In the Linux kernel implementation , I came across this statement on the 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 another _ are reserved for implementation, what does this mean in practice?
This means that if you declare a symbol (variable, function, macro, etc.) in this way, you can conflict with symbols exported by the standard compiler or library implementation.
For example, you could try creating a function:
void minha_funcao(int _Param);
But its implementation declares the macro somewhere:
#define _Param 1
In this way, a syntax error will occur because the preprocessor will replace the macro with a value of 1.
The use of two underscores ('__') in identifiers is reserved for internal compiler use according to the ANSI-C Technical Rule . In practice, this is done to avoid a collision with developer-defined names.
In C, symbols beginning with an underscore followed by a letter capital or other underlining are reserved for implementation. You as a C user should not create any symbols that start with reserved sequences
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