Why am I getting this warning when using extern int errno?

Question

All my C programs are always built on Linux like systems, but this time I'm programming on a Windows system. Every time I use the global variable errno from errno.h I get the following warning:

#include <stdio.h>
#include <errno.h>

extern int errno;

int main()
{
errno = -10;
printf("%d\n", errno);
return 0;
}

Warning: '_errno' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
extern int errno;

Funny, when I comment that line the warning disappear, but why?

I really don't understand why that warning is displayed, because when I compile the same code on a linux system, gcc doesn't display nothing. In both cases, I use flags -g and -Wall to debug program..... I will appreciate if someone explain me this, thank you🤗😀

Answer 1

Why are you redeclaring 'errno'? Is there any purpose of doing that? I think that is the problem, because the compiler warns that '_errno' (errno is a macro which probably expands to '_errno'...?) is "redeclared". It was probably declared with an attribute in the errno.h header, which you are redeclaring without the attribute.

Answer 2

No bro, there isn't any purpose of doing it, it's just that sometimes I write that sentence on linux and I don't get any warnings, that's why I am curious, but you're right, maybe that errno version was declared with some attribute, thank you bro. I will remove that line😀

Answer 3

Hacker-KR4636
You have to remember that 'errno' is a macro
https://en.cppreference.com/w/cpp/error/errno

What it expands to is "implementation-defined", which means it expands to different things on different compilers, which might be the reason why you get a warning on one but not on another.

Answer 4

Azhagesanヾ(˙❥˙)
ChillPill 🌶
XXX

Hope you can help me with this problem. Thanks in advance🤗😀

Answer 5

Yeah, You're right bro. Thank you so much for helping me bro🤗😘🤗🤗😀😀🙏🙏🙏