xref: /third_party/rust/crates/rustix/src/cstr.rs (revision b8a62b91)

/// A macro for [`CStr`] literals.
///
/// This can make passing string literals to rustix APIs more efficient, since
/// most underlying system calls with string arguments expect NUL-terminated
/// strings, and passing strings to rustix as `CStr`s means that rustix doesn't
/// need to copy them into a separate buffer to NUL-terminate them.
///
/// [`CStr`]: crate::ffi::CStr
///
/// # Examples
///
/// ```rust,no_run
/// # #[cfg(feature = "fs")]
/// # fn main() -> rustix::io::Result<()> {
/// use rustix::cstr;
/// use rustix::fs::{cwd, statat, AtFlags};
///
/// let metadata = statat(cwd(), cstr!("test.txt"), AtFlags::empty())?;
/// # Ok(())
/// # }
/// # #[cfg(not(feature = "fs"))]
/// # fn main() {}
/// ```
#[allow(unused_macros)]
#[macro_export]
macro_rules! cstr {
    ($str:literal) => {{
        // Check for NUL manually, to ensure safety.
        //
        // In release builds, with strings that don't contain NULs, this
        // constant-folds away.
        //
        // We don't use std's `CStr::from_bytes_with_nul`; as of this writing,
        // that function isn't defined as `#[inline]` in std and doesn't
        // constant-fold away.
        assert!(
            !$str.bytes().any(|b| b == b'\0'),
            "cstr argument contains embedded NUL bytes",
        );

        #[allow(unsafe_code, unused_unsafe)]
        {
            // Now that we know the string doesn't have embedded NULs, we can call
            // `from_bytes_with_nul_unchecked`, which as of this writing is defined
            // as `#[inline]` and completely optimizes away.
            //
            // Safety: We have manually checked that the string does not contain
            // embedded NULs above, and we append or own NUL terminator here.
            unsafe {
                $crate::ffi::CStr::from_bytes_with_nul_unchecked(concat!($str, "\0").as_bytes())
            }
        }
    }};
}

#[test]
fn test_cstr() {
    use crate::ffi::CString;
    use alloc::borrow::ToOwned;
    assert_eq!(cstr!(""), &*CString::new("").unwrap());
    assert_eq!(cstr!("").to_owned(), CString::new("").unwrap());
    assert_eq!(cstr!("hello"), &*CString::new("hello").unwrap());
    assert_eq!(cstr!("hello").to_owned(), CString::new("hello").unwrap());
}

#[test]
#[should_panic]
fn test_invalid_cstr() {
    let _ = cstr!("hello\0world");
}

#[test]
#[should_panic]
fn test_invalid_empty_cstr() {
    let _ = cstr!("\0");
}