Opengl loaded but ktxTexture_GLUpload fails at glBindTexture

[viseztrance]

I'm using glad with glfw, and can call functions such as glBindTexture(GL_TEXTURE_2D, textureID) just fine. However calling ktxTexture_GLUpload fails, saying Could not load OpenGL command: glBindTexture!.

I looked over the source code, and it seems it's really calling gl.glBindTexture, but its implementation seems to be identical to what glad is already doing. Any ideas on what's going on?

Both glad and ktx are compiled statically.

[MarkCallow]

For a variety of excellent reasons libktx loads the GL function pointers dynamically instead of linking to libgl. The gl struct holds pointers to all the OpenGL functions used by GLUpload. The pointers are initialized the first time GLUpload is called. The message means it could not find the function or symbol glBindTexture when it did a dlsym (GNU/Linux, macOS) or GetProcAddr (Windows) on the OpenGL module. What OS are you using?

It must have found an OpenGL module in your app otherwise you would have seen the message OpenGL lib not linked or loaded by application. and it would not be running the code that prints the message you did see.

I do not know why it couldn't find glBindTexture. Are you building libktx yourself? If so, dId you make any changes? Is there a chance that the #include "GL/glcorearb.h" in glloader.c is somehow finding a modified glcorearb.h from GLAD?

[viseztrance]

Thank you for your fast and thorough reply. I'm on Fedora 38.

gcc --version
gcc (GCC) 13.1.1 20230511 (Red Hat 13.1.1-2)

I made no local changes to ktx. This is how I compiled it:

cmake .. -DCMAKE_CXX_FLAGS="-Wno-error=dangling-reference" -DCMAKE_C_FLAGS="-Wno-error=dangling-reference" \
-DKTX_FEATURE_STATIC_LIBRARY=ON -DCMAKE_BUILD_TYPE=Release -DKTX_FEATURE_VULKAN=OFF && \
make

"Wno-error=dangling-reference" is a fix for this fmtlib/fmt#3415 (used by tools/imageio), basically an unrelated upstream gcc issue.

In addition to vendor/KTX-Software/other_include/GL/glcorearb.h, I do have another glcorearb.h file here, though I assume that's normal:
/usr/include/GL/glcorearb.h

I think I'll try to replicate this issue in a separate repository, and then change glad for glew etc.

[MarkCallow]

I think I'll try to replicate this issue in a separate repository, and then change glad for glew etc.

Please do. I shall be happy to hear what you find out and happy to receive any PR or bug report. Note that glBindTexture is the first function in the list so it could be that the porblem has nothing to do with glBindTexture but is due to a more fundamental issue with the dlsym call retrieving GL function pointers. See Lines 98 - 106 in lib/gl_funcs.c.

[viseztrance]

As it turned out this was a silly mistake on my part, as I wasn't passing -lGL to the compile flag. It was rather annoying that it did work without it everywhere else, which made me jump to the wrong conclusions.