Crate widestring[−][src]

Expand description

A wide string FFI module for converting to and from wide string variants.

This module provides multiple types of wide strings: U16String, U16CString, U32String, and U32CString. These types are backed by two generic implementations parameterized by element size: UString<C> and UCString<C>. The UCString types are analogous to the standard CString FFI type, while the UString types are analogous to OsString. Otherwise, U16 and U32 types differ only in character width.

For U16String and U32String, no guarantees are made about the underlying string data; they are simply a sequence of UTF-16 code units or UTF-32 code points, both of which may be ill-formed or contain nul values. U16CString and U32CString, on the other hand, are aware of nul values and are guaranteed to be terminated with a nul value (unless unchecked methods are used to construct the strings). Because U16CString and U32CString are C-style, nul-terminated strings, they will have no interior nul values. All four string types may still have unpaired UTF-16 surrogates or invalid UTF-32 code points; ill-formed data is preserved until conversion to a basic Rust String.

Use U16String or U32String when you simply need to pass-through strings, or when you know or don’t care if you’re not dealing with a nul-terminated string, such as when string lengths are provided and you are only reading strings from FFI, not writing them out to a FFI.

Use U16CString or U32CString when you must properly handle nul values, and must deal with nul-terminated C-style wide strings, such as when you pass strings into FFI functions.

Relationship to other Rust Strings

Standard Rust strings String and str are well-formed Unicode data encoded as UTF-8. The standard strings provide proper handling of Unicode and ensure strong safety guarantees.

CString and CStr are strings used for C FFI. They handle nul-terminated C-style strings. However, they do not have a builtin encoding, and conversions between C-style and other Rust strings must specifically encode and decode the strings, and handle possibly invalid encoding data. They are safe to use only in passing string-like data back and forth from C APIs but do not provide any other guarantees, so may not be well-formed.

OsString and OsStr are also strings for use with FFI. Unlike CString, they do no special handling of nul values, but instead have an OS-specified encoding. While, for example, on Linux systems this is usually the UTF-8 encoding, this is not the case for every platform. The encoding may not even be 8-bit: on Windows, OsString uses a malformed encoding sometimes referred to as “WTF-8”. In any case, like CString, OsString has no additional guarantees and may not be well-formed.

Due to the loss of safety of these other string types, conversion to standard Rust String is lossy, and may require knowledge of the underlying encoding, including platform-specific quirks.

The wide strings in this crate are roughly based on the principles of the string types in std::ffi, though there are differences. U16String, U32String, U16Str, and U32Str are roughly similar in role to OsString and OsStr, while U16CString, U32CString, U16CStr, and U32CStr are roughly similar in role to CString and CStr. Conversion to FFI string types is generally very straight forward and safe, while conversion directly between standard Rust String is a lossy conversion just as OsString is.

U16String and U16CString are treated as though they use UTF-16 encoding, even if they may contain unpaired surrogates. U32String and U32CString are treated as though they use UTF-32 encoding, even if they may contain values outside the valid Unicode character range.

Remarks on UTF-16 Code Units

Code units are the 16-bit units that comprise UTF-16 sequences. Code units can specify Unicode code points either as single units or in surrogate pairs. Because every code unit might be part of a surrogate pair, many regular string operations, including indexing into a wide string, writing to a wide string, or even iterating a wide string should be handled with care and are greatly discouraged. Some operations have safer alternatives provided, such as Unicode code point iteration instead of code unit iteration. Always keep in mind that the number of code units (len()) of a wide string is not equivalent to the number of Unicode characters in the string, merely the length of the UTF-16 encoding sequence. In fact, Unicode code points do not even have a one-to-one mapping with characters!

UTF-32 simply encodes Unicode code points as-is in 32-bit values, but Unicode character code points are reserved only for 21-bits. Again, Unicode code points do not have a one-to-one mapping with the concept of a visual character glyph.

FFI with C/C++ `wchar_t`

C/C++’s wchar_t (and C++’s corresponding widestring) varies in size depending on compiler and platform. Typically, wchar_t is 16-bits on Windows and 32-bits on most Unix-based platforms. For convenience when using wchar_t-based FFI’s, type aliases for the corresponding string types are provided: WideString aliases U16String on Windows or U32String elsewhere, WideCString aliases U16CString or U32CString, etc. The WideChar alias is also provided, aliasing u16 or u32.

When not interacting with a FFI using wchar_t, it is recommended to use the string types directly rather than via the wide alias.

This crate supports no_std when default features are disabled. The std and alloc features (enabled by default) enable the U16String, U32String, U16CString, and U32CString types and aliases. Other types do not require allocation and can be used in a no_std environment.

Nul values

This crate uses the term legacy ASCII term “nul” to refer to Unicode code point U+0000 NULL and its associated code unit representation as zero-value bytes. This is to disambiguate this zero value from null pointer values. C-style strings end in a nul value, while regular Rust strings allow interior nul values and are not terminated with nul.

Examples

The following example uses U16String to get Windows error messages, since FormatMessageW returns a string length for us and we don’t need to pass error messages into other FFI functions so we don’t need to worry about nul values.

use winapi::um::winbase::{FormatMessageW, LocalFree, FORMAT_MESSAGE_FROM_SYSTEM,
                          FORMAT_MESSAGE_ALLOCATE_BUFFER, FORMAT_MESSAGE_IGNORE_INSERTS};
use winapi::shared::ntdef::LPWSTR;
use winapi::shared::minwindef::HLOCAL;
use std::ptr;
use widestring::U16String;

let s: U16String;
unsafe {
    // First, get a string buffer from some windows api such as FormatMessageW...
    let mut buffer: LPWSTR = ptr::null_mut();
    let strlen = FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM |
                                FORMAT_MESSAGE_ALLOCATE_BUFFER |
                                FORMAT_MESSAGE_IGNORE_INSERTS,
                                ptr::null(),
                                error_code, // error code from GetLastError()
                                0,
                                (&mut buffer as *mut LPWSTR) as LPWSTR,
                                0,
                                ptr::null_mut());

    // Get the buffer as a wide string
    s = U16String::from_ptr(buffer, strlen as usize);
    // Since U16String creates an owned copy, it's safe to free original buffer now
    // If you didn't want an owned copy, you could use &U16Str.
    LocalFree(buffer as HLOCAL);
}
// Convert to a regular Rust String and use it to your heart's desire!
let message = s.to_string_lossy();

The following example is the functionally the same, only using U16CString instead.

use winapi::um::winbase::{FormatMessageW, LocalFree, FORMAT_MESSAGE_FROM_SYSTEM,
                          FORMAT_MESSAGE_ALLOCATE_BUFFER, FORMAT_MESSAGE_IGNORE_INSERTS};
use winapi::shared::ntdef::LPWSTR;
use winapi::shared::minwindef::HLOCAL;
use std::ptr;
use widestring::U16CString;

let s: U16CString;
unsafe {
    // First, get a string buffer from some windows api such as FormatMessageW...
    let mut buffer: LPWSTR = ptr::null_mut();
    FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM |
                   FORMAT_MESSAGE_ALLOCATE_BUFFER |
                   FORMAT_MESSAGE_IGNORE_INSERTS,
                   ptr::null(),
                   error_code, // error code from GetLastError()
                   0,
                   (&mut buffer as *mut LPWSTR) as LPWSTR,
                   0,
                   ptr::null_mut());

    // Get the buffer as a wide string
    s = U16CString::from_ptr_str(buffer);
    // Since U16CString creates an owned copy, it's safe to free original buffer now
    // If you didn't want an owned copy, you could use &U16CStr.
    LocalFree(buffer as HLOCAL);
}
// Convert to a regular Rust String and use it to your heart's desire!
let message = s.to_string_lossy();