Access errno for system calls from Rust code

[jesse99]

There should be a way to get at errno from rust code.

[Blub]

It's important that we get functionality for this by either a keyword or a function in std or core. #553 cannot solve this because errno cannot reliably be expected to exist as a global variable. The manpage states that it can be implemented as a macro, and that it is thread-local.

errno  is  defined by the ISO C standard to be a modifiable lvalue of type int, and must not be explicitly declared; errno may be a macro.  errno is thread-local;

Currently you can only use a getter-function defined in an external C library, but since most C-library wrapper crates will want to use this, it should be available in the default libraries. (I was told the cargo system doesn't handle additional C-library code, so providing such a wrapper with your crate is not a good solution. Even if cargo was to allow this, it's still an ugly one.)

[erickt]

How about we add a fn with_errno<T>(f: extern fn() -> T) -> (uint, T) function to libc. It could portably capture the errno and also leaves us a convenient place to put any "block task migration while running" infrastructure if we implement task stealing.

[luqmana]

This can be closed now. os::errno() should give you the last system error code (GetLastError() on windows). Also, os::last_os_error() will give you the associated error string.