core/ffi/c_str.rs
1//! [`CStr`] and its related types.
2
3use crate::cmp::Ordering;
4use crate::error::Error;
5use crate::ffi::c_char;
6use crate::intrinsics::const_eval_select;
7use crate::iter::FusedIterator;
8use crate::marker::PhantomData;
9use crate::ptr::NonNull;
10use crate::slice::memchr;
11use crate::{fmt, ops, slice, str};
12
13// FIXME: because this is doc(inline)d, we *have* to use intra-doc links because the actual link
14// depends on where the item is being documented. however, since this is libcore, we can't
15// actually reference libstd or liballoc in intra-doc links. so, the best we can do is remove the
16// links to `CString` and `String` for now until a solution is developed
17
18/// Representation of a borrowed C string.
19///
20/// This type represents a borrowed reference to a nul-terminated
21/// array of bytes. It can be constructed safely from a <code>&[[u8]]</code>
22/// slice, or unsafely from a raw `*const c_char`. It can be expressed as a
23/// literal in the form `c"Hello world"`.
24///
25/// The `CStr` can then be converted to a Rust <code>&[str]</code> by performing
26/// UTF-8 validation, or into an owned `CString`.
27///
28/// `&CStr` is to `CString` as <code>&[str]</code> is to `String`: the former
29/// in each pair are borrowed references; the latter are owned
30/// strings.
31///
32/// Note that this structure does **not** have a guaranteed layout (the `repr(transparent)`
33/// notwithstanding) and should not be placed in the signatures of FFI functions.
34/// Instead, safe wrappers of FFI functions may leverage [`CStr::as_ptr`] and the unsafe
35/// [`CStr::from_ptr`] constructor to provide a safe interface to other consumers.
36///
37/// # Examples
38///
39/// Inspecting a foreign C string:
40///
41/// ```
42/// use std::ffi::CStr;
43/// use std::os::raw::c_char;
44///
45/// # /* Extern functions are awkward in doc comments - fake it instead
46/// extern "C" { fn my_string() -> *const c_char; }
47/// # */ unsafe extern "C" fn my_string() -> *const c_char { c"hello".as_ptr() }
48///
49/// unsafe {
50/// let slice = CStr::from_ptr(my_string());
51/// println!("string buffer size without nul terminator: {}", slice.to_bytes().len());
52/// }
53/// ```
54///
55/// Passing a Rust-originating C string:
56///
57/// ```
58/// use std::ffi::CStr;
59/// use std::os::raw::c_char;
60///
61/// fn work(data: &CStr) {
62/// unsafe extern "C" fn work_with(s: *const c_char) {}
63/// unsafe { work_with(data.as_ptr()) }
64/// }
65///
66/// let s = c"Hello world!";
67/// work(&s);
68/// ```
69///
70/// Converting a foreign C string into a Rust `String`:
71///
72/// ```
73/// use std::ffi::CStr;
74/// use std::os::raw::c_char;
75///
76/// # /* Extern functions are awkward in doc comments - fake it instead
77/// extern "C" { fn my_string() -> *const c_char; }
78/// # */ unsafe extern "C" fn my_string() -> *const c_char { c"hello".as_ptr() }
79///
80/// fn my_string_safe() -> String {
81/// let cstr = unsafe { CStr::from_ptr(my_string()) };
82/// // Get copy-on-write Cow<'_, str>, then guarantee a freshly-owned String allocation
83/// String::from_utf8_lossy(cstr.to_bytes()).to_string()
84/// }
85///
86/// println!("string: {}", my_string_safe());
87/// ```
88///
89/// [str]: prim@str "str"
90#[derive(PartialEq, Eq, Hash)]
91#[stable(feature = "core_c_str", since = "1.64.0")]
92#[rustc_diagnostic_item = "cstr_type"]
93#[rustc_has_incoherent_inherent_impls]
94#[lang = "CStr"]
95// `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies
96// on `CStr` being layout-compatible with `[u8]`.
97// However, `CStr` layout is considered an implementation detail and must not be relied upon. We
98// want `repr(transparent)` but we don't want it to show up in rustdoc, so we hide it under
99// `cfg(doc)`. This is an ad-hoc implementation of attribute privacy.
100#[repr(transparent)]
101pub struct CStr {
102 // FIXME: this should not be represented with a DST slice but rather with
103 // just a raw `c_char` along with some form of marker to make
104 // this an unsized type. Essentially `sizeof(&CStr)` should be the
105 // same as `sizeof(&c_char)` but `CStr` should be an unsized type.
106 inner: [c_char],
107}
108
109/// An error indicating that a nul byte was not in the expected position.
110///
111/// The slice used to create a [`CStr`] must have one and only one nul byte,
112/// positioned at the end.
113///
114/// This error is created by the [`CStr::from_bytes_with_nul`] method.
115/// See its documentation for more.
116///
117/// # Examples
118///
119/// ```
120/// use std::ffi::{CStr, FromBytesWithNulError};
121///
122/// let _: FromBytesWithNulError = CStr::from_bytes_with_nul(b"f\0oo").unwrap_err();
123/// ```
124#[derive(Clone, Copy, PartialEq, Eq, Debug)]
125#[stable(feature = "core_c_str", since = "1.64.0")]
126pub enum FromBytesWithNulError {
127 /// Data provided contains an interior nul byte at byte `position`.
128 InteriorNul {
129 /// The position of the interior nul byte.
130 position: usize,
131 },
132 /// Data provided is not nul terminated.
133 NotNulTerminated,
134}
135
136#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
137impl Error for FromBytesWithNulError {
138 #[allow(deprecated)]
139 fn description(&self) -> &str {
140 match self {
141 Self::InteriorNul { .. } => "data provided contains an interior nul byte",
142 Self::NotNulTerminated => "data provided is not nul terminated",
143 }
144 }
145}
146
147/// An error indicating that no nul byte was present.
148///
149/// A slice used to create a [`CStr`] must contain a nul byte somewhere
150/// within the slice.
151///
152/// This error is created by the [`CStr::from_bytes_until_nul`] method.
153///
154#[derive(Clone, PartialEq, Eq, Debug)]
155#[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
156pub struct FromBytesUntilNulError(());
157
158#[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
159impl fmt::Display for FromBytesUntilNulError {
160 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
161 write!(f, "data provided does not contain a nul")
162 }
163}
164
165#[stable(feature = "cstr_debug", since = "1.3.0")]
166impl fmt::Debug for CStr {
167 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
168 write!(f, "\"{}\"", self.to_bytes().escape_ascii())
169 }
170}
171
172#[stable(feature = "cstr_default", since = "1.10.0")]
173impl Default for &CStr {
174 #[inline]
175 fn default() -> Self {
176 const SLICE: &[c_char] = &[0];
177 // SAFETY: `SLICE` is indeed pointing to a valid nul-terminated string.
178 unsafe { CStr::from_ptr(SLICE.as_ptr()) }
179 }
180}
181
182#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
183impl fmt::Display for FromBytesWithNulError {
184 #[allow(deprecated, deprecated_in_future)]
185 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
186 f.write_str(self.description())?;
187 if let Self::InteriorNul { position } = self {
188 write!(f, " at byte pos {position}")?;
189 }
190 Ok(())
191 }
192}
193
194impl CStr {
195 /// Wraps a raw C string with a safe C string wrapper.
196 ///
197 /// This function will wrap the provided `ptr` with a `CStr` wrapper, which
198 /// allows inspection and interoperation of non-owned C strings. The total
199 /// size of the terminated buffer must be smaller than [`isize::MAX`] **bytes**
200 /// in memory (a restriction from [`slice::from_raw_parts`]).
201 ///
202 /// # Safety
203 ///
204 /// * The memory pointed to by `ptr` must contain a valid nul terminator at the
205 /// end of the string.
206 ///
207 /// * `ptr` must be [valid] for reads of bytes up to and including the nul terminator.
208 /// This means in particular:
209 ///
210 /// * The entire memory range of this `CStr` must be contained within a single allocated object!
211 /// * `ptr` must be non-null even for a zero-length cstr.
212 ///
213 /// * The memory referenced by the returned `CStr` must not be mutated for
214 /// the duration of lifetime `'a`.
215 ///
216 /// * The nul terminator must be within `isize::MAX` from `ptr`
217 ///
218 /// > **Note**: This operation is intended to be a 0-cost cast but it is
219 /// > currently implemented with an up-front calculation of the length of
220 /// > the string. This is not guaranteed to always be the case.
221 ///
222 /// # Caveat
223 ///
224 /// The lifetime for the returned slice is inferred from its usage. To prevent accidental misuse,
225 /// it's suggested to tie the lifetime to whichever source lifetime is safe in the context,
226 /// such as by providing a helper function taking the lifetime of a host value for the slice,
227 /// or by explicit annotation.
228 ///
229 /// # Examples
230 ///
231 /// ```
232 /// use std::ffi::{c_char, CStr};
233 ///
234 /// fn my_string() -> *const c_char {
235 /// c"hello".as_ptr()
236 /// }
237 ///
238 /// unsafe {
239 /// let slice = CStr::from_ptr(my_string());
240 /// assert_eq!(slice.to_str().unwrap(), "hello");
241 /// }
242 /// ```
243 ///
244 /// ```
245 /// use std::ffi::{c_char, CStr};
246 ///
247 /// const HELLO_PTR: *const c_char = {
248 /// const BYTES: &[u8] = b"Hello, world!\0";
249 /// BYTES.as_ptr().cast()
250 /// };
251 /// const HELLO: &CStr = unsafe { CStr::from_ptr(HELLO_PTR) };
252 ///
253 /// assert_eq!(c"Hello, world!", HELLO);
254 /// ```
255 ///
256 /// [valid]: core::ptr#safety
257 #[inline] // inline is necessary for codegen to see strlen.
258 #[must_use]
259 #[stable(feature = "rust1", since = "1.0.0")]
260 #[rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0")]
261 pub const unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
262 // SAFETY: The caller has provided a pointer that points to a valid C
263 // string with a NUL terminator less than `isize::MAX` from `ptr`.
264 let len = unsafe { strlen(ptr) };
265
266 // SAFETY: The caller has provided a valid pointer with length less than
267 // `isize::MAX`, so `from_raw_parts` is safe. The content remains valid
268 // and doesn't change for the lifetime of the returned `CStr`. This
269 // means the call to `from_bytes_with_nul_unchecked` is correct.
270 //
271 // The cast from c_char to u8 is ok because a c_char is always one byte.
272 unsafe { Self::from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr.cast(), len + 1)) }
273 }
274
275 /// Creates a C string wrapper from a byte slice with any number of nuls.
276 ///
277 /// This method will create a `CStr` from any byte slice that contains at
278 /// least one nul byte. Unlike with [`CStr::from_bytes_with_nul`], the caller
279 /// does not need to know where the nul byte is located.
280 ///
281 /// If the first byte is a nul character, this method will return an
282 /// empty `CStr`. If multiple nul characters are present, the `CStr` will
283 /// end at the first one.
284 ///
285 /// If the slice only has a single nul byte at the end, this method is
286 /// equivalent to [`CStr::from_bytes_with_nul`].
287 ///
288 /// # Examples
289 /// ```
290 /// use std::ffi::CStr;
291 ///
292 /// let mut buffer = [0u8; 16];
293 /// unsafe {
294 /// // Here we might call an unsafe C function that writes a string
295 /// // into the buffer.
296 /// let buf_ptr = buffer.as_mut_ptr();
297 /// buf_ptr.write_bytes(b'A', 8);
298 /// }
299 /// // Attempt to extract a C nul-terminated string from the buffer.
300 /// let c_str = CStr::from_bytes_until_nul(&buffer[..]).unwrap();
301 /// assert_eq!(c_str.to_str().unwrap(), "AAAAAAAA");
302 /// ```
303 ///
304 #[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
305 #[rustc_const_stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
306 pub const fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError> {
307 let nul_pos = memchr::memchr(0, bytes);
308 match nul_pos {
309 Some(nul_pos) => {
310 // FIXME(const-hack) replace with range index
311 // SAFETY: nul_pos + 1 <= bytes.len()
312 let subslice = unsafe { crate::slice::from_raw_parts(bytes.as_ptr(), nul_pos + 1) };
313 // SAFETY: We know there is a nul byte at nul_pos, so this slice
314 // (ending at the nul byte) is a well-formed C string.
315 Ok(unsafe { CStr::from_bytes_with_nul_unchecked(subslice) })
316 }
317 None => Err(FromBytesUntilNulError(())),
318 }
319 }
320
321 /// Creates a C string wrapper from a byte slice with exactly one nul
322 /// terminator.
323 ///
324 /// This function will cast the provided `bytes` to a `CStr`
325 /// wrapper after ensuring that the byte slice is nul-terminated
326 /// and does not contain any interior nul bytes.
327 ///
328 /// If the nul byte may not be at the end,
329 /// [`CStr::from_bytes_until_nul`] can be used instead.
330 ///
331 /// # Examples
332 ///
333 /// ```
334 /// use std::ffi::CStr;
335 ///
336 /// let cstr = CStr::from_bytes_with_nul(b"hello\0");
337 /// assert_eq!(cstr, Ok(c"hello"));
338 /// ```
339 ///
340 /// Creating a `CStr` without a trailing nul terminator is an error:
341 ///
342 /// ```
343 /// use std::ffi::{CStr, FromBytesWithNulError};
344 ///
345 /// let cstr = CStr::from_bytes_with_nul(b"hello");
346 /// assert_eq!(cstr, Err(FromBytesWithNulError::NotNulTerminated));
347 /// ```
348 ///
349 /// Creating a `CStr` with an interior nul byte is an error:
350 ///
351 /// ```
352 /// use std::ffi::{CStr, FromBytesWithNulError};
353 ///
354 /// let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
355 /// assert_eq!(cstr, Err(FromBytesWithNulError::InteriorNul { position: 2 }));
356 /// ```
357 #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
358 #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
359 pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> {
360 let nul_pos = memchr::memchr(0, bytes);
361 match nul_pos {
362 Some(nul_pos) if nul_pos + 1 == bytes.len() => {
363 // SAFETY: We know there is only one nul byte, at the end
364 // of the byte slice.
365 Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
366 }
367 Some(position) => Err(FromBytesWithNulError::InteriorNul { position }),
368 None => Err(FromBytesWithNulError::NotNulTerminated),
369 }
370 }
371
372 /// Unsafely creates a C string wrapper from a byte slice.
373 ///
374 /// This function will cast the provided `bytes` to a `CStr` wrapper without
375 /// performing any sanity checks.
376 ///
377 /// # Safety
378 /// The provided slice **must** be nul-terminated and not contain any interior
379 /// nul bytes.
380 ///
381 /// # Examples
382 ///
383 /// ```
384 /// use std::ffi::CStr;
385 ///
386 /// let bytes = b"Hello world!\0";
387 ///
388 /// let cstr = unsafe { CStr::from_bytes_with_nul_unchecked(bytes) };
389 /// assert_eq!(cstr.to_bytes_with_nul(), bytes);
390 /// ```
391 #[inline]
392 #[must_use]
393 #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
394 #[rustc_const_stable(feature = "const_cstr_unchecked", since = "1.59.0")]
395 #[rustc_allow_const_fn_unstable(const_eval_select)]
396 pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
397 const_eval_select!(
398 @capture { bytes: &[u8] } -> &CStr:
399 if const {
400 // Saturating so that an empty slice panics in the assert with a good
401 // message, not here due to underflow.
402 let mut i = bytes.len().saturating_sub(1);
403 assert!(!bytes.is_empty() && bytes[i] == 0, "input was not nul-terminated");
404
405 // Ending nul byte exists, skip to the rest.
406 while i != 0 {
407 i -= 1;
408 let byte = bytes[i];
409 assert!(byte != 0, "input contained interior nul");
410 }
411
412 // SAFETY: See runtime cast comment below.
413 unsafe { &*(bytes as *const [u8] as *const CStr) }
414 } else {
415 // Chance at catching some UB at runtime with debug builds.
416 debug_assert!(!bytes.is_empty() && bytes[bytes.len() - 1] == 0);
417
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) }
424 }
425 )
426 }
427
428 /// Returns the inner pointer to this C string.
429 ///
430 /// The returned pointer will be valid for as long as `self` is, and points
431 /// to a contiguous region of memory terminated with a 0 byte to represent
432 /// the end of the string.
433 ///
434 /// The type of the returned pointer is
435 /// [`*const c_char`][crate::ffi::c_char], and whether it's
436 /// an alias for `*const i8` or `*const u8` is platform-specific.
437 ///
438 /// **WARNING**
439 ///
440 /// The returned pointer is read-only; writing to it (including passing it
441 /// to C code that writes to it) causes undefined behavior.
442 ///
443 /// It is your responsibility to make sure that the underlying memory is not
444 /// freed too early. For example, the following code will cause undefined
445 /// behavior when `ptr` is used inside the `unsafe` block:
446 ///
447 /// ```no_run
448 /// # #![expect(dangling_pointers_from_temporaries)]
449 /// use std::ffi::{CStr, CString};
450 ///
451 /// // 💀 The meaning of this entire program is undefined,
452 /// // 💀 and nothing about its behavior is guaranteed,
453 /// // 💀 not even that its behavior resembles the code as written,
454 /// // 💀 just because it contains a single instance of undefined behavior!
455 ///
456 /// // 🚨 creates a dangling pointer to a temporary `CString`
457 /// // 🚨 that is deallocated at the end of the statement
458 /// let ptr = CString::new("Hi!".to_uppercase()).unwrap().as_ptr();
459 ///
460 /// // without undefined behavior, you would expect that `ptr` equals:
461 /// dbg!(CStr::from_bytes_with_nul(b"HI!\0").unwrap());
462 ///
463 /// // 🙏 Possibly the program behaved as expected so far,
464 /// // 🙏 and this just shows `ptr` is now garbage..., but
465 /// // 💀 this violates `CStr::from_ptr`'s safety contract
466 /// // 💀 leading to a dereference of a dangling pointer,
467 /// // 💀 which is immediate undefined behavior.
468 /// // 💀 *BOOM*, you're dead, you're entire program has no meaning.
469 /// dbg!(unsafe { CStr::from_ptr(ptr) });
470 /// ```
471 ///
472 /// This happens because, the pointer returned by `as_ptr` does not carry any
473 /// lifetime information, and the `CString` is deallocated immediately after
474 /// the expression that it is part of has been evaluated.
475 /// To fix the problem, bind the `CString` to a local variable:
476 ///
477 /// ```
478 /// use std::ffi::{CStr, CString};
479 ///
480 /// let c_str = CString::new("Hi!".to_uppercase()).unwrap();
481 /// let ptr = c_str.as_ptr();
482 ///
483 /// assert_eq!(unsafe { CStr::from_ptr(ptr) }, c"HI!");
484 /// ```
485 #[inline]
486 #[must_use]
487 #[stable(feature = "rust1", since = "1.0.0")]
488 #[rustc_const_stable(feature = "const_str_as_ptr", since = "1.32.0")]
489 #[rustc_as_ptr]
490 #[rustc_never_returns_null_ptr]
491 pub const fn as_ptr(&self) -> *const c_char {
492 self.inner.as_ptr()
493 }
494
495 /// We could eventually expose this publicly, if we wanted.
496 #[inline]
497 #[must_use]
498 const fn as_non_null_ptr(&self) -> NonNull<c_char> {
499 // FIXME(const_trait_impl) replace with `NonNull::from`
500 // SAFETY: a reference is never null
501 unsafe { NonNull::new_unchecked(&self.inner as *const [c_char] as *mut [c_char]) }
502 .as_non_null_ptr()
503 }
504
505 /// Returns the length of `self`. Like C's `strlen`, this does not include the nul terminator.
506 ///
507 /// > **Note**: This method is currently implemented as a constant-time
508 /// > cast, but it is planned to alter its definition in the future to
509 /// > perform the length calculation whenever this method is called.
510 ///
511 /// # Examples
512 ///
513 /// ```
514 /// use std::ffi::CStr;
515 ///
516 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").unwrap();
517 /// assert_eq!(cstr.count_bytes(), 3);
518 ///
519 /// let cstr = CStr::from_bytes_with_nul(b"\0").unwrap();
520 /// assert_eq!(cstr.count_bytes(), 0);
521 /// ```
522 #[inline]
523 #[must_use]
524 #[doc(alias("len", "strlen"))]
525 #[stable(feature = "cstr_count_bytes", since = "1.79.0")]
526 #[rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0")]
527 pub const fn count_bytes(&self) -> usize {
528 self.inner.len() - 1
529 }
530
531 /// Returns `true` if `self.to_bytes()` has a length of 0.
532 ///
533 /// # Examples
534 ///
535 /// ```
536 /// use std::ffi::CStr;
537 /// # use std::ffi::FromBytesWithNulError;
538 ///
539 /// # fn main() { test().unwrap(); }
540 /// # fn test() -> Result<(), FromBytesWithNulError> {
541 /// let cstr = CStr::from_bytes_with_nul(b"foo\0")?;
542 /// assert!(!cstr.is_empty());
543 ///
544 /// let empty_cstr = CStr::from_bytes_with_nul(b"\0")?;
545 /// assert!(empty_cstr.is_empty());
546 /// assert!(c"".is_empty());
547 /// # Ok(())
548 /// # }
549 /// ```
550 #[inline]
551 #[stable(feature = "cstr_is_empty", since = "1.71.0")]
552 #[rustc_const_stable(feature = "cstr_is_empty", since = "1.71.0")]
553 pub const fn is_empty(&self) -> bool {
554 // SAFETY: We know there is at least one byte; for empty strings it
555 // is the NUL terminator.
556 // FIXME(const-hack): use get_unchecked
557 unsafe { *self.inner.as_ptr() == 0 }
558 }
559
560 /// Converts this C string to a byte slice.
561 ///
562 /// The returned slice will **not** contain the trailing nul terminator that this C
563 /// string has.
564 ///
565 /// > **Note**: This method is currently implemented as a constant-time
566 /// > cast, but it is planned to alter its definition in the future to
567 /// > perform the length calculation whenever this method is called.
568 ///
569 /// # Examples
570 ///
571 /// ```
572 /// use std::ffi::CStr;
573 ///
574 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
575 /// assert_eq!(cstr.to_bytes(), b"foo");
576 /// ```
577 #[inline]
578 #[must_use = "this returns the result of the operation, \
579 without modifying the original"]
580 #[stable(feature = "rust1", since = "1.0.0")]
581 #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
582 pub const fn to_bytes(&self) -> &[u8] {
583 let bytes = self.to_bytes_with_nul();
584 // FIXME(const-hack) replace with range index
585 // SAFETY: to_bytes_with_nul returns slice with length at least 1
586 unsafe { slice::from_raw_parts(bytes.as_ptr(), bytes.len() - 1) }
587 }
588
589 /// Converts this C string to a byte slice containing the trailing 0 byte.
590 ///
591 /// This function is the equivalent of [`CStr::to_bytes`] except that it
592 /// will retain the trailing nul terminator instead of chopping it off.
593 ///
594 /// > **Note**: This method is currently implemented as a 0-cost cast, but
595 /// > it is planned to alter its definition in the future to perform the
596 /// > length calculation whenever this method is called.
597 ///
598 /// # Examples
599 ///
600 /// ```
601 /// use std::ffi::CStr;
602 ///
603 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
604 /// assert_eq!(cstr.to_bytes_with_nul(), b"foo\0");
605 /// ```
606 #[inline]
607 #[must_use = "this returns the result of the operation, \
608 without modifying the original"]
609 #[stable(feature = "rust1", since = "1.0.0")]
610 #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
611 pub const fn to_bytes_with_nul(&self) -> &[u8] {
612 // SAFETY: Transmuting a slice of `c_char`s to a slice of `u8`s
613 // is safe on all supported targets.
614 unsafe { &*((&raw const self.inner) as *const [u8]) }
615 }
616
617 /// Iterates over the bytes in this C string.
618 ///
619 /// The returned iterator will **not** contain the trailing nul terminator
620 /// that this C string has.
621 ///
622 /// # Examples
623 ///
624 /// ```
625 /// #![feature(cstr_bytes)]
626 /// use std::ffi::CStr;
627 ///
628 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
629 /// assert!(cstr.bytes().eq(*b"foo"));
630 /// ```
631 #[inline]
632 #[unstable(feature = "cstr_bytes", issue = "112115")]
633 pub fn bytes(&self) -> Bytes<'_> {
634 Bytes::new(self)
635 }
636
637 /// Yields a <code>&[str]</code> slice if the `CStr` contains valid UTF-8.
638 ///
639 /// If the contents of the `CStr` are valid UTF-8 data, this
640 /// function will return the corresponding <code>&[str]</code> slice. Otherwise,
641 /// it will return an error with details of where UTF-8 validation failed.
642 ///
643 /// [str]: prim@str "str"
644 ///
645 /// # Examples
646 ///
647 /// ```
648 /// use std::ffi::CStr;
649 ///
650 /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
651 /// assert_eq!(cstr.to_str(), Ok("foo"));
652 /// ```
653 #[stable(feature = "cstr_to_str", since = "1.4.0")]
654 #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
655 pub const fn to_str(&self) -> Result<&str, str::Utf8Error> {
656 // N.B., when `CStr` is changed to perform the length check in `.to_bytes()`
657 // instead of in `from_ptr()`, it may be worth considering if this should
658 // be rewritten to do the UTF-8 check inline with the length calculation
659 // instead of doing it afterwards.
660 str::from_utf8(self.to_bytes())
661 }
662}
663
664// `.to_bytes()` representations are compared instead of the inner `[c_char]`s,
665// because `c_char` is `i8` (not `u8`) on some platforms.
666// That is why this is implemented manually and not derived.
667#[stable(feature = "rust1", since = "1.0.0")]
668impl PartialOrd for CStr {
669 #[inline]
670 fn partial_cmp(&self, other: &CStr) -> Option<Ordering> {
671 self.to_bytes().partial_cmp(&other.to_bytes())
672 }
673}
674#[stable(feature = "rust1", since = "1.0.0")]
675impl Ord for CStr {
676 #[inline]
677 fn cmp(&self, other: &CStr) -> Ordering {
678 self.to_bytes().cmp(&other.to_bytes())
679 }
680}
681
682#[stable(feature = "cstr_range_from", since = "1.47.0")]
683impl ops::Index<ops::RangeFrom<usize>> for CStr {
684 type Output = CStr;
685
686 #[inline]
687 fn index(&self, index: ops::RangeFrom<usize>) -> &CStr {
688 let bytes = self.to_bytes_with_nul();
689 // we need to manually check the starting index to account for the null
690 // byte, since otherwise we could get an empty string that doesn't end
691 // in a null.
692 if index.start < bytes.len() {
693 // SAFETY: Non-empty tail of a valid `CStr` is still a valid `CStr`.
694 unsafe { CStr::from_bytes_with_nul_unchecked(&bytes[index.start..]) }
695 } else {
696 panic!(
697 "index out of bounds: the len is {} but the index is {}",
698 bytes.len(),
699 index.start
700 );
701 }
702 }
703}
704
705#[stable(feature = "cstring_asref", since = "1.7.0")]
706impl AsRef<CStr> for CStr {
707 #[inline]
708 fn as_ref(&self) -> &CStr {
709 self
710 }
711}
712
713/// Calculate the length of a nul-terminated string. Defers to C's `strlen` when possible.
714///
715/// # Safety
716///
717/// The pointer must point to a valid buffer that contains a NUL terminator. The NUL must be
718/// located within `isize::MAX` from `ptr`.
719#[inline]
720#[unstable(feature = "cstr_internals", issue = "none")]
721#[rustc_allow_const_fn_unstable(const_eval_select)]
722const unsafe fn strlen(ptr: *const c_char) -> usize {
723 const_eval_select!(
724 @capture { s: *const c_char = ptr } -> usize:
725 if const {
726 let mut len = 0;
727
728 // SAFETY: Outer caller has provided a pointer to a valid C string.
729 while unsafe { *s.add(len) } != 0 {
730 len += 1;
731 }
732
733 len
734 } else {
735 unsafe extern "C" {
736 /// Provided by libc or compiler_builtins.
737 fn strlen(s: *const c_char) -> usize;
738 }
739
740 // SAFETY: Outer caller has provided a pointer to a valid C string.
741 unsafe { strlen(s) }
742 }
743 )
744}
745
746/// An iterator over the bytes of a [`CStr`], without the nul terminator.
747///
748/// This struct is created by the [`bytes`] method on [`CStr`].
749/// See its documentation for more.
750///
751/// [`bytes`]: CStr::bytes
752#[must_use = "iterators are lazy and do nothing unless consumed"]
753#[unstable(feature = "cstr_bytes", issue = "112115")]
754#[derive(Clone, Debug)]
755pub struct Bytes<'a> {
756 // since we know the string is nul-terminated, we only need one pointer
757 ptr: NonNull<u8>,
758 phantom: PhantomData<&'a [c_char]>,
759}
760
761#[unstable(feature = "cstr_bytes", issue = "112115")]
762unsafe impl Send for Bytes<'_> {}
763
764#[unstable(feature = "cstr_bytes", issue = "112115")]
765unsafe impl Sync for Bytes<'_> {}
766
767impl<'a> Bytes<'a> {
768 #[inline]
769 fn new(s: &'a CStr) -> Self {
770 Self { ptr: s.as_non_null_ptr().cast(), phantom: PhantomData }
771 }
772
773 #[inline]
774 fn is_empty(&self) -> bool {
775 // SAFETY: We uphold that the pointer is always valid to dereference
776 // by starting with a valid C string and then never incrementing beyond
777 // the nul terminator.
778 unsafe { self.ptr.read() == 0 }
779 }
780}
781
782#[unstable(feature = "cstr_bytes", issue = "112115")]
783impl Iterator for Bytes<'_> {
784 type Item = u8;
785
786 #[inline]
787 fn next(&mut self) -> Option<u8> {
788 // SAFETY: We only choose a pointer from a valid C string, which must
789 // be non-null and contain at least one value. Since we always stop at
790 // the nul terminator, which is guaranteed to exist, we can assume that
791 // the pointer is non-null and valid. This lets us safely dereference
792 // it and assume that adding 1 will create a new, non-null, valid
793 // pointer.
794 unsafe {
795 let ret = self.ptr.read();
796 if ret == 0 {
797 None
798 } else {
799 self.ptr = self.ptr.add(1);
800 Some(ret)
801 }
802 }
803 }
804
805 #[inline]
806 fn size_hint(&self) -> (usize, Option<usize>) {
807 if self.is_empty() { (0, Some(0)) } else { (1, None) }
808 }
809
810 #[inline]
811 fn count(self) -> usize {
812 // SAFETY: We always hold a valid pointer to a C string
813 unsafe { strlen(self.ptr.as_ptr().cast()) }
814 }
815}
816
817#[unstable(feature = "cstr_bytes", issue = "112115")]
818impl FusedIterator for Bytes<'_> {}