Can someone explain what __ means?

Im debugging through some code and saw this:

Code:

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typedef struct {
__GLinterface *(*createContext)(__GLimports *imports,
                                    __GLcontextModes *modes,
                                    __GLinterface *shareGC);
} __GLXscreenInfo;

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Can someone explain whats going on here? What is with the __ thing?

is something like casting going on here: *(*createContext)? I just dont understand whats going on here... But it looks like GLinterface is a type (most likely a struct) and we are declaring a pointer to a pointer?

Names beginning with an underscore are reserved for the compiler. Other than that, they're just part of the names.

Code:

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typedef struct { // begin struct
__GLinterface *(*createContext)(__GLimports *imports,
//return type // function pointer named createContext //first parameter
                                    __GLcontextModes *modes, //second param
                                    __GLinterface *shareGC); //third param
} __GLXscreenInfo; //typedef name

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