C: How to name a struct field `errno`?

Question

I want to define a struct:

struct error
{
    int errno;
    /* other information */
};

And then I want my code to have error paths that look like this:

struct error my_error;
my_error.errno = errno;
/* set other fields on the error */

But if the same translation unit includes #include <errno.h>, then errno is normally a macro which gets replaced with something else.

Assuming you actually need to access errno in that translation unit so you can't just undefine it, is there a way to still have that struct field name?

I don't care if it's by suppressing the expansion for a specific token, or by somehow getting the errno token to get generated by other macro expansions which don't then expand further, or whatever method.

Ultimately it doesn't matter and I can just name the field something else like error or error_number instead. I just don't like being unable to name things what seems to be the most appropriate name.

Answer 1

Because you commented:

I want to achieve source code where stuff like this is possible ... because I think errno is self-evidently the most clear and optimal name for that field and it's offensive that C isn't letting me use the most appropriate name for something.

It's generally a bad idea to override common identifiers that are part of the language, because it confuses people. And, it will confuse yourself at some point, when you've forgotten you did this; so it's not even advisable in private own projects.

Writing meaningful bug free code is already a very demanding task for humans. Tricks like this make things more difficult to understand. Meaning matters the most in programming!

Then, I don't think that errno is such a great name that asks to be used elsewhere:

• consists of two abbreviated parts: err and no.
• doesn't use common camel-case errNo
• good alternatives are easy/obvious (e.g. errorNumber)
• in the example I see struct error, not struct err, so it seems you don't like the abbr. err that much

Finally, if errno is a macro, a line like: my_error.errno = errno;, which is at the heart of why you want this, will never be possible because the CPP won't be able to differentiate between the two errnos. QED.