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

# Struct std::ffi::CString 1.0.0 [−] [src]

    pub struct CString { /* fields omitted */ }

A type representing an owned, C-compatible, nul-terminated string with no nul bytes in the middle.

This type serves the purpose of being able to safely generate a C-compatible string from a Rust byte slice or vector. An instance of this type is a static guarantee that the underlying bytes contain no interior 0 bytes ("nul characters") and that the final byte is 0 ("nul terminator").

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

# Creating a `CString`

A `CString` is created from either a byte slice or a byte vector, or anything that implements `Into``<``Vec``<``u8``>>` (for example, you can build a `CString` straight out of a `String` or a `&str`, since both implement that trait).

The `new` method will actually check that the provided `&[u8]` does not have 0 bytes in the middle, and return an error if it finds one.

# Extracting a raw pointer to the whole C string

`CString` implements a `as_ptr` method through the `Deref` trait. This method will give you a `*const c_char` which you can feed directly to extern functions that expect a nul-terminated string, like C's `strdup()`.

# Extracting a slice of the whole C string

Alternatively, you can obtain a `&[``u8``]` slice from a `CString` with the `as_bytes` method. Slices produced in this way do not contain the trailing nul terminator. This is useful when you will be calling an extern function that takes a `*const u8` argument which is not necessarily nul-terminated, plus another argument with the length of the string — like C's `strndup()`. You can of course get the slice's length with its `len` method.

If you need a `&[``u8``]` slice with the nul terminator, you can use `as_bytes_with_nul` instead.

Once you have the kind of slice you need (with or without a nul terminator), you can call the slice's own `as_ptr` method to get a raw pointer to pass to extern functions. See the documentation for that function for a discussion on ensuring the lifetime of the raw pointer.

# Examples

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

    extern {
        fn my_printer(s: *const c_char);
    }

    // We are certain that our string doesn't have 0 bytes in the middle,
    // so we can .unwrap()
    let c_to_print = CString::new("Hello, world!").unwrap();
    unsafe {
        my_printer(c_to_print.as_ptr());
    }Run

# Safety

`CString` is intended for working with traditional C-style strings (a sequence of non-nul bytes terminated by a single nul byte); the primary use case for these kinds of strings is interoperating with C-like code. Often you will need to transfer ownership to/from that external code. It is strongly recommended that you thoroughly read through the documentation of `CString` before use, as improper ownership management of `CString` instances can lead to invalid memory accesses, memory leaks, and other memory errors.

## Methods

### `impl CString`
[src]

#### `pub fn new<T: Into<Vec<u8>>>(t: T) -> Result<CString, NulError>`
[src]

Creates a new C-compatible string from a container of bytes.

This function will consume the provided data and use the underlying bytes to construct a new string, ensuring that there is a trailing 0 byte. This trailing 0 byte will be appended by this function; the provided data should not contain any 0 bytes in it.

# Examples

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

    extern { fn puts(s: *const c_char); }

    let to_print = CString::new("Hello!").unwrap();
    unsafe {
        puts(to_print.as_ptr());
    }Run

# Errors

This function will return an error if the supplied bytes contain an internal 0 byte. The `NulError` returned will contain the bytes as well as the position of the nul byte.

#### `pub unsafe fn from_vec_unchecked(v: Vec<u8>) -> CString`
[src]

Creates a C-compatible string by consuming a byte vector, without checking for interior 0 bytes.

This method is equivalent to `new` except that no runtime assertion is made that `v` contains no 0 bytes, and it requires an actual byte vector, not anything that can be converted to one with Into.

# Examples

    use std::ffi::CString;

    let raw = b"foo".to_vec();
    unsafe {
        let c_string = CString::from_vec_unchecked(raw);
    }Run

#### `pub unsafe fn from_raw(ptr: *mut c_char) -> CString`
1.4.0
[src]

Retakes ownership of a `CString` that was transferred to C via `into_raw`.

Additionally, the length of the string will be recalculated from the pointer.

# Safety

This should only ever be called with a pointer that was earlier obtained by calling `into_raw` on a `CString`. Other usage (e.g. trying to take ownership of a string that was allocated by foreign code) is likely to lead to undefined behavior or allocator corruption.

> Note: If you need to borrow a string that was allocated by foreign code, use `CStr`. If you need to take ownership of a string that was allocated by foreign code, you will need to make your own provisions for freeing it appropriately, likely with the foreign code's API to do that.

# Examples

Create a `CString`, pass ownership to an `extern` function (via raw pointer), then retake ownership with `from_raw`:

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

    extern {
        fn some_extern_function(s: *mut c_char);
    }

    let c_string = CString::new("Hello!").unwrap();
    let raw = c_string.into_raw();
    unsafe {
        some_extern_function(raw);
        let c_string = CString::from_raw(raw);
    }Run

#### `pub fn into_raw(self) -> *mut c_char`
1.4.0
[src]

Consumes the `CString` and transfers ownership of the string to a C caller.

The pointer which this function returns must be returned to Rust and reconstituted using `from_raw` to be properly deallocated. Specifically, one should not use the standard C `free()` function to deallocate this string.

Failure to call `from_raw` will lead to a memory leak.

# Examples

    use std::ffi::CString;

    let c_string = CString::new("foo").unwrap();

    let ptr = c_string.into_raw();

    unsafe {
        assert_eq!(b'f', *ptr as u8);
        assert_eq!(b'o', *ptr.offset(1) as u8);
        assert_eq!(b'o', *ptr.offset(2) as u8);
        assert_eq!(b'\0', *ptr.offset(3) as u8);

        // retake pointer to free memory
        let _ = CString::from_raw(ptr);
    }Run

#### `pub fn into_string(self) -> Result<String, IntoStringError>`
1.7.0
[src]

Converts the `CString` into a `String` if it contains valid UTF-8 data.

On failure, ownership of the original `CString` is returned.

# Examples

    use std::ffi::CString;

    let valid_utf8 = vec![b'f', b'o', b'o'];
    let cstring = CString::new(valid_utf8).unwrap();
    assert_eq!(cstring.into_string().unwrap(), "foo");

    let invalid_utf8 = vec![b'f', 0xff, b'o', b'o'];
    let cstring = CString::new(invalid_utf8).unwrap();
    let err = cstring.into_string().err().unwrap();
    assert_eq!(err.utf8_error().valid_up_to(), 1);Run

####
ⓘImportant traits for Vec<u8>

### Important traits for Vec<u8>

`impl Write for Vec<u8>`
`pub fn into_bytes(self) -> Vec<u8>`
1.7.0
[src]

Consumes the `CString` and returns the underlying byte buffer.

The returned buffer does not contain the trailing nul terminator, and it is guaranteed to not have any interior nul bytes.

# Examples

    use std::ffi::CString;

    let c_string = CString::new("foo").unwrap();
    let bytes = c_string.into_bytes();
    assert_eq!(bytes, vec![b'f', b'o', b'o']);Run

####
ⓘImportant traits for Vec<u8>

### Important traits for Vec<u8>

`impl Write for Vec<u8>`
`pub fn into_bytes_with_nul(self) -> Vec<u8>`
1.7.0
[src]

Equivalent to the `into_bytes` function except that the returned vector includes the trailing nul terminator.

# Examples

    use std::ffi::CString;

    let c_string = CString::new("foo").unwrap();
    let bytes = c_string.into_bytes_with_nul();
    assert_eq!(bytes, vec![b'f', b'o', b'o', b'\0']);Run

####
ⓘImportant traits for &'a [u8]

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

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

Returns the contents of this `CString` as a slice of bytes.

The returned slice does not contain the trailing nul terminator, and it is guaranteed to not have any interior nul bytes. If you need the nul terminator, use `as_bytes_with_nul` instead.

# Examples

    use std::ffi::CString;

    let c_string = CString::new("foo").unwrap();
    let bytes = c_string.as_bytes();
    assert_eq!(bytes, &[b'f', b'o', b'o']);Run

####
ⓘImportant traits for &'a [u8]

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

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

Equivalent to the `as_bytes` function except that the returned slice includes the trailing nul terminator.

# Examples

    use std::ffi::CString;

    let c_string = CString::new("foo").unwrap();
    let bytes = c_string.as_bytes_with_nul();
    assert_eq!(bytes, &[b'f', b'o', b'o', b'\0']);Run

#### `pub fn as_c_str(&self) -> &CStr`
1.20.0
[src]

Extracts a `CStr` slice containing the entire string.

# Examples

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

    let c_string = CString::new(b"foo".to_vec()).unwrap();
    let c_str = c_string.as_c_str();
    assert_eq!(c_str, CStr::from_bytes_with_nul(b"foo\0").unwrap());Run

####
ⓘ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<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`
`pub fn into_boxed_c_str(self) -> Box<CStr>`
1.20.0
[src]

Converts this `CString` into a boxed `CStr`.

# Examples

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

    let c_string = CString::new(b"foo".to_vec()).unwrap();
    let boxed = c_string.into_boxed_c_str();
    assert_eq!(&*boxed, CStr::from_bytes_with_nul(b"foo\0").unwrap());Run

## Methods from Deref<Target = CStr>

#### `pub 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

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").unwrap().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").unwrap().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").unwrap();
    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 &'a [u8]

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

`impl<'a> Read for &'a [u8]impl<'a> Write for &'a 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 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 c_str = CStr::from_bytes_with_nul(b"foo\0").unwrap();
    assert_eq!(c_str.to_bytes(), b"foo");Run

####
ⓘImportant traits for &'a [u8]

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

`impl<'a> Read for &'a [u8]impl<'a> Write for &'a 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 c_str = CStr::from_bytes_with_nul(b"foo\0").unwrap();
    assert_eq!(c_str.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 0-cost 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 c_str = CStr::from_bytes_with_nul(b"foo\0").unwrap();
    assert_eq!(c_str.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 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 0-cost 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 c_str = CStr::from_bytes_with_nul(b"Hello World\0").unwrap();
    assert_eq!(c_str.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 c_str = CStr::from_bytes_with_nul(b"Hello \xF0\x90\x80World\0").unwrap();
    assert_eq!(
        c_str.to_string_lossy(),
        Cow::Owned(String::from("Hello �World")) as Cow<str>
    );Run

## Trait Implementations

### `impl PartialEq for CString`
[src]

#### `fn eq(&self, __arg_0: &CString) -> bool`
[src]

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

#### `fn ne(&self, __arg_0: &CString) -> bool`
[src]

This method tests for `!=`.

### `impl PartialOrd for CString`
[src]

#### `fn partial_cmp(&self, __arg_0: &CString) -> Option<Ordering>`
[src]

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

#### `fn lt(&self, __arg_0: &CString) -> bool`
[src]

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

#### `fn le(&self, __arg_0: &CString) -> bool`
[src]

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

#### `fn gt(&self, __arg_0: &CString) -> bool`
[src]

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

#### `fn ge(&self, __arg_0: &CString) -> bool`
[src]

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

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

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

#### `fn cmp(&self, __arg_0: &CString) -> 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

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

#### `fn hash<__H: Hasher>(&self, __arg_0: &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 Clone for CString`
[src]

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

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`
[src]

Performs copy-assignment from `source`. Read more

### `impl Drop for CString`
1.13.0
[src]

#### `fn drop(&mut self)`
[src]

Executes the destructor for this type. Read more

### `impl Deref for CString`
[src]

#### `type Target = CStr`

The resulting type after dereferencing.

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

Dereferences the value.

### `impl Debug for CString`
[src]

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

Formats the value using the given formatter. Read more

### `impl From<CString> for Vec<u8>`
1.7.0
[src]

####
ⓘImportant traits for Vec<u8>

### Important traits for Vec<u8>

`impl Write for Vec<u8>`
`fn from(s: CString) -> Vec<u8>`
[src]

Performs the conversion.

### `impl Default for CString`
1.10.0
[src]

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

Creates an empty `CString`.

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

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

Immutably borrows from an owned value. Read more

### `impl From<Box<CStr>> for CString`
1.18.0
[src]

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

Performs the conversion.

### `impl From<CString> for Box<CStr>`
1.20.0
[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<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`
`fn from(s: CString) -> Box<CStr>`
[src]

Performs the conversion.

### `impl From<CString> for Arc<CStr>`
1.23.0
[src]

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

Performs the conversion.

### `impl From<CString> for Rc<CStr>`
1.23.0
[src]

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

Performs the conversion.

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

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

Performs the conversion.

### `impl Index<RangeFull> for CString`
1.7.0
[src]

#### `type Output = CStr`

The returned type after indexing.

#### `fn index(&self, _index: RangeFull) -> &CStr`
[src]

Performs the indexing (`container[index]`) operation.

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

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

Performs the conversion.

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