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GLSL has many global variables that are predefined such as gl_LightSource. They are global because any shader can access them.
How do I define custom global variables in GLSL?
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A uniform is a global Shader variable declared with the "uniform" storage qualifier. These act as parameters that the user of a shader program can pass to that program. Their values are stored in a program object.
The GLSL memory model does not allow for recursive function calls. This allows GLSL to execute on hardware that simply doesn't allow for recursion. It allows GLSL to function when there is no ability to write arbitrarily to memory, which is true of most shader hardware (though it is becoming less true with time).
Yes, gl_FragColor is deprecated. You should use the following syntax: layout(location = 0) out vec4 diffuseColor; It is included in the GLSL 4.60 spec under the section 7.1.
Vertex shaders manipulate coordinates in a 3D space and are called once per vertex. The purpose of the vertex shader is to set up the gl_Position variable — this is a special, global, and built-in GLSL variable. gl_Position is used to store the position of the current vertex.
Global variables don't exist in GLSL. The gl_* variables are hardcoded variables (think of them as automatically added compile-time) to access non-programmable elements of the pipeline from a shader. In Core OpenGL 3.0 most of them were removed, including gl_LightSource. The user is now expected to handle his own matrices and lights and send them to the shaders as uniforms. If you want to see a list of all the ones that remain, you should look at the GLSL Reference Pages.
What you want are uniforms. If you want to synchronize uniforms between shaders, store the location of the uniform and iterate through all your programs to upload the uniform to each shader.
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