Expose strerror and strerror_r

[Susurrus]

I'm refactoring some code of mine to use nix where possible but right now I can't get a string description for low-level errors. I used to use errno, but you can't do that easily with nix because once you have an Errno there's no way to get the underlying i32 and pass that to libc::strerror. Instead of supporting extracting this integer, it'd be useful having a safe wrapper around strerror. I'd be happy to do this since I already have code that does it, but I don't know where it should go or if this functionality is desired in nix. Should I add wrappers for strerror and strerror_r in a new /src/string.rs module?

[fiveop]

You want a function strerror(Errno) -> SomeStringType? Sounds good to me. And it does belong in src/string.rs as you suggest.

[Susurrus]

So it looks like this behavior already exists in Errno::desc(). I've used it in my own library to make sure it works correctly, and it does, so I'll just use that instead. I don't see a good reason to expose strerror() or strerror_r() given that that functionality exists, but there should be a way for people trying to use those functions to find the Errno struct and related functionality.

[kamalmarhubi]

once you have an Errno there's no way to get the underlying i32 and pass that to libc::strerror

You can do this via a cast: my_errno_value as i32. It may be nicer to provide a method, however.

[Susurrus]

I don't need this functionality anymore since discovering Errno::desc. Should I close this or leave it open as a TODO item?

[kamalmarhubi]

We can convert it to a doc issue :-)

[Susurrus]

If you have a suggestion for how to document this, I can submit a PR for it. I just don't know where this should go.

[kamalmarhubi]

@Susurrus maybe module level docs on nix::error?

[Susurrus]

Re #545 (comment) would it be accepted to add impl From<i32> for Errno which I think is more ergonomic than a cast? This could possibly replace the from_i32() method as well.

Also, do you mean I should add docs to nix::errno? nix::error doesn't exist though the type nix::Error does.

[Susurrus]

Regarding this, is there any reason to have hard-coded error strings in nix versus uses strerror_r? I'm uncertain why we'd want to bloat the binary size and duplicate the descriptions by hard-coding them.