glsl dot function indirectly causing a segmentation fault in program

Question

Heres my vertex shader code:

#version 330

layout(location = 0) in vec3 vertex_position;
layout(location = 0) in vec3 vertex_normal;

//model space
uniform vec3 toLight;

uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;

smooth out vec3 color;

void main()
{
  gl_Position = projection * view * model * vec4(vertex_position, 1.0f);
  
  vec3 normal = normalize(vertex_normal);
  
  float intensity = dot(normal, toLight);
  intensity = clamp(intensity, 0, 1);
  
  color = vec3(intensity);
}

In this code I am trying to calculate lighting using the tutorial at Learning Modern 3D Graphics Programming site. Which is a great book by the way, however the code here is basically identical to the listed code with a couple of names changed, however through the some experimentation, I have discovered something very peculiar. The segmentation fault occurs on the

color = vec3(intensity);

line, I know that because removing that line causes the program to run normally, (except without lighting of course)

However when I replace the line

float intensity = dot(normal, toLight);

with:

float intensity = 3.14159;

It clamps to 1.0f and everything works fine, the cube is white, same happens if I put a negative number: the number is clamped to zero and the cube is black...

So this is strange, here's the facts:

• The segmentation fault disappears when removing the line that sets the output vector to the color requested.
• The segmentation fault also disappears when removing the calculation of the dot product
• When the intensity is a constant float, everything works fine.
• setting the color to anything not involving the intensity works fine.
• EDIT: found some interesting info, when setting a variable other than the color variable using the intensity variable works fine, for example vec3 temp = vec3(intensity); doesn't cause a segfault, however setting color to something that is not related to the intensity variable works fine also color = vec3(1.0f); although, if I were to set color to temp it would crash too, this is very strange.

Any suggestions are greatly appreciated. Thanks

Also, I would include my opengl, c++ code, but I don't think it's necessary, correct me if I'm wrong.

Answer 1

Oh, god! I made a big mistake, right here:

layout(location = 0) in vec3 vertex_position;
layout(location = 0) in vec3 vertex_normal;

notice how vertex_position and vertex_normal have the same location, I didn't either. Setting the vertex_normal location to 1 fixed everything, sorry I wasted most of your time, but thanks to all of you for suggestions and to Tim who got me to realize that a segfault was occurring with the normal, leading me to the top of the source, thanks all of you just the same... =D

Answer 2

Best guess: you have a bug in the code that sets the toLight uniform -- it looks something like glUniform3fv(toLight_location, 3, pointer), but pointer is bogus (NULL or otherwise invalid). The GL driver code tries to dereference this pointer and crashes. When you remove all uses of toLight in the shader, it doesn't crash because toLight_location becomes -1 as its inactive and the glUniform3fv call becomes a noop.