Module std::ffi

Expand description

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 an O(1) operation (because the length is stored); in C it is an O(n) 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 losslessly represents an owned platform string. However, this representation is not necessarily in a form native to the platform. 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 losslessly represents a borrowed reference to a platform string. However, this representation is not necessarily in a form native to the platform. It can be converted into a UTF-8 Rust string slice in a similar way to OsString.

Conversions

On Unix

On Unix, OsStr implements the std::os::unix::ffi::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 trait, which provides from_vec and into_vec methods that consume their arguments, and take or produce vectors of u8.

On Windows

An OsStr can be losslessly converted to a native Windows string. And a native Windows string can be losslessly converted to an OsString.

On Windows, OsStr implements the std::os::windows::ffi::OsStrExt trait, which provides an encode_wide method. This provides an iterator that can be collected into a vector of u16. After a nul characters is appended, this is the same as a native Windows string.

Additionally, on Windows OsString implements the std::os::windows:ffi::OsStringExt trait, which provides a from_wide method to convert a native Windows string (without the terminating nul character) to an OsString.