Struct std::ffi::CStr

pub struct CStr { /* fields omitted */ }

Representation of a borrowed C string.

This dynamically sized type is only safely constructed via a borrowed version of an instance of CString. This type can be constructed from a raw C string as well and represents a C string borrowed from another location.

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

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 length: {}", slice.to_bytes().len());
}

Passing a Rust-originating C string

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").unwrap();
work(&s);

Converting a foreign C string into a Rust String

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());

Methods

impl CStr

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

Casts a raw C string to a safe C string wrapper.

This function will cast the provided ptr to the CStr wrapper which allows inspection and interoperation of non-owned C strings. 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.

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

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());
}

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

Creates a C string wrapper from a byte slice.

This function will cast the provided bytes to a CStr wrapper after ensuring that it is null 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());

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

Examples

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

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

fn as_ptr(&self) -> *const c_char

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 behaviour 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;
}

This happens because the pointer returned by as_ptr does not carry any lifetime information and the string is deallocated immediately after the CString::new("Hello").unwrap().as_ptr() expression is evaluated. To fix the problem, bind the string 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;
}

fn to_bytes(&self) -> &[u8]

Converts this C string to a byte slice.

This function will calculate the length of this string (which normally requires a linear amount of work to be done) and then return the resulting slice of u8 elements.

The returned slice will not contain the trailing nul 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.

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 to_bytes except that it will retain the trailing nul 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.

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

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

This function will calculate the length of this string and check for UTF-8 validity, and then return the &str if it's valid.

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.

fn to_string_lossy(&self) -> Cow<str>

Converts a CStr into a Cow<str>.

This function will calculate the length of this string (which normally requires a linear amount of work to be done) and then return the resulting slice as a Cow<str>, replacing any invalid UTF-8 sequences with U+FFFD REPLACEMENT CHARACTER.

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.

Trait Implementations

impl Hash for CStr

fn hash<__H: Hasher>(&self, __arg_0: &mut __H)

Feeds this value into the state given, updating the hasher as necessary.

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

Feeds a slice of this type into the state provided.

impl Debug for CStr

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl<'a> Default for &'a CStr

fn default() -> &'a CStr

Returns the "default value" for a type.

impl PartialEq for CStr

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

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

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

This method tests for !=.

impl Eq for CStr

impl PartialOrd for CStr

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

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

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

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

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

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

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

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

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

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

impl Ord for CStr

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

This method returns an Ordering between self and other.

impl ToOwned for CStr

type Owned = CString

fn to_owned(&self) -> CString

Creates owned data from borrowed data, usually by cloning.

impl AsRef<CStr> for CStr

fn as_ref(&self) -> &CStr

Performs the conversion.