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use crate::ffi::OsString;
use crate::io;
use crate::os::unix::ffi::OsStringExt;
use crate::sys::pal::os::errno;
pub fn hostname() -> io::Result<OsString> {
// Query the system for the maximum host name length.
let host_name_max = match unsafe { libc::sysconf(libc::_SC_HOST_NAME_MAX) } {
// If this fails (possibly because there is no maximum length), then
// assume a maximum length of _POSIX_HOST_NAME_MAX (255).
-1 => 255,
max => max as usize,
};
// Reserve space for the nul terminator too.
let mut buf = Vec::<u8>::try_with_capacity(host_name_max + 1)?;
loop {
// SAFETY: `buf.capacity()` bytes of `buf` are writable.
let r = unsafe { libc::gethostname(buf.as_mut_ptr().cast(), buf.capacity()) };
match (r != 0).then(errno) {
None => {
// Unfortunately, the UNIX specification says that the name will
// be truncated if it does not fit in the buffer, without returning
// an error. As additionally, the truncated name may still be null-
// terminated, there is no reliable way to detect truncation.
// Fortunately, most platforms ignore what the specification says
// and return an error (mostly ENAMETOOLONG). Should that not be
// the case, the following detects truncation if the null-terminator
// was omitted. Note that this check does not impact performance at
// all as we need to find the length of the string anyways.
//
// Use `strnlen` as it does not place an initialization requirement
// on the bytes after the nul terminator.
//
// SAFETY: `buf.capacity()` bytes of `buf` are accessible, and are
// initialized up to and including a possible nul terminator.
let len = unsafe { libc::strnlen(buf.as_ptr().cast(), buf.capacity()) };
if len < buf.capacity() {
// If the string is nul-terminated, we assume that is has not
// been truncated, as the capacity *should be* enough to hold
// `HOST_NAME_MAX` bytes.
// SAFETY: `len + 1` bytes have been initialized (we exclude
// the nul terminator from the string).
unsafe { buf.set_len(len) };
return Ok(OsString::from_vec(buf));
}
}
// As `buf.capacity()` is always less than or equal to `isize::MAX`
// (Rust allocations cannot exceed that limit), the only way `EINVAL`
// can be returned is if the system uses `EINVAL` to report that the
// name does not fit in the provided buffer. In that case (or in the
// case of `ENAMETOOLONG`), resize the buffer and try again.
Some(libc::EINVAL | libc::ENAMETOOLONG) => {}
// Other error codes (e.g. EPERM) have nothing to do with the buffer
// size and should be returned to the user.
Some(err) => return Err(io::Error::from_raw_os_error(err)),
}
// Resize the buffer (according to `Vec`'s resizing rules) and try again.
buf.try_reserve(buf.capacity() + 1)?;
}
}
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