std::ffi::CStr - Rust
Description: API documentation for the Rust `CStr` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, CStr

# 1.0.0[−][src]Struct std::ffi::CStr

    pub struct CStr { /* fields omitted */ }

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 is not `repr(C)` and is not recommended to be placed in the signatures of FFI functions. Instead, safe wrappers of FFI functions may leverage the unsafe `from_ptr` constructor to provide a safe interface to other consumers.

# Examples

Inspecting a foreign C string:

ⓘThis example is not tested

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

    extern { 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:

ⓘThis example is not tested

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

    fn work(data: &CStr) {
        extern { 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);Run

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

ⓘThis example is not tested

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

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

    fn my_string_safe() -> String {
        unsafe {
            CStr::from_ptr(my_string()).to_string_lossy().into_owned()
        }
    }

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

## Methods

### `impl CStr`[src]

#### `pub unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr`[src]

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 raw C string must be smaller than `isize::MAX` bytes in memory due to calling the `slice::from_raw_parts` function. This method is unsafe for a number of reasons:

- There is no guarantee to the validity of `ptr`.
- The returned lifetime is not guaranteed to be the actual lifetime of `ptr`.
- There is no guarantee that the memory pointed to by `ptr` contains a valid nul terminator byte at the end of the string.
- It is not guaranteed that the memory pointed by `ptr` won't change before the `CStr` has been destroyed.

> 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.

# Examples

ⓘThis example is not tested

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

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

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

#### `pub fn from_bytes_with_nul(bytes: &[u8]) -> Result<&CStr, FromBytesWithNulError>`1.10.0[src]

Creates a C string wrapper from a byte slice.

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.

# 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

#### `pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr`1.10.0[src]

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. 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

#### `pub const fn as_ptr(&self) -> *const c_char`[src]

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.

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;

    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.

####
ⓘImportant traits for &'_ [u8]

### Important traits for &'_ [u8]

`impl<'_> Read for &'_ [u8]impl<'_> Write for &'_ mut [u8]`
`pub fn to_bytes(&self) -> &[u8]`[src]

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

####
ⓘImportant traits for &'_ [u8]

### Important traits for &'_ [u8]

`impl<'_> Read for &'_ [u8]impl<'_> Write for &'_ mut [u8]`
`pub fn to_bytes_with_nul(&self) -> &[u8]`[src]

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

This function is the equivalent of `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

#### `pub fn to_str(&self) -> Result<&str, Utf8Error>`1.4.0[src]

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.

> Note: This method is currently implemented to check for validity after a constant-time cast, but it is planned to alter its definition in the future to perform the length calculation in addition to the UTF-8 check 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_str(), Ok("foo"));Run

#### `pub fn to_string_lossy(&self) -> Cow<str>`1.4.0[src]

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``(``String``)` with the result.

> Note: This method is currently implemented to check for validity after a constant-time cast, but it is planned to alter its definition in the future to perform the length calculation in addition to the UTF-8 check whenever this method is called.

# 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"));Run

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

#### `pub fn into_c_string(self: Box<CStr>) -> CString`1.20.0[src]

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"));Run

## Trait Implementations

### `impl<'_> From<&'_ CStr> for Box<CStr>`1.17.0[src]

####
ⓘImportant traits for Box<I>

### Important traits for Box<I>

`impl<I> Iterator for Box<I> where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`
`fn from(s: &CStr) -> Box<CStr>`[src]

Performs the conversion.

### `impl<'a> From<&'a CStr> for Cow<'a, CStr>`1.28.0[src]

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

Performs the conversion.

### `impl<'_> From<&'_ CStr> for Arc<CStr>`1.24.0[src]

#### `fn from(s: &CStr) -> Arc<CStr>`[src]

Performs the conversion.

### `impl<'_> From<&'_ CStr> for Rc<CStr>`1.24.0[src]

#### `fn from(s: &CStr) -> Rc<CStr>`[src]

Performs the conversion.

### `impl<'_> From<&'_ CStr> for CString` 1.7.0[src]

#### `fn from(s: &CStr) -> CString`[src]

Performs the conversion.

### `impl Debug for CStr` 1.3.0[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

### `impl PartialEq<CStr> for CStr`[src]

#### `fn eq(&self, other: &CStr) -> bool`[src]

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

#### `#[must_use] fn ne(&self, other: &Rhs) -> bool`[src]

This method tests for `!=`.

### `impl Eq for CStr`[src]

### `impl Ord for CStr`[src]

#### `fn cmp(&self, other: &CStr) -> Ordering`[src]

This method returns an `Ordering` between `self` and `other`. Read more

#### `fn max(self, other: Self) -> Self`1.21.0[src]

Compares and returns the maximum of two values. Read more

#### `fn min(self, other: Self) -> Self`1.21.0[src]

Compares and returns the minimum of two values. Read more

#### `fn clamp(self, min: Self, max: Self) -> Self`[src]

🔬 This is a nightly-only experimental API. (`clamp` #44095)

Restrict a value to a certain interval. Read more

### `impl PartialOrd<CStr> for CStr`[src]

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

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

#### `#[must_use] fn lt(&self, other: &Rhs) -> bool`[src]

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more

#### `#[must_use] fn le(&self, other: &Rhs) -> bool`[src]

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more

#### `#[must_use] fn gt(&self, other: &Rhs) -> bool`[src]

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more

#### `#[must_use] fn ge(&self, other: &Rhs) -> bool`[src]

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more

### `impl Hash for CStr`[src]

#### `fn hash<__H: Hasher>(&self, state: &mut __H)`[src]

Feeds this value into the given [`Hasher`]. Read more

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,` 1.3.0[src]

Feeds a slice of this type into the given [`Hasher`]. Read more

### `impl AsRef<CStr> for CStr` 1.7.0[src]

#### `fn as_ref(&self) -> &CStr`[src]

Performs the conversion.

### `impl AsRef<CStr> for CString` 1.7.0[src]

#### `fn as_ref(&self) -> &CStr`[src]

Performs the conversion.

### `impl<'_> Default for &'_ CStr` 1.10.0[src]

#### `fn default() -> Self`[src]

Returns the "default value" for a type. Read more

### `impl Borrow<CStr> for CString` 1.3.0[src]

#### `fn borrow(&self) -> &CStr`[src]

Immutably borrows from an owned value. Read more

### `impl ToOwned for CStr` 1.3.0[src]

#### `type Owned = CString`

The resulting type after obtaining ownership.

#### `fn to_owned(&self) -> CString`[src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&self, target: &mut Self::Owned)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

## Auto Trait Implementations

### `impl UnwindSafe for CStr`

### `impl RefUnwindSafe for CStr`

### `impl Unpin for CStr`

### `impl Send for CStr`

### `impl Sync for CStr`

## Blanket Implementations

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `fn to_owned(&self) -> T`[src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&self, target: &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more