Understanding C errno

Question

On my system errno defined as:

int *    __error(void);
#define errno    (* __error())

I understand errno is a macro and expands to * __error() function:

1. I searched everywhere (source on my system) but I can't find the definition of the __error() function, can someone show/explain what would/should be the definition of it?

2. How the expression errno = 0 works with the above definition (Assigning 0 to a function?)? Does errno = 0 expands to * __error() = 0?

Thanks

Answer 1

The __error function returns a pointer to the errno variable for the calling thread. The errno macro dereferences that pointer, resulting in an lvalue that can appear on either side of an equals sign.

To answer your questions:

1. The function determines the correct address for the errno variable for that specific thread. Each thread gets its own.

2. Yes, it becomes (* __error()) = 0; which assigns 0 to that thread's errno variable.