What does a double underscore mean in a variable name in the C language? [duplicate]

Answer 1

It means it's a system-reserved name. The C standard says that all names beginning with two underscore, or underscore and capital letter, are reserved for the use of the system or compiler and should not be defined in application code.

Answer 2

The other answers are correct that it's reserved for the implementation. Of course here Linux should advance out of the 20th century and use the standard type uint32_t rather than the myriad of nonstandard names (__u32, u_int32_t, ...) that plagued legacy Unices..

Answer 3

That's a type, defined in here (as well as few other places).

It is by convention that usually a double underscore in front of a type, variable or function name implies a name that is always reserved, as defined in section 7.1.3 of the current standard (C99).