What are the conventions for __func functions?

Question

I've seen a bunch of functions in Linux code named __foo. What does the double underscore stand for and when should it be used?

Answer 1

It means it's a reserved identifer. Both C++ 03 and C99 standard mentioned about this.

C99:

7.1.3 Reserved identifiers

• All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
• All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

C++ 03:

Each name that contains a double underscore (_ _) or begins with an underscore followed by an uppercase letter (2.11) is reserved to the implementation for any use.

You can also refer to:

• What are the rules about using an underscore in a C++ identifier?
• What does a double underscore mean in a variable name in the C language?
• Why do people use __(double underscore) so much in C++