1//! Utilities related to FFI bindings.
2//!
3//! This module provides utilities to handle data across non-Rust
4//! interfaces, like other programming languages and the underlying
5//! operating system. It is mainly of use for FFI (Foreign Function
6//! Interface) bindings and code that needs to exchange C-like strings
7//! with other languages.
8//!
9//! # Overview
10//!
11//! Rust represents owned strings with the [`String`] type, and
12//! borrowed slices of strings with the [`str`] primitive. Both are
13//! always in UTF-8 encoding, and may contain nul bytes in the middle,
14//! i.e., if you look at the bytes that make up the string, there may
15//! be a `\0` among them. Both `String` and `str` store their length
16//! explicitly; there are no nul terminators at the end of strings
17//! like in C.
18//!
19//! C strings are different from Rust strings:
20//!
21//! * **Encodings** - Rust strings are UTF-8, but C strings may use
22//! other encodings. If you are using a string from C, you should
23//! check its encoding explicitly, rather than just assuming that it
24//! is UTF-8 like you can do in Rust.
25//!
26//! * **Character size** - C strings may use `char` or `wchar_t`-sized
27//! characters; please **note** that C's `char` is different from Rust's.
28//! The C standard leaves the actual sizes of those types open to
29//! interpretation, but defines different APIs for strings made up of
30//! each character type. Rust strings are always UTF-8, so different
31//! Unicode characters will be encoded in a variable number of bytes
32//! each. The Rust type [`char`] represents a '[Unicode scalar
33//! value]', which is similar to, but not the same as, a '[Unicode
34//! code point]'.
35//!
36//! * **Nul terminators and implicit string lengths** - Often, C
37//! strings are nul-terminated, i.e., they have a `\0` character at the
38//! end. The length of a string buffer is not stored, but has to be
39//! calculated; to compute the length of a string, C code must
40//! manually call a function like `strlen()` for `char`-based strings,
41//! or `wcslen()` for `wchar_t`-based ones. Those functions return
42//! the number of characters in the string excluding the nul
43//! terminator, so the buffer length is really `len+1` characters.
44//! Rust strings don't have a nul terminator; their length is always
45//! stored and does not need to be calculated. While in Rust
46//! accessing a string's length is an *O*(1) operation (because the
47//! length is stored); in C it is an *O*(*n*) operation because the
48//! length needs to be computed by scanning the string for the nul
49//! terminator.
50//!
51//! * **Internal nul characters** - When C strings have a nul
52//! terminator character, this usually means that they cannot have nul
53//! characters in the middle — a nul character would essentially
54//! truncate the string. Rust strings *can* have nul characters in
55//! the middle, because nul does not have to mark the end of the
56//! string in Rust.
57//!
58//! # Representations of non-Rust strings
59//!
60//! [`CString`] and [`CStr`] are useful when you need to transfer
61//! UTF-8 strings to and from languages with a C ABI, like Python.
62//!
63//! * **From Rust to C:** [`CString`] represents an owned, C-friendly
64//! string: it is nul-terminated, and has no internal nul characters.
65//! Rust code can create a [`CString`] out of a normal string (provided
66//! that the string doesn't have nul characters in the middle), and
67//! then use a variety of methods to obtain a raw <code>\*mut [u8]</code> that can
68//! then be passed as an argument to functions which use the C
69//! conventions for strings.
70//!
71//! * **From C to Rust:** [`CStr`] represents a borrowed C string; it
72//! is what you would use to wrap a raw <code>\*const [u8]</code> that you got from
73//! a C function. A [`CStr`] is guaranteed to be a nul-terminated array
74//! of bytes. Once you have a [`CStr`], you can convert it to a Rust
75//! <code>&[str]</code> if it's valid UTF-8, or lossily convert it by adding
76//! replacement characters.
77//!
78//! [`OsString`] and [`OsStr`] are useful when you need to transfer
79//! strings to and from the operating system itself, or when capturing
80//! the output of external commands. Conversions between [`OsString`],
81//! [`OsStr`] and Rust strings work similarly to those for [`CString`]
82//! and [`CStr`].
83//!
84//! * [`OsString`] losslessly represents an owned platform string. However, this
85//! representation is not necessarily in a form native to the platform.
86//! In the Rust standard library, various APIs that transfer strings to/from the operating
87//! system use [`OsString`] instead of plain strings. For example,
88//! [`env::var_os()`] is used to query environment variables; it
89//! returns an <code>[Option]<[OsString]></code>. If the environment variable
90//! exists you will get a <code>[Some]\(os_string)</code>, which you can
91//! *then* try to convert to a Rust string. This yields a [`Result`], so that
92//! your code can detect errors in case the environment variable did
93//! not in fact contain valid Unicode data.
94//!
95//! * [`OsStr`] losslessly represents a borrowed reference to a platform string.
96//! However, this representation is not necessarily in a form native to the platform.
97//! It can be converted into a UTF-8 Rust string slice in a similar way to
98//! [`OsString`].
99//!
100//! # Conversions
101//!
102//! ## On Unix
103//!
104//! On Unix, [`OsStr`] implements the
105//! <code>std::os::unix::ffi::[OsStrExt][unix.OsStrExt]</code> trait, which
106//! augments it with two methods, [`from_bytes`] and [`as_bytes`].
107//! These do inexpensive conversions from and to byte slices.
108//!
109//! Additionally, on Unix [`OsString`] implements the
110//! <code>std::os::unix::ffi::[OsStringExt][unix.OsStringExt]</code> trait,
111//! which provides [`from_vec`] and [`into_vec`] methods that consume
112//! their arguments, and take or produce vectors of [`u8`].
113//!
114//! ## On Windows
115//!
116//! An [`OsStr`] can be losslessly converted to a native Windows string. And
117//! a native Windows string can be losslessly converted to an [`OsString`].
118//!
119//! On Windows, [`OsStr`] implements the
120//! <code>std::os::windows::ffi::[OsStrExt][windows.OsStrExt]</code> trait,
121//! which provides an [`encode_wide`] method. This provides an
122//! iterator that can be [`collect`]ed into a vector of [`u16`]. After a nul
123//! characters is appended, this is the same as a native Windows string.
124//!
125//! Additionally, on Windows [`OsString`] implements the
126//! <code>std::os::windows:ffi::[OsStringExt][windows.OsStringExt]</code>
127//! trait, which provides a [`from_wide`] method to convert a native Windows
128//! string (without the terminating nul character) to an [`OsString`].
129//!
130//! ## Other platforms
131//!
132//! Many other platforms provide their own extension traits in a
133//! `std::os::*::ffi` module.
134//!
135//! ## On all platforms
136//!
137//! On all platforms, [`OsStr`] consists of a sequence of bytes that is encoded as a superset of
138//! UTF-8; see [`OsString`] for more details on its encoding on different platforms.
139//!
140//! For limited, inexpensive conversions from and to bytes, see [`OsStr::as_encoded_bytes`] and
141//! [`OsStr::from_encoded_bytes_unchecked`].
142//!
143//! For basic string processing, see [`OsStr::slice_encoded_bytes`].
144//!
145//! [Unicode scalar value]: https://www.unicode.org/glossary/#unicode_scalar_value
146//! [Unicode code point]: https://www.unicode.org/glossary/#code_point
147//! [`env::set_var()`]: crate::env::set_var "env::set_var"
148//! [`env::var_os()`]: crate::env::var_os "env::var_os"
149//! [unix.OsStringExt]: crate::os::unix::ffi::OsStringExt "os::unix::ffi::OsStringExt"
150//! [`from_vec`]: crate::os::unix::ffi::OsStringExt::from_vec "os::unix::ffi::OsStringExt::from_vec"
151//! [`into_vec`]: crate::os::unix::ffi::OsStringExt::into_vec "os::unix::ffi::OsStringExt::into_vec"
152//! [unix.OsStrExt]: crate::os::unix::ffi::OsStrExt "os::unix::ffi::OsStrExt"
153//! [`from_bytes`]: crate::os::unix::ffi::OsStrExt::from_bytes "os::unix::ffi::OsStrExt::from_bytes"
154//! [`as_bytes`]: crate::os::unix::ffi::OsStrExt::as_bytes "os::unix::ffi::OsStrExt::as_bytes"
155//! [`OsStrExt`]: crate::os::unix::ffi::OsStrExt "os::unix::ffi::OsStrExt"
156//! [windows.OsStrExt]: crate::os::windows::ffi::OsStrExt "os::windows::ffi::OsStrExt"
157//! [`encode_wide`]: crate::os::windows::ffi::OsStrExt::encode_wide "os::windows::ffi::OsStrExt::encode_wide"
158//! [`collect`]: crate::iter::Iterator::collect "iter::Iterator::collect"
159//! [windows.OsStringExt]: crate::os::windows::ffi::OsStringExt "os::windows::ffi::OsStringExt"
160//! [`from_wide`]: crate::os::windows::ffi::OsStringExt::from_wide "os::windows::ffi::OsStringExt::from_wide"
161
162#![stable(feature = "rust1", since = "1.0.0")]
163
164#[stable(feature = "c_str_module", since = "1.88.0")]
165pub mod c_str;
166
167#[stable(feature = "core_c_void", since = "1.30.0")]
168pub use core::ffi::c_void;
169#[unstable(
170 feature = "c_variadic",
171 reason = "the `c_variadic` feature has not been properly tested on \
172 all supported platforms",
173 issue = "44930"
174)]
175pub use core::ffi::{VaArgSafe, VaList, VaListImpl};
176#[stable(feature = "core_ffi_c", since = "1.64.0")]
177pub use core::ffi::{
178 c_char, c_double, c_float, c_int, c_long, c_longlong, c_schar, c_short, c_uchar, c_uint,
179 c_ulong, c_ulonglong, c_ushort,
180};
181#[unstable(feature = "c_size_t", issue = "88345")]
182pub use core::ffi::{c_ptrdiff_t, c_size_t, c_ssize_t};
183
184#[doc(inline)]
185#[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
186pub use self::c_str::FromBytesUntilNulError;
187#[doc(inline)]
188#[stable(feature = "cstr_from_bytes", since = "1.10.0")]
189pub use self::c_str::FromBytesWithNulError;
190#[doc(inline)]
191#[stable(feature = "cstring_from_vec_with_nul", since = "1.58.0")]
192pub use self::c_str::FromVecWithNulError;
193#[doc(inline)]
194#[stable(feature = "cstring_into", since = "1.7.0")]
195pub use self::c_str::IntoStringError;
196#[doc(inline)]
197#[stable(feature = "rust1", since = "1.0.0")]
198pub use self::c_str::NulError;
199#[doc(inline)]
200#[stable(feature = "rust1", since = "1.0.0")]
201pub use self::c_str::{CStr, CString};
202#[stable(feature = "rust1", since = "1.0.0")]
203#[doc(inline)]
204pub use self::os_str::{OsStr, OsString};
205
206#[stable(feature = "os_str_display", since = "1.87.0")]
207pub mod os_str;
208