mod.rs.html -- source
Description: Source to the Rust file `src/libstd/ffi/mod.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Utilities related to FFI bindings.
    //!
    //! This module provides utilities to handle data across non-Rust
    //! interfaces, like other programming languages and the underlying
    //! operating system. It is mainly of use for FFI (Foreign Function
    //! Interface) bindings and code that needs to exchange C-like strings
    //! with other languages.
    //!
    //! # Overview
    //!
    //! Rust represents owned strings with the [`String`] type, and
    //! borrowed slices of strings with the [`str`] primitive. Both are
    //! always in UTF-8 encoding, and may contain nul bytes in the middle,
    //! i.e., if you look at the bytes that make up the string, there may
    //! be a `\0` among them. Both `String` and `str` store their length
    //! explicitly; there are no nul terminators at the end of strings
    //! like in C.
    //!
    //! C strings are different from Rust strings:
    //!
    //! * **Encodings** - Rust strings are UTF-8, but C strings may use
    //! other encodings. If you are using a string from C, you should
    //! check its encoding explicitly, rather than just assuming that it
    //! is UTF-8 like you can do in Rust.
    //!
    //! * **Character size** - C strings may use `char` or `wchar_t`-sized
    //! characters; please **note** that C's `char` is different from Rust's.
    //! The C standard leaves the actual sizes of those types open to
    //! interpretation, but defines different APIs for strings made up of
    //! each character type. Rust strings are always UTF-8, so different
    //! Unicode characters will be encoded in a variable number of bytes
    //! each. The Rust type [`char`] represents a '[Unicode scalar
    //! value]', which is similar to, but not the same as, a '[Unicode
    //! code point]'.
    //!
    //! * **Nul terminators and implicit string lengths** - Often, C
    //! strings are nul-terminated, i.e., they have a `\0` character at the
    //! end. The length of a string buffer is not stored, but has to be
    //! calculated; to compute the length of a string, C code must
    //! manually call a function like `strlen()` for `char`-based strings,
    //! or `wcslen()` for `wchar_t`-based ones. Those functions return
    //! the number of characters in the string excluding the nul
    //! terminator, so the buffer length is really `len+1` characters.
    //! Rust strings don't have a nul terminator; their length is always
    //! stored and does not need to be calculated. While in Rust
    //! accessing a string's length is a O(1) operation (because the
    //! length is stored); in C it is an O(length) operation because the
    //! length needs to be computed by scanning the string for the nul
    //! terminator.
    //!
    //! * **Internal nul characters** - When C strings have a nul
    //! terminator character, this usually means that they cannot have nul
    //! characters in the middle — a nul character would essentially
    //! truncate the string. Rust strings *can* have nul characters in
    //! the middle, because nul does not have to mark the end of the
    //! string in Rust.
    //!
    //! # Representations of non-Rust strings
    //!
    //! [`CString`] and [`CStr`] are useful when you need to transfer
    //! UTF-8 strings to and from languages with a C ABI, like Python.
    //!
    //! * **From Rust to C:** [`CString`] represents an owned, C-friendly
    //! string: it is nul-terminated, and has no internal nul characters.
    //! Rust code can create a [`CString`] out of a normal string (provided
    //! that the string doesn't have nul characters in the middle), and
    //! then use a variety of methods to obtain a raw `*mut `[`u8`] that can
    //! then be passed as an argument to functions which use the C
    //! conventions for strings.
    //!
    //! * **From C to Rust:** [`CStr`] represents a borrowed C string; it
    //! is what you would use to wrap a raw `*const `[`u8`] that you got from
    //! a C function. A [`CStr`] is guaranteed to be a nul-terminated array
    //! of bytes. Once you have a [`CStr`], you can convert it to a Rust
    //! [`&str`][`str`] if it's valid UTF-8, or lossily convert it by adding
    //! replacement characters.
    //!
    //! [`OsString`] and [`OsStr`] are useful when you need to transfer
    //! strings to and from the operating system itself, or when capturing
    //! the output of external commands. Conversions between [`OsString`],
    //! [`OsStr`] and Rust strings work similarly to those for [`CString`]
    //! and [`CStr`].
    //!
    //! * [`OsString`] represents an owned string in whatever
    //! representation the operating system prefers. In the Rust standard
    //! library, various APIs that transfer strings to/from the operating
    //! system use [`OsString`] instead of plain strings. For example,
    //! [`env::var_os()`] is used to query environment variables; it
    //! returns an [`Option`]`<`[`OsString`]`>`. If the environment variable
    //! exists you will get a [`Some`]`(os_string)`, which you can *then* try to
    //! convert to a Rust string. This yields a [`Result<>`], so that
    //! your code can detect errors in case the environment variable did
    //! not in fact contain valid Unicode data.
    //!
    //! * [`OsStr`] represents a borrowed reference to a string in a
    //! format that can be passed to the operating system. It can be
    //! converted into an UTF-8 Rust string slice in a similar way to
    //! [`OsString`].
    //!
    //! # Conversions
    //!
    //! ## On Unix
    //!
    //! On Unix, [`OsStr`] implements the
    //! `std::os::unix::ffi::`[`OsStrExt`][unix.OsStrExt] trait, which
    //! augments it with two methods, [`from_bytes`] and [`as_bytes`].
    //! These do inexpensive conversions from and to UTF-8 byte slices.
    //!
    //! Additionally, on Unix [`OsString`] implements the
    //! `std::os::unix::ffi::`[`OsStringExt`][unix.OsStringExt] trait,
    //! which provides [`from_vec`] and [`into_vec`] methods that consume
    //! their arguments, and take or produce vectors of [`u8`].
    //!
    //! ## On Windows
    //!
    //! On Windows, [`OsStr`] implements the
    //! `std::os::windows::ffi::`[`OsStrExt`][windows.OsStrExt] trait,
    //! which provides an [`encode_wide`] method. This provides an
    //! iterator that can be [`collect`]ed into a vector of [`u16`].
    //!
    //! Additionally, on Windows [`OsString`] implements the
    //! `std::os::windows:ffi::`[`OsStringExt`][windows.OsStringExt]
    //! trait, which provides a [`from_wide`] method. The result of this
    //! method is an [`OsString`] which can be round-tripped to a Windows
    //! string losslessly.
    //!
    //! [`String`]: ../string/struct.String.html
    //! [`str`]: ../primitive.str.html
    //! [`char`]: ../primitive.char.html
    //! [`u8`]: ../primitive.u8.html
    //! [`u16`]: ../primitive.u16.html
    //! [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
    //! [Unicode code point]: http://www.unicode.org/glossary/#code_point
    //! [`CString`]: struct.CString.html
    //! [`CStr`]: struct.CStr.html
    //! [`OsString`]: struct.OsString.html
    //! [`OsStr`]: struct.OsStr.html
    //! [`env::set_var()`]: ../env/fn.set_var.html
    //! [`env::var_os()`]: ../env/fn.var_os.html
    //! [`Result<>`]: ../result/enum.Result.html
    //! [unix.OsStringExt]: ../os/unix/ffi/trait.OsStringExt.html
    //! [`from_vec`]: ../os/unix/ffi/trait.OsStringExt.html#tymethod.from_vec
    //! [`into_vec`]: ../os/unix/ffi/trait.OsStringExt.html#tymethod.into_vec
    //! [unix.OsStrExt]: ../os/unix/ffi/trait.OsStrExt.html
    //! [`from_bytes`]: ../os/unix/ffi/trait.OsStrExt.html#tymethod.from_bytes
    //! [`as_bytes`]: ../os/unix/ffi/trait.OsStrExt.html#tymethod.as_bytes
    //! [`OsStrExt`]: ../os/unix/ffi/trait.OsStrExt.html
    //! [windows.OsStrExt]: ../os/windows/ffi/trait.OsStrExt.html
    //! [`encode_wide`]: ../os/windows/ffi/trait.OsStrExt.html#tymethod.encode_wide
    //! [`collect`]: ../iter/trait.Iterator.html#method.collect
    //! [windows.OsStringExt]: ../os/windows/ffi/trait.OsStringExt.html
    //! [`from_wide`]: ../os/windows/ffi/trait.OsStringExt.html#tymethod.from_wide
    //! [`Option`]: ../option/enum.Option.html
    //! [`Some`]: ../option/enum.Option.html#variant.Some

    #![stable(feature = "rust1", since = "1.0.0")]

    #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
    pub use self::c_str::FromBytesWithNulError;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::c_str::{CStr, CString, IntoStringError, NulError};

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::os_str::{OsStr, OsString};

    #[stable(feature = "core_c_void", since = "1.30.0")]
    pub use core::ffi::c_void;

    #[unstable(
        feature = "c_variadic",
        reason = "the `c_variadic` feature has not been properly tested on \
                  all supported platforms",
        issue = "44930"
    )]
    pub use core::ffi::{VaList, VaListImpl};

    mod c_str;
    mod os_str;