Struct std::ffi::CString1.0.0[][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

This example is not tested
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]

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

This example is not tested
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.

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

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:

This example is not tested
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

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

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>

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>

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]

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]

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

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>

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>

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]

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 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]

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

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 c_str = CStr::from_bytes_with_nul(b"foo\0").unwrap();
assert_eq!(c_str.to_str(), Ok("foo"));Run

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 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 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]

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

This method tests for !=.

impl PartialOrd for CString
[src]

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

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

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

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

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]

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

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

impl Hash for CString
[src]

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

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

impl Clone for CString
[src]

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl Drop for CString
1.13.0
[src]

Executes the destructor for this type. Read more

impl Deref for CString
[src]

The resulting type after dereferencing.

Dereferences the value.

impl Debug for CString
[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>

Performs the conversion.

impl Default for CString
1.10.0
[src]

Creates an empty CString.

impl Borrow<CStr> for CString
1.3.0
[src]

Immutably borrows from an owned value. Read more

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

Performs the conversion.

impl From<CString> for Box<CStr>
1.20.0
[src]

Important traits for Box<I>

Performs the conversion.

impl From<CString> for Arc<CStr>
1.24.0
[src]

Performs the conversion.

impl From<CString> for Rc<CStr>
1.24.0
[src]

Performs the conversion.

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

Performs the conversion.

impl Index<RangeFull> for CString
1.7.0
[src]

The returned type after indexing.

Performs the indexing (container[index]) operation.

impl AsRef<CStr> for CString
1.7.0
[src]

Performs the conversion.

Auto Trait Implementations

impl Send for CString

impl Sync for CString