Fix potential segmentation fault on glBindSampler

[matrefeytontias]

As per the OpenGL 4 specification, glBindSampler is available only when the GL version is 3.3 or higher. This fixes a potential segmentation fault when trying to run the OpenGL 3 example with another extension wrangler than the one included in the examples package.

[ocornut]

OpenGL version are tricky to get right in a cross platform way, see e.g. #1466 where the version was already changed from 3.3 to 3.2 on January 7 (read the comments about OS X).
Did you actually get a segmentation fault? Could you specify which OpenGL loader/extension wrangler/OS/GPU/driver?

[matrefeytontias]

I did get a segmentation fault on Windows with MSYS2 (MSYS shell environment with -mwin32 passed to gcc), GLAD set to OpenGL 4.6 compatibility mode with latest GLFW, running on an Nvidia GeForce GTX 1050 with latest drivers. I have asserted that printf("%p", (void*)glBindSampler); displays 0x0 with the various GLFW hints set to OpenGL 3.2 and that the program is working fine with the various hints pointing to OpenGL 3.3 .

[ocornut]

Thank for the details! I don’t really know what’s the best course of action, will think about it.

[matrefeytontias]

A little older, but the exact same situation appeared there (see the only comment on the only answer) : https://stackoverflow.com/questions/48582444/imgui-with-the-glad-opengl-loader-throws-segmentation-fault-core-dumped

[ocornut]

@matrefeytontias I guess we could do make those calls optionals:

if (glBindSampler != NULL) 
   glBindSampler(0, 0);

Are you aware of an OpenGL function that which would setup the functions in a way that would pass the if() test but still crash inside?

[matrefeytontias]

I doubt it. As far as I know, segfaults related to OpenGL that aren't a programmer's mistake are always due to null function pointers, be it because extension wrangling has been done wrong or because the function is flat out not available in the context. However, I think that this test is more a work-around than a real fix, because if the program still works when the function isn't called, then what's the point in calling it to begin with ?

[ocornut]

If the OpenGL context has this functionality it needs to explicitly be set up, otherwise an application that sets a different sampler and run the imgui render function will get broken rendering.

Also note that main.cpp is purely demo code while imgui_impl_opengl etc. are meant to be shared code.

[ocornut]

Closing as fixed by 1954462