Question

While using the errno, I once read words that

It is important to include errno.h rather than providing your own
extern int errno, since errno is implemented in a thread-safe
manner that is more than just a simple global int.

How does the thread-safe manner impact the difference between #include and extern ?

Answer 1

It’s not the extern vs #include that matters

errno is defined in the header as

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

Each thread has it’s own errno in it’s own address space