Function _readlinkat

src/fs/at.rs:112–159

(dirfd: BorrowedFd<'_>, path: &CStr, mut buffer: Vec<u8>)

#[cfg(all(feature = "alloc", not(target_os = "redox")))]
#[allow(unsafe_code)]
fn _readlinkat(dirfd: BorrowedFd<'_>, path: &CStr, mut buffer: Vec<u8>) -> io::Result<CString> {
    buffer.clear();
    buffer.reserve(SMALL_PATH_BUFFER_SIZE);

    loop {
        let buf = buffer.spare_capacity_mut();

        // SAFETY: `readlinkat` behaves.
        let nread = unsafe {
            backend::fs::syscalls::readlinkat(
                dirfd.as_fd(),
                path,
                (buf.as_mut_ptr().cast(), buf.len()),
            )?
        };

        debug_assert!(nread <= buffer.capacity());
        if nread < buffer.capacity() {
            // SAFETY: From the [documentation]: “On success, these calls
            // return the number of bytes placed in buf.”
            //
            // [documentation]: https://man7.org/linux/man-pages/man2/readlinkat.2.html
            unsafe {
                buffer.set_len(nread);
            }

            // SAFETY:
            // - “readlink places the contents of the symbolic link pathname
            //   in the buffer buf”
            // - [POSIX definition 3.271: Pathname]: “A string that is used
            //   to identify a file.”
            // - [POSIX definition 3.375: String]: “A contiguous sequence of
            //   bytes terminated by and including the first null byte.”
            // - “readlink does not append a terminating null byte to buf.”
            //
            // Thus, there will be no NUL bytes in the string.
            //
            // [POSIX definition 3.271: Pathname]: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap03.html#tag_03_271
            // [POSIX definition 3.375: String]: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap03.html#tag_03_375
            unsafe {
                return Ok(CString::from_vec_unchecked(buffer));
            }
        }

        // Use `Vec` reallocation strategy to grow capacity exponentially.
        buffer.reserve(buffer.capacity() + 1);
    }
}

/// `readlinkat(fd, path)`—Reads the contents of a symlink, without
/// allocating.