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copied!<p>There has been a change in how this sort of thing is done in OpenGL. So let's present <a href="http://www.opengl.org/wiki/Program_Introspection" rel="nofollow noreferrer">the old way and the new way</a>.</p> <h3>Old Way</h3> <p>Linked shaders have the concept of a number of active uniforms and active attributes (vertex shader stage inputs). These are the uniforms/attributes that are in use by that shader. The number of these (as well as quite a few other things) can be queried with <a href="http://www.opengl.org/wiki/GLAPI/glGetProgram" rel="nofollow noreferrer">glGetProgramiv</a>:</p> <pre><code>GLint numActiveAttribs = 0; GLint numActiveUniforms = 0; glGetProgramiv(prog, GL_ACTIVE_ATTRIBUTES, &numActiveAttribs); glGetProgramiv(prog, GL_ACTIVE_UNIFORMS, &numActiveUniforms); </code></pre> <p>You can query active uniform blocks, transform feedback varyings, atomic counters, and similar things in this way.</p> <p>Once you have the number of active attributes/uniforms, you can start querying information about them. To get info about an attribute, you use <a href="http://www.opengl.org/wiki/GLAPI/glGetActiveAttrib" rel="nofollow noreferrer"><code>glGetActiveAttrib</code></a>; to get info about a uniform, you use <a href="http://www.opengl.org/wiki/GLAPI/glGetActiveUniform" rel="nofollow noreferrer"><code>glGetActiveUniform</code></a>. As an example, extended from the above:</p> <pre><code>GLint maxAttribNameLength = 0; glGetProgramiv(prog, GL_ACTIVE_ATTRIBUTE_MAX_LENGTH, &maxAttribNameLength); std::vector<GLchar> nameData(maxAttribNameLength) for(int attrib = 0; attrib < numActiveAttribs; ++attrib) { GLint arraySize = 0; GLenum type = 0; GLsizei actualLength = 0; glGetActiveAttrib(prog, attrib, nameData.size(), &actualLength, &arraySize, &type, &nameData[0]); std::string name((char*)&nameData[0], actualLength - 1); } </code></pre> <p>Something similar can be done for uniforms. However, the <code>GL_ACTIVE_UNIFORM_MAX_LENGTH</code> trick can be buggy on some drivers. So I would suggest this:</p> <pre><code>std::vector<GLchar> nameData(256); for(int unif = 0; unif < numActiveUniforms; ++unif) { GLint arraySize = 0; GLenum type = 0; GLsizei actualLength = 0; glGetActiveUniform(prog, unif, nameData.size(), &actualLength, &arraySize, &type, &nameData[0]); std::string name((char*)&nameData[0], actualLength - 1); } </code></pre> <p>Also, for uniforms, there's <code>glGetActiveUniforms</code>, which can query all of the name lengths for every uniform all at once (as well as all of the types, array sizes, strides, and other parameters).</p> <h3>New Way</h3> <p>This way lets you access pretty much <em>everything</em> about active variables in a successfully linked program (except for regular globals). The <a href="http://www.opengl.org/registry/specs/ARB/program_interface_query.txt" rel="nofollow noreferrer">ARB_program_interface_query</a> extension is not widely available yet, but it'll get there.</p> <p>It starts with a call to <a href="http://www.opengl.org/wiki/GLAPI/glGetProgramInterface" rel="nofollow noreferrer"><code>glGetProgramInterfaceiv</code></a>, to query the number of active attributes/uniforms. Or whatever else you may want.</p> <pre><code>GLint numActiveAttribs = 0; GLint numActiveUniforms = 0; glGetProgramInterfaceiv(prog, GL_PROGRAM_INPUT, GL_ACTIVE_RESOURCES, &numActiveAttribs); glGetProgramInterfaceiv(prog, GL_UNIFORM, GL_ACTIVE_RESOURCES, &numActiveUniforms); </code></pre> <p>Attributes are just vertex shader inputs; <code>GL_PROGRAM_INPUT</code> means the inputs to the first program in the program object.</p> <p>You can then loop over the number of active resources, asking for info on each one in turn, from <a href="http://www.opengl.org/wiki/GLAPI/glGetProgramResource" rel="nofollow noreferrer"><code>glGetProgramResourceiv</code></a> and <a href="http://www.opengl.org/wiki/GLAPI/glGetProgramResourceName" rel="nofollow noreferrer"><code>glGetProgramResourceName</code></a>:</p> <pre><code>std::vector<GLchar> nameData(256); std::vector<GLenum> properties; properties.push_back(GL_NAME_LENGTH); properties.push_back(GL_TYPE); properties.push_back(GL_ARRAY_SIZE); std::vector<GLint> values(properties.size()); for(int attrib = 0; attrib < numActiveAttribs; ++attrib) { glGetProgramResourceiv(prog, GL_PROGRAM_INPUT, attrib, properties.size(), &properties[0], values.size(), NULL, &values[0]); nameData.resize(values[0]); //The length of the name. glGetProgramResourceName(prog, GL_PROGRAM_INPUT, attrib, nameData.size(), NULL, &nameData[0]); std::string name((char*)&nameData[0], nameData.size() - 1); } </code></pre> <p>The exact same code would work for <code>GL_UNIFORM</code>; just swap <code>numActiveAttribs</code> with <code>numActiveUniforms</code>.</p>

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