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