How to get errno value from rust libc


I have been trying to get errno value from the libc. For some reason, the API available in is not present. (using libc = "0.2")

Any clues how to get the errno value?

extern crate libc;

fn main() {
    println!("{}", *libc::___errno());

Fails compilation with the following output,

error[E0425]: cannot find function `___errno` in crate `libc`
 --> src/
4 |     println!("{}", *libc::___errno());
  |                           ^^^^^^^^ not found in `libc`


Seems like libc::__errno_location() is a better choice. But any better solution is welcome.

You might consider std::io::Error::last_os_error().raw_os_error().unwrap().


Thanks very much ! Does it panic if i make a call directly? I went ahead with __errno_location mostly because it doesn't panic.

io::Error::raw_os_error() is guaranteed to always return Some if you call it on the result of last_os_error():

Returns the OS error that this error represents (if any).

If this Error was constructed via last_os_error or from_raw_os_error, then this function will return Some, otherwise it will return None.

So the unwrap() there can never actually panic.

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Or you can write .unwrap_or(0) for a guaranteed-not-panicking unwrap, where 0 means "no error", since errno is guaranteed to be non-zero in case of error (NB: two links).

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Thanks. So far ok with libc::__errno_location. But curios as to why there are __ before errno_location. What does that signify?

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It's there to say that you don't need to call it directly but have to use errno.h macro.

Not an option for Rust user, of course.

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libc::__errno_location() is an internal platform-specific function that exists on the major Linux libc implementations. It does not necessarily exist on other Unix-like operating systems, nor on Windows. (For instance, the same function is instead called __errno() on OpenBSD and __error() on macOS.) The best cross-platform way to get the value of errno is to use io::Error::last_os_error(), which is guaranteed to return an io::Error representing an OS error.

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