guys, strangely, if I don't call one of the built-in uniform matrices (no matter which) in my shader, my application crashes with access violation. as I tried the variations, I realized it doesn't matter which one of the matrices called. I don't have to assign em to something or use in a operation. so if I add "just one" of following lines, it works just fine. otherwise it crashes the application: mat4 dummy = gl_ProjectionMatrix; or gl_Position = gl_ModelViewProjectionMatrix * vec4(vertexpos, 1.0f); or just gl_ModelViewMatrix; gl_ProjectionMatrix; etc... any one familiar? or am I missing something? (Card's ATI mobility radeon x700)

as I dig more I realized actually it crashes if I don't call at least one of the built-in uniforms of vertex shaders listed in http://www.opengl.org/sdk/libs/OpenSceneGraph/glsl_quickref.pdf in some point of vertex shader. but I try to create everything custom and don't want to use these built ins. any idea?

Is anything output at the shader validation stage? I've had problems with shaders that run fine on NVidia but crash horribly on Ati...

if you mean the info log, the answer is no. it says the fragment and vertex shaders are compiled succesfully and program linked succesfully.

by the way during the debug crash, access violation creates a breakpoint in the line that I pass my sampler to shader with glUniform call. but as I said before, if I access one of these built-ins in vertex shader, sampler works just fine.

Yes the info log. Do you actually try to validate the shader (not just compile and link)? It may give some clue to what's happening. Shaders get very aggressively optimised which can lead to some seemingly strange behaviour.

You get the crash when binding a sampler - this leads me to think you may get a "Validation failed - samplers of different types are bound to the same texture image unit" error when validating...

Could be many things going on... for example an output variable from the vertex shader that is input for the fragment shader that doesn't have the exact same format or qualifiers. An example I encountered a few days ago (only applies to GLSL version 1.30 and higher), I got similar behavior you described (strange things seemed to fix the program), the cause:

vertex shader:

#version 130;out vec4 color;void main(void){  gl_Position = ... // fill the position  color = vec4(1.0, 1.0, 1.0, 1.0);}

fragment shader:

#version 130;precision mediump float; // otherwise the compiler complained not specifying a precisionin vec4 color;out vec4 fragmentColor;void main(void){  fragmentColor = color;}

As soon as I added the precision qualifier in the vertex shader to the out variable as well it worked flawlessly (while the program compiled, linked and validated). These kind of things can be very strict I guess.

Other possible causes:
Have you checked all attributes (you are probably using as you are trying not to use the built-in attributes) are bound properly? Same goes for custom uniforms you have bound.