# Errno

Native implementation of Errno

Provides system errno constants for Linux. In real Perl, `Errno.pm` is
auto-generated from system headers. We provide the same constants via
native Rust implementation using the `libc` crate.

The module exports constant subs like `ENOENT()`, `EACCES()`, etc. that
return the errno integer value. It also provides `TIEHASH`, `FETCH`,
`STORE`, `EXISTS`, `FIRSTKEY`, `NEXTKEY` methods for the `%!` tied hash.

Perl equivalent: Errno

# Functions

## EPERM, ENOENT, ESRCH, … (errno constants)

Each errno name is a constant sub returning its integer value.
Over 100 Linux errno constants are provided (see `ERRNO_CONSTANTS`).

```none
use Errno qw(ENOENT EACCES);
if ($! == ENOENT) { ... }
```

## TIEHASH

Ties the `%!` hash to the Errno module. Called implicitly by the runtime.

```none
tie %!, 'Errno';
```

## FETCH

Looks up an errno constant by name through the tied `%!` hash.

```none
my $val = $!{ENOENT};  # returns 2
```

## EXISTS

Checks whether a given name is a known errno constant.

```none
if (exists $!{ENOENT}) { ... }
```

## FIRSTKEY

Returns the first errno constant name. Used by `each %!` and `keys %!`.

```none
my $first = (keys %!)[0];  # "EPERM"
```

## NEXTKEY

Returns the next errno constant name after a given key. Drives hash
iteration over `%!`.

```none
while (my ($k, $v) = each %!) { ... }
```