how can a function return an lvalue in c?

Question

i read in The Linux Programming Interface 29-2 Threads and errno the next :

On Linux, a thread-specific errno is achieved in a similar manner to most other UNIX implementations: errno is defined as a macro that expands into a function call returning a modifiable lvalue that is distinct for each thread.

and i wondered how can a function return a modifiable lvalue.

Answer 1

It is the macro that "returns" the modifiable lvalue, not the function call itself. The function returns a pointer and the macro dereferences this pointer. For example, errno.h of the glibc source code defines the macro errno like this:

# define errno (*__errno_location ())

This wording is a bit misleading:

errno is defined as a macro that expands into a function call returning a modifiable lvalue that is distinct for each thread.

A more accurate wording would be:

errno is defined as a macro that expands into an expression that contains a function call. This expression evaluates to a modifiable lvalue that is distinct for each thread.