1/// A macro for [`CStr`] literals.
2///
3/// This can make passing string literals to rustix APIs more efficient, since
4/// most underlying system calls with string arguments expect NUL-terminated
5/// strings, and passing strings to rustix as `CStr`s means that rustix doesn't
6/// need to copy them into a separate buffer to NUL-terminate them.
7///
8/// In Rust ≥ 1.77, users can use [C-string literals] instead of this macro.
9///
10/// [`CStr`]: crate::ffi::CStr
11/// [C-string literals]: https://blog.rust-lang.org/2024/03/21/Rust-1.77.0.html#c-string-literals
12///
13/// # Examples
14///
15/// ```
16/// # #[cfg(feature = "fs")]
17/// # fn main() -> rustix::io::Result<()> {
18/// use rustix::cstr;
19/// use rustix::fs::{statat, AtFlags, CWD};
20///
21/// let metadata = statat(CWD, cstr!("Cargo.toml"), AtFlags::empty())?;
22/// # Ok(())
23/// # }
24/// # #[cfg(not(feature = "fs"))]
25/// # fn main() {}
26/// ```
27#[allow(unused_macros)]
28#[macro_export]
29macro_rules! cstr {
30 ($str:literal) => {{
31 // Check for NUL manually, to ensure safety.
32 //
33 // In release builds, with strings that don't contain NULs, this
34 // constant-folds away.
35 //
36 // We don't use std's `CStr::from_bytes_with_nul`; as of this writing,
37 // that function isn't defined as `#[inline]` in std and doesn't
38 // constant-fold away.
39 assert!(
40 !$str.bytes().any(|b| b == b'\0'),
41 "cstr argument contains embedded NUL bytes",
42 );
43
44 #[allow(unsafe_code, unused_unsafe)]
45 {
46 // Now that we know the string doesn't have embedded NULs, we can
47 // call `from_bytes_with_nul_unchecked`, which as of this writing
48 // is defined as `#[inline]` and completely optimizes away.
49 //
50 // SAFETY: We have manually checked that the string does not
51 // contain embedded NULs above, and we append or own NUL terminator
52 // here.
53 unsafe {
54 $crate::ffi::CStr::from_bytes_with_nul_unchecked(concat!($str, "\0").as_bytes())
55 }
56 }
57 }};
58}
59
60#[cfg(test)]
61mod tests {
62 #[allow(unused_imports)]
63 use super::*;
64
65 #[test]
66 fn test_cstr() {
67 use crate::ffi::CString;
68 use alloc::borrow::ToOwned as _;
69 assert_eq!(cstr!(""), &*CString::new("").unwrap());
70 assert_eq!(cstr!("").to_owned(), CString::new("").unwrap());
71 assert_eq!(cstr!("hello"), &*CString::new("hello").unwrap());
72 assert_eq!(cstr!("hello").to_owned(), CString::new("hello").unwrap());
73 }
74
75 #[test]
76 #[should_panic]
77 fn test_invalid_cstr() {
78 let _ = cstr!("hello\0world");
79 }
80
81 #[test]
82 #[should_panic]
83 fn test_invalid_empty_cstr() {
84 let _ = cstr!("\0");
85 }
86}
87