impl CStr

unsafe const fn from_ptr<'a>(ptr: *const c_char) -> &'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 nul terminator. This means in particular:

  • The entire memory range of this CStr must be contained within a single allocation!
  • 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};

fn my_string() -> *const c_char {
    c"hello".as_ptr()
}

unsafe {
    let slice = CStr::from_ptr(my_string());
    assert_eq!(slice.to_str().unwrap(), "hello");
}

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) };

assert_eq!(c"Hello, world!", HELLO);

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");

const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, 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_eq!(cstr, Ok(c"hello"));

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

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

let cstr = CStr::from_bytes_with_nul(b"hello");
assert_eq!(cstr, Err(FromBytesWithNulError::NotNulTerminated));

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

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

let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
assert_eq!(cstr, Err(FromBytesWithNulError::InteriorNul { position: 2 }));

unsafe const 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;

let bytes = b"Hello world!\0";

let cstr = unsafe { CStr::from_bytes_with_nul_unchecked(bytes) };
assert_eq!(cstr.to_bytes_with_nul(), bytes);

const fn as_ptr(self: &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.

The type of the returned pointer is [*const c_char][crate::ffi::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:

# #![expect(dangling_pointers_from_temporaries)]
use std::ffi::{CStr, CString};

// 💀 The meaning of this entire program is undefined,
// 💀 and nothing about its behavior is guaranteed,
// 💀 not even that its behavior resembles the code as written,
// 💀 just because it contains a single instance of undefined behavior!

// 🚨 creates a dangling pointer to a temporary `CString`
// 🚨 that is deallocated at the end of the statement
let ptr = CString::new("Hi!".to_uppercase()).unwrap().as_ptr();

// without undefined behavior, you would expect that `ptr` equals:
dbg!(CStr::from_bytes_with_nul(b"HI!\0").unwrap());

// 🙏 Possibly the program behaved as expected so far,
// 🙏 and this just shows `ptr` is now garbage..., but
// 💀 this violates `CStr::from_ptr`'s safety contract
// 💀 leading to a dereference of a dangling pointer,
// 💀 which is immediate undefined behavior.
// 💀 *BOOM*, you're dead, your entire program has no meaning.
dbg!(unsafe { CStr::from_ptr(ptr) });

This happens because, the pointer returned by as_ptr does not carry any lifetime information, and the CString is deallocated immediately after the expression that it is part of has been evaluated. To fix the problem, bind the CString to a local variable:

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

let c_str = CString::new("Hi!".to_uppercase()).unwrap();
let ptr = c_str.as_ptr();

assert_eq!(unsafe { CStr::from_ptr(ptr) }, c"HI!");

const fn count_bytes(self: &Self) -> usize

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

assert_eq!(c"foo".count_bytes(), 3);
assert_eq!(c"".count_bytes(), 0);

const fn is_empty(self: &Self) -> bool

Returns true if self.to_bytes() has a length of 0.

Examples

assert!(!c"foo".is_empty());
assert!(c"".is_empty());

const fn to_bytes(self: &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

assert_eq!(c"foo".to_bytes(), b"foo");

const fn to_bytes_with_nul(self: &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

assert_eq!(c"foo".to_bytes_with_nul(), b"foo\0");

fn bytes(self: &Self) -> Bytes<'_>

Iterates over the bytes in this C string.

The returned iterator will not contain the trailing nul terminator that this C string has.

Examples

#![feature(cstr_bytes)]

assert!(c"foo".bytes().eq(*b"foo"));

const fn to_str(self: &Self) -> Result<&str, 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

assert_eq!(c"foo".to_str(), Ok("foo"));

fn display(self: &Self) -> impl fmt::Display

Returns an object that implements Display for safely printing a CStr that may contain non-Unicode data.

Behaves as if self were first lossily converted to a str, with invalid UTF-8 presented as the Unicode replacement character: �.

Examples

#![feature(cstr_display)]

let cstr = c"Hello, world!";
println!("{}", cstr.display());

const fn as_c_str(self: &Self) -> &CStr

Returns the same string as a string slice &CStr.

This method is redundant when used directly on &CStr, but it helps dereferencing other string-like types to string slices, for example references to Box<CStr> or Arc<CStr>.

impl AsRef for CStr

fn as_ref(self: &Self) -> &CStr

impl CloneToUninit for crate::ffi::CStr

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl Debug for CStr

fn fmt(self: &Self, f: &mut fmt::Formatter<'_>) -> fmt::Result

impl Eq for CStr

impl Freeze for CStr

impl Hash for CStr

fn hash<__H: $crate::hash::Hasher>(self: &Self, state: &mut __H)

impl Index for CStr

fn index(self: &Self, index: ops::RangeFrom<usize>) -> &CStr

impl Ord for CStr

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

impl PartialEq for CStr

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

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

impl PartialEq for CStr

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

impl PartialOrd for CStr

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

impl RefUnwindSafe for CStr

impl Send for CStr

impl Sized for CStr

impl StructuralPartialEq for CStr

impl Sync for CStr

impl Unpin for CStr

impl UnwindSafe for CStr

impl<T> Any for CStr

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for CStr

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for CStr

fn borrow_mut(self: &mut Self) -> &mut T