Meaning of "__"

Question

In the implementation of the Linux kernel, I came across this statement on line 89:

#define _THIS_IP_ ({ __label__ __here; __here: (unsigned long)&&__here; })

I know that in C, symbols starting with a _ followed by a capital letter or other _ are reserved for implementation, which means this in practice?

Answer 1

The use of two underscores ('__') in identifiers is reserved for internal compiler use according to the ANSI-C Technical Standard. In practice, this is done to avoid a collision with developer-defined names.

In C, symbols starting with an underscore followed by a letter uppercase or other underline are reserved for implementation. You as a C user should not create any symbols that begin with reserved sequences

Details

• Programming Languages - C
• Programming in ANSI C Language

Answer 2

This means that if you declare a symbol (variable, function, macro, etc.) in this way, you may conflict with symbols exported by the standard compiler or library implementation.

For example, you can try to create a function:

void minha_funcao(int _Param);

But its implementation somewhere declares the macro:

#define _Param 1

In this way a syntax error will occur, because the Preprocessor will replace the macro with the value 1.