1 use crate::cmp::Ordering;
2 use crate::ffi::c_char;
7 use crate::slice::memchr;
10 /// Representation of a borrowed C string.
12 /// This type represents a borrowed reference to a nul-terminated
13 /// array of bytes. It can be constructed safely from a <code>&[[u8]]</code>
14 /// slice, or unsafely from a raw `*const c_char`. It can then be
15 /// converted to a Rust <code>&[str]</code> by performing UTF-8 validation, or
16 /// into an owned `CString`.
18 /// `&CStr` is to `CString` as <code>&[str]</code> is to `String`: the former
19 /// in each pair are borrowed references; the latter are owned
22 /// Note that this structure is **not** `repr(C)` and is not recommended to be
23 /// placed in the signatures of FFI functions. Instead, safe wrappers of FFI
24 /// functions may leverage the unsafe [`CStr::from_ptr`] constructor to provide
25 /// a safe interface to other consumers.
29 /// Inspecting a foreign C string:
31 /// ```ignore (extern-declaration)
32 /// use std::ffi::CStr;
33 /// use std::os::raw::c_char;
35 /// extern "C" { fn my_string() -> *const c_char; }
38 /// let slice = CStr::from_ptr(my_string());
39 /// println!("string buffer size without nul terminator: {}", slice.to_bytes().len());
43 /// Passing a Rust-originating C string:
45 /// ```ignore (extern-declaration)
46 /// use std::ffi::{CString, CStr};
47 /// use std::os::raw::c_char;
49 /// fn work(data: &CStr) {
50 /// extern "C" { fn work_with(data: *const c_char); }
52 /// unsafe { work_with(data.as_ptr()) }
55 /// let s = CString::new("data data data data").expect("CString::new failed");
59 /// Converting a foreign C string into a Rust `String`:
61 /// ```ignore (extern-declaration)
62 /// use std::ffi::CStr;
63 /// use std::os::raw::c_char;
65 /// extern "C" { fn my_string() -> *const c_char; }
67 /// fn my_string_safe() -> String {
68 /// let cstr = unsafe { CStr::from_ptr(my_string()) };
69 /// // Get copy-on-write Cow<'_, str>, then guarantee a freshly-owned String allocation
70 /// String::from_utf8_lossy(cstr.to_bytes()).to_string()
73 /// println!("string: {}", my_string_safe());
76 /// [str]: prim@str "str"
78 #[cfg_attr(not(test), rustc_diagnostic_item = "CStr")]
79 #[stable(feature = "core_c_str", since = "1.64.0")]
80 #[rustc_has_incoherent_inherent_impls]
82 // `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies
83 // on `CStr` being layout-compatible with `[u8]`.
84 // When attribute privacy is implemented, `CStr` should be annotated as `#[repr(transparent)]`.
85 // Anyway, `CStr` representation and layout are considered implementation detail, are
86 // not documented and must not be relied upon.
88 // FIXME: this should not be represented with a DST slice but rather with
89 // just a raw `c_char` along with some form of marker to make
90 // this an unsized type. Essentially `sizeof(&CStr)` should be the
91 // same as `sizeof(&c_char)` but `CStr` should be an unsized type.
95 /// An error indicating that a nul byte was not in the expected position.
97 /// The slice used to create a [`CStr`] must have one and only one nul byte,
98 /// positioned at the end.
100 /// This error is created by the [`CStr::from_bytes_with_nul`] method.
101 /// See its documentation for more.
106 /// use std::ffi::{CStr, FromBytesWithNulError};
108 /// let _: FromBytesWithNulError = CStr::from_bytes_with_nul(b"f\0oo").unwrap_err();
110 #[derive(Clone, PartialEq, Eq, Debug)]
111 #[stable(feature = "core_c_str", since = "1.64.0")]
112 pub struct FromBytesWithNulError {
113 kind: FromBytesWithNulErrorKind,
116 #[derive(Clone, PartialEq, Eq, Debug)]
117 enum FromBytesWithNulErrorKind {
122 impl FromBytesWithNulError {
123 const fn interior_nul(pos: usize) -> FromBytesWithNulError {
124 FromBytesWithNulError { kind: FromBytesWithNulErrorKind::InteriorNul(pos) }
126 const fn not_nul_terminated() -> FromBytesWithNulError {
127 FromBytesWithNulError { kind: FromBytesWithNulErrorKind::NotNulTerminated }
131 #[unstable(feature = "cstr_internals", issue = "none")]
132 pub fn __description(&self) -> &str {
134 FromBytesWithNulErrorKind::InteriorNul(..) => {
135 "data provided contains an interior nul byte"
137 FromBytesWithNulErrorKind::NotNulTerminated => "data provided is not nul terminated",
142 /// An error indicating that no nul byte was present.
144 /// A slice used to create a [`CStr`] must contain a nul byte somewhere
145 /// within the slice.
147 /// This error is created by the [`CStr::from_bytes_until_nul`] method.
149 #[derive(Clone, PartialEq, Eq, Debug)]
150 #[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
151 pub struct FromBytesUntilNulError(());
153 #[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
154 impl fmt::Display for FromBytesUntilNulError {
155 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
156 write!(f, "data provided does not contain a nul")
160 #[stable(feature = "cstr_debug", since = "1.3.0")]
161 impl fmt::Debug for CStr {
162 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
163 write!(f, "\"{}\"", self.to_bytes().escape_ascii())
167 #[stable(feature = "cstr_default", since = "1.10.0")]
168 impl Default for &CStr {
169 fn default() -> Self {
170 const SLICE: &[c_char] = &[0];
171 // SAFETY: `SLICE` is indeed pointing to a valid nul-terminated string.
172 unsafe { CStr::from_ptr(SLICE.as_ptr()) }
176 #[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
177 impl fmt::Display for FromBytesWithNulError {
178 #[allow(deprecated, deprecated_in_future)]
179 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
180 f.write_str(self.__description())?;
181 if let FromBytesWithNulErrorKind::InteriorNul(pos) = self.kind {
182 write!(f, " at byte pos {pos}")?;
189 /// Wraps a raw C string with a safe C string wrapper.
191 /// This function will wrap the provided `ptr` with a `CStr` wrapper, which
192 /// allows inspection and interoperation of non-owned C strings. The total
193 /// size of the raw C string must be smaller than `isize::MAX` **bytes**
194 /// in memory due to calling the `slice::from_raw_parts` function.
198 /// * The memory pointed to by `ptr` must contain a valid nul terminator at the
199 /// end of the string.
201 /// * `ptr` must be [valid] for reads of bytes up to and including the null terminator.
202 /// This means in particular:
204 /// * The entire memory range of this `CStr` must be contained within a single allocated object!
205 /// * `ptr` must be non-null even for a zero-length cstr.
207 /// * The memory referenced by the returned `CStr` must not be mutated for
208 /// the duration of lifetime `'a`.
210 /// > **Note**: This operation is intended to be a 0-cost cast but it is
211 /// > currently implemented with an up-front calculation of the length of
212 /// > the string. This is not guaranteed to always be the case.
216 /// The lifetime for the returned slice is inferred from its usage. To prevent accidental misuse,
217 /// it's suggested to tie the lifetime to whichever source lifetime is safe in the context,
218 /// such as by providing a helper function taking the lifetime of a host value for the slice,
219 /// or by explicit annotation.
223 /// ```ignore (extern-declaration)
224 /// use std::ffi::{c_char, CStr};
227 /// fn my_string() -> *const c_char;
231 /// let slice = CStr::from_ptr(my_string());
232 /// println!("string returned: {}", slice.to_str().unwrap());
237 /// #![feature(const_cstr_methods)]
239 /// use std::ffi::{c_char, CStr};
241 /// const HELLO_PTR: *const c_char = {
242 /// const BYTES: &[u8] = b"Hello, world!\0";
243 /// BYTES.as_ptr().cast()
245 /// const HELLO: &CStr = unsafe { CStr::from_ptr(HELLO_PTR) };
248 /// [valid]: core::ptr#safety
251 #[stable(feature = "rust1", since = "1.0.0")]
252 #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
253 pub const unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
254 // SAFETY: The caller has provided a pointer that points to a valid C
255 // string with a NUL terminator of size less than `isize::MAX`, whose
256 // content remain valid and doesn't change for the lifetime of the
259 // Thus computing the length is fine (a NUL byte exists), the call to
260 // from_raw_parts is safe because we know the length is at most `isize::MAX`, meaning
261 // the call to `from_bytes_with_nul_unchecked` is correct.
263 // The cast from c_char to u8 is ok because a c_char is always one byte.
265 const fn strlen_ct(s: *const c_char) -> usize {
268 // SAFETY: Outer caller has provided a pointer to a valid C string.
269 while unsafe { *s.add(len) } != 0 {
276 fn strlen_rt(s: *const c_char) -> usize {
278 /// Provided by libc or compiler_builtins.
279 fn strlen(s: *const c_char) -> usize;
282 // SAFETY: Outer caller has provided a pointer to a valid C string.
286 let len = intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt);
287 Self::from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr.cast(), len + 1))
291 /// Creates a C string wrapper from a byte slice.
293 /// This method will create a `CStr` from any byte slice that contains at
294 /// least one nul byte. The caller does not need to know or specify where
295 /// the nul byte is located.
297 /// If the first byte is a nul character, this method will return an
298 /// empty `CStr`. If multiple nul characters are present, the `CStr` will
299 /// end at the first one.
301 /// If the slice only has a single nul byte at the end, this method is
302 /// equivalent to [`CStr::from_bytes_with_nul`].
306 /// #![feature(cstr_from_bytes_until_nul)]
308 /// use std::ffi::CStr;
310 /// let mut buffer = [0u8; 16];
312 /// // Here we might call an unsafe C function that writes a string
313 /// // into the buffer.
314 /// let buf_ptr = buffer.as_mut_ptr();
315 /// buf_ptr.write_bytes(b'A', 8);
317 /// // Attempt to extract a C nul-terminated string from the buffer.
318 /// let c_str = CStr::from_bytes_until_nul(&buffer[..]).unwrap();
319 /// assert_eq!(c_str.to_str().unwrap(), "AAAAAAAA");
322 #[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
323 #[rustc_const_unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
324 pub const fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError> {
325 let nul_pos = memchr::memchr(0, bytes);
328 let subslice = &bytes[..nul_pos + 1];
329 // SAFETY: We know there is a nul byte at nul_pos, so this slice
330 // (ending at the nul byte) is a well-formed C string.
331 Ok(unsafe { CStr::from_bytes_with_nul_unchecked(subslice) })
333 None => Err(FromBytesUntilNulError(())),
337 /// Creates a C string wrapper from a byte slice.
339 /// This function will cast the provided `bytes` to a `CStr`
340 /// wrapper after ensuring that the byte slice is nul-terminated
341 /// and does not contain any interior nul bytes.
343 /// If the nul byte may not be at the end,
344 /// [`CStr::from_bytes_until_nul`] can be used instead.
349 /// use std::ffi::CStr;
351 /// let cstr = CStr::from_bytes_with_nul(b"hello\0");
352 /// assert!(cstr.is_ok());
355 /// Creating a `CStr` without a trailing nul terminator is an error:
358 /// use std::ffi::CStr;
360 /// let cstr = CStr::from_bytes_with_nul(b"hello");
361 /// assert!(cstr.is_err());
364 /// Creating a `CStr` with an interior nul byte is an error:
367 /// use std::ffi::CStr;
369 /// let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
370 /// assert!(cstr.is_err());
372 #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
373 #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
374 pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> {
375 let nul_pos = memchr::memchr(0, bytes);
377 Some(nul_pos) if nul_pos + 1 == bytes.len() => {
378 // SAFETY: We know there is only one nul byte, at the end
379 // of the byte slice.
380 Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
382 Some(nul_pos) => Err(FromBytesWithNulError::interior_nul(nul_pos)),
383 None => Err(FromBytesWithNulError::not_nul_terminated()),
387 /// Unsafely creates a C string wrapper from a byte slice.
389 /// This function will cast the provided `bytes` to a `CStr` wrapper without
390 /// performing any sanity checks.
393 /// The provided slice **must** be nul-terminated and not contain any interior
399 /// use std::ffi::{CStr, CString};
402 /// let cstring = CString::new("hello").expect("CString::new failed");
403 /// let cstr = CStr::from_bytes_with_nul_unchecked(cstring.to_bytes_with_nul());
404 /// assert_eq!(cstr, &*cstring);
409 #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
410 #[rustc_const_stable(feature = "const_cstr_unchecked", since = "1.59.0")]
411 #[rustc_allow_const_fn_unstable(const_eval_select)]
412 pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
414 fn rt_impl(bytes: &[u8]) -> &CStr {
415 // Chance at catching some UB at runtime with debug builds.
416 debug_assert!(!bytes.is_empty() && bytes[bytes.len() - 1] == 0);
418 // SAFETY: Casting to CStr is safe because its internal representation
419 // is a [u8] too (safe only inside std).
420 // Dereferencing the obtained pointer is safe because it comes from a
421 // reference. Making a reference is then safe because its lifetime
422 // is bound by the lifetime of the given `bytes`.
423 unsafe { &*(bytes as *const [u8] as *const CStr) }
426 const fn const_impl(bytes: &[u8]) -> &CStr {
427 // Saturating so that an empty slice panics in the assert with a good
428 // message, not here due to underflow.
429 let mut i = bytes.len().saturating_sub(1);
430 assert!(!bytes.is_empty() && bytes[i] == 0, "input was not nul-terminated");
432 // Ending null byte exists, skip to the rest.
436 assert!(byte != 0, "input contained interior nul");
439 // SAFETY: See `rt_impl` cast.
440 unsafe { &*(bytes as *const [u8] as *const CStr) }
443 // SAFETY: The const and runtime versions have identical behavior
444 // unless the safety contract of `from_bytes_with_nul_unchecked` is
445 // violated, which is UB.
446 unsafe { intrinsics::const_eval_select((bytes,), const_impl, rt_impl) }
449 /// Returns the inner pointer to this C string.
451 /// The returned pointer will be valid for as long as `self` is, and points
452 /// to a contiguous region of memory terminated with a 0 byte to represent
453 /// the end of the string.
457 /// The returned pointer is read-only; writing to it (including passing it
458 /// to C code that writes to it) causes undefined behavior.
460 /// It is your responsibility to make sure that the underlying memory is not
461 /// freed too early. For example, the following code will cause undefined
462 /// behavior when `ptr` is used inside the `unsafe` block:
465 /// # #![allow(unused_must_use)] #![allow(temporary_cstring_as_ptr)]
466 /// use std::ffi::CString;
468 /// let ptr = CString::new("Hello").expect("CString::new failed").as_ptr();
470 /// // `ptr` is dangling
475 /// This happens because the pointer returned by `as_ptr` does not carry any
476 /// lifetime information and the `CString` is deallocated immediately after
477 /// the `CString::new("Hello").expect("CString::new failed").as_ptr()`
478 /// expression is evaluated.
479 /// To fix the problem, bind the `CString` to a local variable:
482 /// # #![allow(unused_must_use)]
483 /// use std::ffi::CString;
485 /// let hello = CString::new("Hello").expect("CString::new failed");
486 /// let ptr = hello.as_ptr();
488 /// // `ptr` is valid because `hello` is in scope
493 /// This way, the lifetime of the `CString` in `hello` encompasses
494 /// the lifetime of `ptr` and the `unsafe` block.
497 #[stable(feature = "rust1", since = "1.0.0")]
498 #[rustc_const_stable(feature = "const_str_as_ptr", since = "1.32.0")]
499 pub const fn as_ptr(&self) -> *const c_char {
503 /// Returns `true` if `self.to_bytes()` has a length of 0.
508 /// #![feature(cstr_is_empty)]
510 /// use std::ffi::CStr;
511 /// # use std::ffi::FromBytesWithNulError;
513 /// # fn main() { test().unwrap(); }
514 /// # fn test() -> Result<(), FromBytesWithNulError> {
515 /// let cstr = CStr::from_bytes_with_nul(b"foo\0")?;
516 /// assert!(!cstr.is_empty());
518 /// let empty_cstr = CStr::from_bytes_with_nul(b"\0")?;
519 /// assert!(empty_cstr.is_empty());
524 #[unstable(feature = "cstr_is_empty", issue = "102444")]
525 pub const fn is_empty(&self) -> bool {
526 // SAFETY: We know there is at least one byte; for empty strings it
527 // is the NUL terminator.
528 (unsafe { self.inner.get_unchecked(0) }) == &0
531 /// Converts this C string to a byte slice.
533 /// The returned slice will **not** contain the trailing nul terminator that this C
536 /// > **Note**: This method is currently implemented as a constant-time
537 /// > cast, but it is planned to alter its definition in the future to
538 /// > perform the length calculation whenever this method is called.
543 /// use std::ffi::CStr;
545 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
546 /// assert_eq!(cstr.to_bytes(), b"foo");
549 #[must_use = "this returns the result of the operation, \
550 without modifying the original"]
551 #[stable(feature = "rust1", since = "1.0.0")]
552 #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
553 pub const fn to_bytes(&self) -> &[u8] {
554 let bytes = self.to_bytes_with_nul();
555 // SAFETY: to_bytes_with_nul returns slice with length at least 1
556 unsafe { bytes.get_unchecked(..bytes.len() - 1) }
559 /// Converts this C string to a byte slice containing the trailing 0 byte.
561 /// This function is the equivalent of [`CStr::to_bytes`] except that it
562 /// will retain the trailing nul terminator instead of chopping it off.
564 /// > **Note**: This method is currently implemented as a 0-cost cast, but
565 /// > it is planned to alter its definition in the future to perform the
566 /// > length calculation whenever this method is called.
571 /// use std::ffi::CStr;
573 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
574 /// assert_eq!(cstr.to_bytes_with_nul(), b"foo\0");
577 #[must_use = "this returns the result of the operation, \
578 without modifying the original"]
579 #[stable(feature = "rust1", since = "1.0.0")]
580 #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
581 pub const fn to_bytes_with_nul(&self) -> &[u8] {
582 // SAFETY: Transmuting a slice of `c_char`s to a slice of `u8`s
583 // is safe on all supported targets.
584 unsafe { &*(&self.inner as *const [c_char] as *const [u8]) }
587 /// Yields a <code>&[str]</code> slice if the `CStr` contains valid UTF-8.
589 /// If the contents of the `CStr` are valid UTF-8 data, this
590 /// function will return the corresponding <code>&[str]</code> slice. Otherwise,
591 /// it will return an error with details of where UTF-8 validation failed.
593 /// [str]: prim@str "str"
598 /// use std::ffi::CStr;
600 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
601 /// assert_eq!(cstr.to_str(), Ok("foo"));
603 #[stable(feature = "cstr_to_str", since = "1.4.0")]
604 #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
605 pub const fn to_str(&self) -> Result<&str, str::Utf8Error> {
606 // N.B., when `CStr` is changed to perform the length check in `.to_bytes()`
607 // instead of in `from_ptr()`, it may be worth considering if this should
608 // be rewritten to do the UTF-8 check inline with the length calculation
609 // instead of doing it afterwards.
610 str::from_utf8(self.to_bytes())
614 #[stable(feature = "rust1", since = "1.0.0")]
615 impl PartialEq for CStr {
616 fn eq(&self, other: &CStr) -> bool {
617 self.to_bytes().eq(other.to_bytes())
620 #[stable(feature = "rust1", since = "1.0.0")]
622 #[stable(feature = "rust1", since = "1.0.0")]
623 impl PartialOrd for CStr {
624 fn partial_cmp(&self, other: &CStr) -> Option<Ordering> {
625 self.to_bytes().partial_cmp(&other.to_bytes())
628 #[stable(feature = "rust1", since = "1.0.0")]
630 fn cmp(&self, other: &CStr) -> Ordering {
631 self.to_bytes().cmp(&other.to_bytes())
635 #[stable(feature = "cstr_range_from", since = "1.47.0")]
636 impl ops::Index<ops::RangeFrom<usize>> for CStr {
639 fn index(&self, index: ops::RangeFrom<usize>) -> &CStr {
640 let bytes = self.to_bytes_with_nul();
641 // we need to manually check the starting index to account for the null
642 // byte, since otherwise we could get an empty string that doesn't end
644 if index.start < bytes.len() {
645 // SAFETY: Non-empty tail of a valid `CStr` is still a valid `CStr`.
646 unsafe { CStr::from_bytes_with_nul_unchecked(&bytes[index.start..]) }
649 "index out of bounds: the len is {} but the index is {}",
657 #[stable(feature = "cstring_asref", since = "1.7.0")]
658 impl AsRef<CStr> for CStr {
660 fn as_ref(&self) -> &CStr {