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.

Thanks for your time, and any help!

-Tony