[rust.git] / src / libstd / ffi / mod.rs

// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! 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` 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

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

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

#[stable(feature = "raw_os", since = "1.1.0")]
pub use core::ffi::c_void;

#[cfg(not(stage0))]
#[unstable(feature = "c_variadic",
           reason = "the `c_variadic` feature has not been properly tested on \
                     all supported platforms",
           issue = "27745")]
pub use core::ffi::VaList;

mod c_str;
mod os_str;