Semantics of `errno`

Question

I tried searching the standards but couldn't find anything specific regarding errno.

What I mean is some implementations define errno as a global variable while some define it as a macro

#define errno (*_err_no())

The standard doesn't include it in the list of reserved keywords for identifiers.

This means that I should be able to use errno as a local variable. While it should be fine for the first implementation (that defines it as a global), the second one wouldn't be fine.

Is use of errno implementation defined or am I missing something?

Answer 1

From the latest C11 draft (N1570), 7.5 Errors <errno.h> (emphasis mine):

The header <errno.h> defines several macros, all relating to the reporting of error conditions.

The macros are

EDOM
EILSEQ
ERANGE

which expand to integer constant expressions with type int, distinct positive values, and which are suitable for use in #if preprocessing directives; and

errno

which expands to a modifiable lvalue that has type int and thread local storage duration, the value of which is set to a positive error number by several library functions. If a macro definition is suppressed in order to access an actual object, or a program defines an identifier with the name errno, the behavior is undefined.

This pretty clearly defines errno as a macro, and reserves its name in all scopes.