This parameter is confusing me

Question

I'm learning opengl, and one of the parameters of the function glShaderSource() is:

const GLchar* const* string

The only thing I can understand from this expression is that neither the address pointed to by the pointer nor the value of the address can be changed (because there are 2 "const" expressions). But why are there 2 asterisks?

Answer 1

what you need to know is that glShaderSource expects two related sequences which are the C-style arrays of values.

The #1 sequence is an array of C-strings, either zero-terminated or not and the #2 sequence is an array of integers, indicating the length of each of the strings in the first sequence. Note that the sequence is optional if the strings are Zero-terminated as the library will find the strings by itself.
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Let me try to simplify by tackling it in steps:
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#1 const GLchar is an immutable (constant) character.

#2 const GLchar * is a pointer to immutable GLchars, in this case your shader source.

#3 const GLchar * const is an immutable pointer to immutable GLchars, that means the pointer itself can not be changed to point somewhere else.

#4 const GLchar * const * is a pointer to an immutable pointer to immutable GLchars.

#5 string is the name of the parameter.

I don't know if it make a little sense now.

The GL-prefixed types are because the OpenGL specification needs to talk about types without tying itself into a particular language, so it introduces aliases of common C types.

Answer 2

Thanks for clarifying this concept for me!