Struct std::ffi::CStr

1.64.0 · source · 
pub struct CStr { /* private fields */ }
Expand description

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 CStr::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 "C" { 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:

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

fn work(data: &CStr) {
    extern "C" { 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:

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

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

fn my_string_safe() -> String {
    let cstr = unsafe { CStr::from_ptr(my_string()) };
    // Get copy-on-write Cow<'_, str>, then guarantee a freshly-owned String allocation
    String::from_utf8_lossy(cstr.to_bytes()).to_string()
}

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

Implementations§

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.

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 null terminator. This means in particular:

    • The entire memory range of this CStr must be contained within a single allocated object!
    • 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.

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};

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

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

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) };
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🔬This is a nightly-only experimental API. (cstr_from_bytes_until_nul #95027)

Creates a C string wrapper from a byte slice.

This method will create a CStr from any byte slice that contains at least one nul byte. The caller does not need to know or specify 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
#![feature(cstr_from_bytes_until_nul)]

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");
Run

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.

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!(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

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, 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

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.

🔬This is a nightly-only experimental API. (cstr_is_empty #102444)

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

Examples
#![feature(cstr_is_empty)]

use std::ffi::CStr;

let cstr = CStr::from_bytes_with_nul(b"foo\0")?;
assert!(!cstr.is_empty());

let empty_cstr = CStr::from_bytes_with_nul(b"\0")?;
assert!(empty_cstr.is_empty());
Run

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");
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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
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

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
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

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(&str) with the result.

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

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§

Converts this type into a shared reference of the (usually inferred) input type.
Converts this type into a shared reference of the (usually inferred) input type.
Immutably borrows from an owned value. Read more
Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more
Returns the “default value” for a type. Read more
Returns the “default value” for a type. Read more

Converts a CStr into a borrowed Cow without copying or allocating.

Converts a &CStr into a Arc<CStr>, by copying the contents into a newly allocated Arc.

Converts a &CStr into a Box<CStr>, by copying the contents into a newly allocated Box.

Converts to this type from the input type.

Converts a &CStr into a Rc<CStr>, by copying the contents into a newly allocated Rc.

Converts a CString into a Box<CStr> without copying or allocating.

Converts a Cow<'a, CStr> into a Box<CStr>, by copying the contents if they are borrowed.

Feeds this value into the given Hasher. Read more
The returned type after indexing.
Performs the indexing (container[index]) operation. Read more
This method returns an Ordering between self and other. Read more
This method tests for self and other values to be equal, and is used by ==.
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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
The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more

Auto Trait Implementations§

Blanket Implementations§

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more