# Struct std::ffi::CStr

1.64.0 · source ·

    pub struct CStr { /* private fields */ }

Expand description

Representation of a borrowed C string.

This type represents a borrowed reference to a nul-terminated array of bytes. It can be constructed safely from a `&[u8]` slice, or unsafely from a raw `*const c_char`. It 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 borrowed references; the latter are owned strings.

Note that this structure does not have a guaranteed layout (the `repr(transparent)` notwithstanding) and is not recommended to be placed in the signatures of FFI functions. Instead, safe wrappers of FFI functions may leverage 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());
    }

Run

Passing a Rust-originating C string:

ⓘ

    use std::ffi::{CString, CStr};
    use std::os::raw::c_char;

    fn work(data: &CStr) {
        extern "C" { fn work_with(data: *const c_char); }

        unsafe { work_with(data.as_ptr()) }
    }

    let s = CString::new("data data data data").expect("CString::new failed");
    work(&s);

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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 copy-on-write Cow<'_, str>, then guarantee a freshly-owned String allocation
        String::from_utf8_lossy(cstr.to_bytes()).to_string()
    }

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

Run

## Implementations§

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### impl CStr

1.0.0 (const: unstable) · source

#### pub 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 null terminator. This means in particular:

  - The entire memory range of this `CStr` must be contained within a single allocated object!
  - `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};

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

    unsafe {
        let slice = CStr::from_ptr(my_string());
        println!("string returned: {}", slice.to_str().unwrap());
    }

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    #![feature(const_cstr_from_ptr)]

    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) };

Run
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");

Run
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!(cstr.is_ok());

Run

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

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"hello");
    assert!(cstr.is_err());

Run

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

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
    assert!(cstr.is_err());

Run
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, CString};

    unsafe {
        let cstring = CString::new("hello").expect("CString::new failed");
        let cstr = CStr::from_bytes_with_nul_unchecked(cstring.to_bytes_with_nul());
        assert_eq!(cstr, &*cstring);
    }

Run
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::CString;

    // Do not do this:
    let ptr = CString::new("Hello").expect("CString::new failed").as_ptr();
    unsafe {
        // `ptr` is dangling
        *ptr;
    }

Run

This happens because the pointer returned by `as_ptr` does not carry any lifetime information and the `CString` is deallocated immediately after the `CString::new("Hello").expect("CString::new failed").as_ptr()` expression is evaluated. To fix the problem, bind the `CString` to a local variable:

    use std::ffi::CString;

    let hello = CString::new("Hello").expect("CString::new failed");
    let ptr = hello.as_ptr();
    unsafe {
        // `ptr` is valid because `hello` is in scope
        *ptr;
    }

Run

This way, the lifetime of the `CString` in `hello` encompasses the lifetime of `ptr` and the `unsafe` block.

const: unstable · source

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

🔬This is a nightly-only experimental API. (`cstr_count_bytes` #114441)

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

    #![feature(cstr_count_bytes)]

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"foo\0").unwrap();
    assert_eq!(cstr.count_bytes(), 3);

    let cstr = CStr::from_bytes_with_nul(b"\0").unwrap();
    assert_eq!(cstr.count_bytes(), 0);

Run
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

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"foo\0")?;
    assert!(!cstr.is_empty());

    let empty_cstr = CStr::from_bytes_with_nul(b"\0")?;
    assert!(empty_cstr.is_empty());

Run
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

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
    assert_eq!(cstr.to_bytes(), b"foo");

Run
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

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
    assert_eq!(cstr.to_bytes_with_nul(), b"foo\0");

Run
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

    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
    assert_eq!(cstr.to_str(), Ok("foo"));

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source§

### impl CStr

1.4.0 · source

#### pub fn to_string_lossy(&self) -> Cow<'_, str>

Converts a `CStr` into a `Cow<str>`.

If the contents of the `CStr` are valid UTF-8 data, this function will return a `Cow::Borrowed(&str)` with the corresponding `&str` slice. Otherwise, it will replace any invalid UTF-8 sequences with `U+FFFD REPLACEMENT CHARACTER` and return a `Cow::Owned(&str)` with the result.

##### Examples

Calling `to_string_lossy` on a `CStr` containing valid UTF-8:

    use std::borrow::Cow;
    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"Hello World\0")
                     .expect("CStr::from_bytes_with_nul failed");
    assert_eq!(cstr.to_string_lossy(), Cow::Borrowed("Hello World"));

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Calling `to_string_lossy` on a `CStr` containing invalid UTF-8:

    use std::borrow::Cow;
    use std::ffi::CStr;

    let cstr = CStr::from_bytes_with_nul(b"Hello \xF0\x90\x80World\0")
                     .expect("CStr::from_bytes_with_nul failed");
    assert_eq!(
        cstr.to_string_lossy(),
        Cow::Owned(String::from("Hello �World")) as Cow<'_, str>
    );

Run
1.20.0 · source

#### pub fn into_c_string(self: Box<CStr>) -> CString

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

##### Examples

    use std::ffi::CString;

    let c_string = CString::new(b"foo".to_vec()).expect("CString::new failed");
    let boxed = c_string.into_boxed_c_str();
    assert_eq!(boxed.into_c_string(), CString::new("foo").expect("CString::new failed"));

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## Trait Implementations§

1.7.0 · 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 copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from `source`. Read more
1.3.0 · source§

### impl Debug for CStr

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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>

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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 to this type from the input type.
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`.

1.20.0 · 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.

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### 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
1.0.0 · source§

### impl PartialEq for CStr

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

This method tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · source§

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

This method 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

This method 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

This method 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

This method 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

This method 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
1.0.0 · source§

### impl Eq for CStr

## Auto Trait Implementations§

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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 Twhere 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 Twhere 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 Twhere T: ?Sized,

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

Mutably borrows from an owned value. Read more