CStr in std::ffi::c_str - Rust
Description: A dynamically-sized view of a C string.

## CStr

# Struct CStr

1.88.0 · Source

    pub struct CStr { /* private fields */ }

Expand description

A dynamically-sized view of a C string.

The type `&CStr` represents a reference to a borrowed nul-terminated array of bytes. It can be constructed safely from a `&[u8]` slice, or unsafely from a raw `*const c_char`. It can be expressed as a literal in the form `c"Hello world"`.

The `&CStr` can then be converted to a Rust `&str` by performing UTF-8 validation, or into an owned `CString`.

`&CStr` is to `CString` as `&str` is to `String`: the former in each pair are borrowing references; the latter are owned strings.

Note that this structure does not have a guaranteed layout (the `repr(transparent)` notwithstanding) and should not be placed in the signatures of FFI functions. Instead, safe wrappers of FFI functions may leverage `CStr::as_ptr` and the unsafe `CStr::from_ptr` constructor to provide a safe interface to other consumers.

## §Examples

Inspecting a foreign C string:

    use std::ffi::CStr;
    use std::os::raw::c_char;

    extern "C" { fn my_string() -> *const c_char; }

    unsafe {
        let slice = CStr::from_ptr(my_string());
        println!("string buffer size without nul terminator: {}", slice.to_bytes().len());
    }

Passing a Rust-originating C string:

    use std::ffi::CStr;
    use std::os::raw::c_char;

    fn work(data: &CStr) {
        unsafe extern "C" fn work_with(s: *const c_char) {}
        unsafe { work_with(data.as_ptr()) }
    }

    let s = c"Hello world!";
    work(&s);

Converting a foreign C string into a Rust `String`:

    use std::ffi::CStr;
    use std::os::raw::c_char;

    extern "C" { fn my_string() -> *const c_char; }

    fn my_string_safe() -> String {
        let cstr = unsafe { CStr::from_ptr(my_string()) };
        // Get a copy-on-write Cow<'_, str>, then extract the
        // allocated String (or allocate a fresh one if needed).
        cstr.to_string_lossy().into_owned()
    }

    println!("string: {}", my_string_safe());

## Implementations§

Source§

### impl CStr

1.0.0 (const: 1.81.0) · Source

#### pub const unsafe fn from_ptr<'a>(ptr: *const i8) -> &'a CStr

Wraps a raw C string with a safe C string wrapper.

This function will wrap the provided `ptr` with a `CStr` wrapper, which allows inspection and interoperation of non-owned C strings. The total size of the terminated buffer must be smaller than `isize::MAX` bytes in memory (a restriction from `slice::from_raw_parts`).

##### §Safety

- The memory pointed to by `ptr` must contain a valid nul terminator at the end of the string.

- `ptr` must be valid for reads of bytes up to and including the nul terminator. This means in particular:

  - The entire memory range of this `CStr` must be contained within a single allocation!
  - `ptr` must be non-null even for a zero-length cstr.

- The memory referenced by the returned `CStr` must not be mutated for the duration of lifetime `'a`.

- The nul terminator must be within `isize::MAX` from `ptr`

> Note: This operation is intended to be a 0-cost cast but it is currently implemented with an up-front calculation of the length of the string. This is not guaranteed to always be the case.

##### §Caveat

The lifetime for the returned slice is inferred from its usage. To prevent accidental misuse, it’s suggested to tie the lifetime to whichever source lifetime is safe in the context, such as by providing a helper function taking the lifetime of a host value for the slice, or by explicit annotation.

##### §Examples

    use std::ffi::{c_char, CStr};

    fn my_string() -> *const c_char {
        c"hello".as_ptr()
    }

    unsafe {
        let slice = CStr::from_ptr(my_string());
        assert_eq!(slice.to_str().unwrap(), "hello");
    }

    use std::ffi::{c_char, CStr};

    const HELLO_PTR: *const c_char = {
        const BYTES: &[u8] = b"Hello, world!\0";
        BYTES.as_ptr().cast()
    };
    const HELLO: &CStr = unsafe { CStr::from_ptr(HELLO_PTR) };

    assert_eq!(c"Hello, world!", HELLO);

1.69.0 (const: 1.69.0) · Source

#### pub const fn from_bytes_until_nul( bytes: &[u8], ) -> Result<&CStr, FromBytesUntilNulError>

Creates a C string wrapper from a byte slice with any number of nuls.

This method will create a `CStr` from any byte slice that contains at least one nul byte. Unlike with `CStr::from_bytes_with_nul`, the caller does not need to know where the nul byte is located.

If the first byte is a nul character, this method will return an empty `CStr`. If multiple nul characters are present, the `CStr` will end at the first one.

If the slice only has a single nul byte at the end, this method is equivalent to `CStr::from_bytes_with_nul`.

##### §Examples

    use std::ffi::CStr;

    let mut buffer = [0u8; 16];
    unsafe {
        // Here we might call an unsafe C function that writes a string
        // into the buffer.
        let buf_ptr = buffer.as_mut_ptr();
        buf_ptr.write_bytes(b'A', 8);
    }
    // Attempt to extract a C nul-terminated string from the buffer.
    let c_str = CStr::from_bytes_until_nul(&buffer[..]).unwrap();
    assert_eq!(c_str.to_str().unwrap(), "AAAAAAAA");

1.10.0 (const: 1.72.0) · Source

#### pub const fn from_bytes_with_nul( bytes: &[u8], ) -> Result<&CStr, FromBytesWithNulError>

Creates a C string wrapper from a byte slice with exactly one nul terminator.

This function will cast the provided `bytes` to a `CStr` wrapper after ensuring that the byte slice is nul-terminated and does not contain any interior nul bytes.

If the nul byte may not be at the end, `CStr::from_bytes_until_nul` can be used instead.

##### §Examples

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"hello\0");
    assert_eq!(cstr, Ok(c"hello"));

Creating a `CStr` without a trailing nul terminator is an error:

    use std::ffi::{CStr, FromBytesWithNulError};

    let cstr = CStr::from_bytes_with_nul(b"hello");
    assert_eq!(cstr, Err(FromBytesWithNulError::NotNulTerminated));

Creating a `CStr` with an interior nul byte is an error:

    use std::ffi::{CStr, FromBytesWithNulError};

    let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
    assert_eq!(cstr, Err(FromBytesWithNulError::InteriorNul { position: 2 }));

1.10.0 (const: 1.59.0) · Source

#### pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr

Unsafely creates a C string wrapper from a byte slice.

This function will cast the provided `bytes` to a `CStr` wrapper without performing any sanity checks.

##### §Safety

The provided slice must be nul-terminated and not contain any interior nul bytes.

##### §Examples

    use std::ffi::CStr;

    let bytes = b"Hello world!\0";

    let cstr = unsafe { CStr::from_bytes_with_nul_unchecked(bytes) };
    assert_eq!(cstr.to_bytes_with_nul(), bytes);

1.0.0 (const: 1.32.0) · Source

#### pub const fn as_ptr(&self) -> *const i8

Returns the inner pointer to this C string.

The returned pointer will be valid for as long as `self` is, and points to a contiguous region of memory terminated with a 0 byte to represent the end of the string.

The type of the returned pointer is `*const c_char`, and whether it’s an alias for `*const i8` or `*const u8` is platform-specific.

WARNING

The returned pointer is read-only; writing to it (including passing it to C code that writes to it) causes undefined behavior.

It is your responsibility to make sure that the underlying memory is not freed too early. For example, the following code will cause undefined behavior when `ptr` is used inside the `unsafe` block:

    use std::ffi::{CStr, CString};

    // 💀 The meaning of this entire program is undefined,
    // 💀 and nothing about its behavior is guaranteed,
    // 💀 not even that its behavior resembles the code as written,
    // 💀 just because it contains a single instance of undefined behavior!

    // 🚨 creates a dangling pointer to a temporary `CString`
    // 🚨 that is deallocated at the end of the statement
    let ptr = CString::new("Hi!".to_uppercase()).unwrap().as_ptr();

    // without undefined behavior, you would expect that `ptr` equals:
    dbg!(CStr::from_bytes_with_nul(b"HI!\0").unwrap());

    // 🙏 Possibly the program behaved as expected so far,
    // 🙏 and this just shows `ptr` is now garbage..., but
    // 💀 this violates `CStr::from_ptr`'s safety contract
    // 💀 leading to a dereference of a dangling pointer,
    // 💀 which is immediate undefined behavior.
    // 💀 *BOOM*, you're dead, your entire program has no meaning.
    dbg!(unsafe { CStr::from_ptr(ptr) });

This happens because, the pointer returned by `as_ptr` does not carry any lifetime information, and the `CString` is deallocated immediately after the expression that it is part of has been evaluated. To fix the problem, bind the `CString` to a local variable:

    use std::ffi::{CStr, CString};

    let c_str = CString::new("Hi!".to_uppercase()).unwrap();
    let ptr = c_str.as_ptr();

    assert_eq!(unsafe { CStr::from_ptr(ptr) }, c"HI!");

1.79.0 (const: 1.81.0) · Source

#### pub const fn count_bytes(&self) -> usize

Returns the length of `self`. Like C’s `strlen`, this does not include the nul terminator.

> Note: This method is currently implemented as a constant-time cast, but it is planned to alter its definition in the future to perform the length calculation whenever this method is called.

##### §Examples

    assert_eq!(c"foo".count_bytes(), 3);
    assert_eq!(c"".count_bytes(), 0);

1.71.0 (const: 1.71.0) · Source

#### pub const fn is_empty(&self) -> bool

Returns `true` if `self.to_bytes()` has a length of 0.

##### §Examples

    assert!(!c"foo".is_empty());
    assert!(c"".is_empty());

1.0.0 (const: 1.72.0) · Source

#### pub const fn to_bytes(&self) -> &[u8] ⓘ

Converts this C string to a byte slice.

The returned slice will not contain the trailing nul terminator that this C string has.

> Note: This method is currently implemented as a constant-time cast, but it is planned to alter its definition in the future to perform the length calculation whenever this method is called.

##### §Examples

    assert_eq!(c"foo".to_bytes(), b"foo");

1.0.0 (const: 1.72.0) · Source

#### pub const fn to_bytes_with_nul(&self) -> &[u8] ⓘ

Converts this C string to a byte slice containing the trailing 0 byte.

This function is the equivalent of `CStr::to_bytes` except that it will retain the trailing nul terminator instead of chopping it off.

> Note: This method is currently implemented as a 0-cost cast, but it is planned to alter its definition in the future to perform the length calculation whenever this method is called.

##### §Examples

    assert_eq!(c"foo".to_bytes_with_nul(), b"foo\0");

Source

#### pub fn bytes(&self) -> Bytes<'_> ⓘ

🔬This is a nightly-only experimental API. (`cstr_bytes` #112115)

Iterates over the bytes in this C string.

The returned iterator will not contain the trailing nul terminator that this C string has.

##### §Examples

    #![feature(cstr_bytes)]

    assert!(c"foo".bytes().eq(*b"foo"));

1.4.0 (const: 1.72.0) · Source

#### pub const fn to_str(&self) -> Result<&str, Utf8Error>

Yields a `&str` slice if the `CStr` contains valid UTF-8.

If the contents of the `CStr` are valid UTF-8 data, this function will return the corresponding `&str` slice. Otherwise, it will return an error with details of where UTF-8 validation failed.

##### §Examples

    assert_eq!(c"foo".to_str(), Ok("foo"));

Source

#### pub fn display(&self) -> impl Display

🔬This is a nightly-only experimental API. (`cstr_display` #139984)

Returns an object that implements `Display` for safely printing a `CStr` that may contain non-Unicode data.

Behaves as if `self` were first lossily converted to a `str`, with invalid UTF-8 presented as the Unicode replacement character: �.

##### §Examples

    #![feature(cstr_display)]

    let cstr = c"Hello, world!";
    println!("{}", cstr.display());

Source

#### pub const fn as_c_str(&self) -> &CStr

🔬This is a nightly-only experimental API. (`str_as_str` #130366)

Returns the same string as a string slice `&CStr`.

This method is redundant when used directly on `&CStr`, but it helps dereferencing other string-like types to string slices, for example references to `Box<CStr>` or `Arc<CStr>`.

## Trait Implementations§

1.7.0 (const: unstable) · Source§

### impl AsRef<CStr> for CStr

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#### fn as_ref(&self) -> &CStr

Converts this type into a shared reference of the (usually inferred) input type.
1.7.0 · Source§

### impl AsRef<CStr> for CString

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#### fn as_ref(&self) -> &CStr

Converts this type into a shared reference of the (usually inferred) input type.
1.3.0 · Source§

### impl Borrow<CStr> for CString

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#### fn borrow(&self) -> &CStr

Immutably borrows from an owned value. Read more
1.29.0 · Source§

### impl Clone for Box<CStr>

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#### fn clone(&self) -> Box<CStr>

Returns a duplicate of the value. Read more
1.0.0 · Source§

#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl CloneToUninit for CStr

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
1.3.0 · Source§

### impl Debug for CStr

Shows the underlying bytes as a normal string, with invalid UTF-8 presented as hex escape sequences.

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
1.10.0 · Source§

### impl Default for &CStr

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#### fn default() -> &CStr

Returns the “default value” for a type. Read more
1.17.0 · Source§

### impl Default for Box<CStr>

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#### fn default() -> Box<CStr>

Returns the “default value” for a type. Read more
1.24.0 · Source§

### impl From<&CStr> for Arc<CStr>

Available on `target_has_atomic=ptr` only.
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#### fn from(s: &CStr) -> Arc<CStr>

Converts a `&CStr` into a `Arc<CStr>`, by copying the contents into a newly allocated `Arc`.

1.17.0 · Source§

### impl From<&CStr> for Box<CStr>

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#### fn from(s: &CStr) -> Box<CStr>

Converts a `&CStr` into a `Box<CStr>`, by copying the contents into a newly allocated `Box`.

1.7.0 · Source§

### impl From<&CStr> for CString

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#### fn from(s: &CStr) -> CString

Converts a `&CStr` into a `CString` by copying the contents into a new allocation.

1.28.0 · Source§

### impl<'a> From<&'a CStr> for Cow<'a, CStr>

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#### fn from(s: &'a CStr) -> Cow<'a, CStr>

Converts a `CStr` into a borrowed `Cow` without copying or allocating.

1.24.0 · Source§

### impl From<&CStr> for Rc<CStr>

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#### fn from(s: &CStr) -> Rc<CStr>

Converts a `&CStr` into a `Rc<CStr>`, by copying the contents into a newly allocated `Rc`.

1.84.0 · Source§

### impl From<&mut CStr> for Arc<CStr>

Available on `target_has_atomic=ptr` only.
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#### fn from(s: &mut CStr) -> Arc<CStr>

Converts a `&mut CStr` into a `Arc<CStr>`, by copying the contents into a newly allocated `Arc`.

1.84.0 · Source§

### impl From<&mut CStr> for Box<CStr>

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#### fn from(s: &mut CStr) -> Box<CStr>

Converts a `&mut CStr` into a `Box<CStr>`, by copying the contents into a newly allocated `Box`.

1.84.0 · Source§

### impl From<&mut CStr> for Rc<CStr>

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#### fn from(s: &mut CStr) -> Rc<CStr>

Converts a `&mut CStr` into a `Rc<CStr>`, by copying the contents into a newly allocated `Rc`.

1.20.0 · Source§

### impl From<CString> for Box<CStr>

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#### fn from(s: CString) -> Box<CStr>

Converts a `CString` into a `Box<CStr>` without copying or allocating.

1.45.0 · Source§

### impl From<Cow<'_, CStr>> for Box<CStr>

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#### fn from(cow: Cow<'_, CStr>) -> Box<CStr>

Converts a `Cow<'a, CStr>` into a `Box<CStr>`, by copying the contents if they are borrowed.

1.64.0 · Source§

### impl Hash for CStr

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#### fn hash<__H>(&self, state: &mut __H)
where __H: Hasher,

Feeds this value into the given `Hasher`. Read more
1.47.0 · Source§

### impl Index<RangeFrom<usize>> for CStr

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#### type Output = CStr

The returned type after indexing.
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#### fn index(&self, index: RangeFrom<usize>) -> &CStr

Performs the indexing (`container[index]`) operation. Read more
1.0.0 · Source§

### impl Ord for CStr

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#### fn cmp(&self, other: &CStr) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more
1.90.0 · Source§

### impl PartialEq<&CStr> for CStr

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#### fn eq(&self, other: &&CStr) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &&CStr) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.90.0 · Source§

### impl PartialEq<&CStr> for CString

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#### fn eq(&self, other: &&CStr) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &&CStr) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.90.0 · Source§

### impl PartialEq<&CStr> for Cow<'_, CStr>

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#### fn eq(&self, other: &&CStr) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &&CStr) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.90.0 · Source§

### impl PartialEq<CStr> for CString

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#### fn eq(&self, other: &CStr) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &CStr) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.90.0 · Source§

### impl PartialEq<CStr> for Cow<'_, CStr>

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#### fn eq(&self, other: &CStr) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &CStr) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.90.0 · Source§

### impl PartialEq<CString> for CStr

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#### fn eq(&self, other: &CString) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &CString) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.90.0 · Source§

### impl PartialEq<Cow<'_, CStr>> for CStr

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#### fn eq(&self, other: &Cow<'_, CStr>) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Cow<'_, CStr>) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.64.0 · Source§

### impl PartialEq for CStr

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#### fn eq(&self, other: &CStr) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · Source§

#### fn ne(&self, other: &Rhs) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.0.0 · Source§

### impl PartialOrd for CStr

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#### fn partial_cmp(&self, other: &CStr) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more
1.0.0 · Source§

#### fn lt(&self, other: &Rhs) -> bool

Tests less than (for `self` and `other`) and is used by the `<` operator. Read more
1.0.0 · Source§

#### fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
1.0.0 · Source§

#### fn gt(&self, other: &Rhs) -> bool

Tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
1.0.0 · Source§

#### fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
1.3.0 · Source§

### impl ToOwned for CStr

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#### type Owned = CString

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> CString

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut CString)

Uses borrowed data to replace owned data, usually by cloning. Read more
1.64.0 · Source§

### impl Eq for CStr

1.64.0 · Source§

### impl StructuralPartialEq for CStr

## Auto Trait Implementations§

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### impl Freeze for CStr

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### impl RefUnwindSafe for CStr

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### impl Send for CStr

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### impl !Sized for CStr

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### impl Sync for CStr

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### impl Unpin for CStr

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### impl UnwindSafe for CStr

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more