpenGL calls from the running program the result is
> usually like this:
>
>     glGetUniformLocation(program = 3, name = "fontColor") = 3
>
> But on occasion (once in several hundred calls) has garbage:
>
>     glGetUniformLocation(program = 3, name = "\220\552\663\1") = -1
>
> The really odd thing to me is the structure of these strings (some more, not
> all from "fontColor"):
>     "\220    \774\1"
>     "\220\332\663\1"
>     "\440\\\665\1"
>     "\220\552\663\1"
>     "\220J\663\1"
>     "\660\117\553\2"
>     "\440\220w\2"
>
> For reference, Gl.get_uniform_location is implemented thusly (in tgls):
>
>   let get_uniform_location =
>     foreign ~stub "glGetUniformLocation"
>       (int_as_uint @-> string @-> returning int)
>
>
> I am using OCaml 4.03+flambda. The problem occurs with -O3 or no
> optimizations. The problem might have happened with prior versions (I
> haven't been able to work with OCaml much in the past year), and my own code
> is of course suspect. :)
>
> I'm posting in case this rings a bell for someone, that they might know
> what's going on. I'm continuing to debug.

If you have a reproducible test-case I'd be interested to take a look at this.