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GL_ARB_shader_storage_buffer_object ".length()" for unsized arrays returns erroneous value

Question asked by fletcheb on Oct 17, 2014

I am using OpenGL 4.3 core profile to do various things in OpenGL under Linux.  I am having trouble when using shader storage buffers where the last component of a shader storage block is an unsized array.  I do not have this problem when using the NVIDIA driver.  Is this a driver bug, or am I doing something wrong??

 

 

Description of problem:

 

It appears that the ".length()" function for unsized arrays at the end of shader storage blocks always returns 0. This is contrary to the specification for GL_ARB_shader_storage_buffer_object where "Issue (19)" describes ".length()" as returning the size of unsized arrays based on the size of the provided buffer object.

 

 

Steps to reproduce:

1. Include a shader with a shader storage block containing an unsized array as the last component

 

...

layout(std430, binding=0) buffer Test

{

  float test[];

}

...

 

2. Bind a shader storage buffer with a size equivalent to the desired shader storage block unsized array size in bytes

 

...

GLuint testBuffer;

glGenBuffers(1, &testBuffer);

glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, testBuffer);

glBufferData(GL_SHADER_STORAGE_BUFFER, sizeof(float)*4, NULL, GL_DYNAMIC_DRAW);

...

 

3. In the shader, access the shader storage block unsized array ".length()" function and check that it contains the number of elements we allocated via

glBufferData.

 

...

int elements = test.length();

 

if(elements!=4)

{

  // the .length() function has returned a value outside the

  // GL_ARB_shader_storage_buffer_object specification

}

...

 

 

Actual result:

 

elements = 0

 

Expected result:

 

elements = 4

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