Difference between revisions of "FPC Unicode support"

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* [[unicode use cases]]
* [[unicode use cases]]
* [[LCL Unicode Support]]
* [[LCL Unicode Support]]
* [[not Delphi compatible enhancement for Unicode Support]]
* Suggestion: [[not Delphi compatible enhancement for Unicode Support]]

Revision as of 10:45, 26 November 2014

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Up to and including FPC 2.6.x, the RTL was based on the ones of Turbo Pascal and Delphi 7. This means it was primarily based around the shortstring, ansistring and pchar types. None of these types had any encoding information associated with them, but were implicitly assumed to be encoded in the "default system encoding" and were passed on to OS API calls without any conversion.

In Delphi 2009, Embarcadero switched the entire RTL over to the UnicodeString type, which represents strings using UTF-16. Additionally, they also made the AnsiString type "code page-aware". This means that AnsiStrings from then on contain the code page according to which their data should be interpreted.

FPC's language-level support for these string types is already available in current development versions of the compiler (FPC 2.7.1/trunk). The RTL level support is not yet complete. This page gives an overview of the code page-related behaviour of these string types, the current level of support in the RTL, and possible future ways of how this support may be improved.

Backward compatibility

If you have existing code that works in a defined way (*) with a previous version of FPC and make no changes to it, it should continue to work unmodified with the new FPC version. Guaranteeing this is the main purpose of the multitude of Default*CodePage variables and their default values as described below.

(*) this primarily means: you do not store data in an ansistring that has been encoded using something else than the system's default code page, and subsequently pass this string as-is to an FPC RTL routine. E.g., current Lazarus code is generally fine, as you are supposed to call UTF8ToAnsi() before passing its strings to FPC RTL routines.

Code pages

A code page defines how the individual bytes of a string should be interpreted, i.e., which letter, symbol or other graphic character corresponds to every byte or sequence of bytes.

Code page identifiers

A code page identifier is always stored as a TSystemCodePage, which is an alias for Word. The value represents the corresponding code page as defined by Microsoft Windows. Additionally, there are 3 special code page values:

  • CP_ACP: this value represents the currently set "default system code page". See #Code page settings for more information.
  • CP_OEM: this value represents the OEM code page. On Windows platforms this corresponds to the code page used by the console (e.g. cmd.exe windows). On other platforms this value is interpreted the same as CP_ACP.
  • CP_NONE: this value indicates that no code page information has been associated with the string data. The result of any explicit or implicit operation that converts this data to another code page is undefined.

Note: code page identifiers are different from codepage names as used in the {$codepage xxx} directives (which is available in current stable FPC already). Codepage names are the names of individual codepage units exposed by the charset unit, which have names such as cp866 and cp1251 and utf8.

Code page settings

The system unit contains several global variables that indicate the default code page used for certain operations.


  • Purpose: determines how CP_ACP is interpreted
  • Initial value:
    • Windows: The result of the GetACP OS call, which returns the Windows ANSI code page.
    • iOS: UTF-8
    • Unix (excluding iOS): Based on the currently set LANG or LC_CTYPE environment variables. This is usually UTF-8, but that is not guaranteed to be the case.
    • Other platforms: CP_ACP (these platforms currently do not support multiple code pages, and are hardcoded to use their OS-specific code page in all cases)
  • Modifications: you can modify this value by calling SetMultiByteConversionCodePage(CodePage: TSystemCodePage)
  • Notes: Since the value of this variable can be changed, it is not a good idea to use its value to determine the real OS "default system code page" (unless you do it at program startup and are certain no other unit has changed it in its initialisation code).


  • Purpose: defines the code page to which file/path names are translated before they are passed to OS API calls, if the RTL uses a single byte OS API for this purpose on the current platform. This code page is also used for intermediate operations on file paths inside the RTL before making OS API calls. This variable does not exist in Delphi, and has been introduced in FPC to make it possible to change the value of DefaultSystemCodePage without breaking RTL interfaces with the OS file system API calls.
  • Initial value:
    • Windows: UTF-8, because the RTL uses UTF-16 OS API calls (so no data is lost in intermediate operations).
    • OS X and iOS: UTF-8 (as defined by Apple)
    • Unix (excluding OS X and iOS): DefaultSystemCodePage, because the encoding of file names is undefined on Unix platforms (it's an untyped array of bytes that can be interpreted in any way; it is not guaranteed to be valid UTF-8)
    • Other platforms: same as DefaultSystemCodePage
  • Modifications: you can modify this value by calling SetMultiByteFileSystemCodePage(CodePage: TSystemCodePage)
  • Notes: the Unix/OS X/iOS settings only apply in case the cwstring widestring manager is installed, otherwise DefaultFileSystemCodePage will have the same value as DefaultSystemCodePage after program startup.


  • Purpose: defines the code page to which file/path names are translated before they are returned from RawByteString file/path RTL routines. Examples include the file/path names returned by the RawbyteString versions of SysUtils.FindFirst and System.GetDir. The main reason for its existence is to enable the RTL to provide backward compatibility with earlier versions of FPC, as these always returned strings encoded in whatever the OS' single byte API used (which was usually what is now known as DefaultSystemCodePage).
  • Initial value
    • Windows: DefaultSystemCodePage, for backward compatibility.
    • OS X and iOS: UTF-8, for backward compatibility (it was already always UTF-8 in the past, since that's what the OS file APIs return and we did not convert this data).
    • Unix (excluding OS X and iOS): DefaultSystemCodePage, for the same reason as with DefaultFileSystemCodePage. Setting this to a different value than DefaultFileSystemCodePage is a bad idea on these platforms, since any code page conversion can corrupt these strings as their initial encoding is unknown.
    • Other platforms: same as DefaultSystemCodePage
  • Modifications: you can modify this value by calling SetMultiByteRTLFileSystemCodePage(CodePage: TSystemCodePage)
  • Notes: same as for DefaultFileSystemCodePage.


String/character types


The code page of a shortstring is implicitly CP_ACP and hence will always be equal to the current value of DefaultSystemCodePage.


These types are the same as the old PChar/Char types. In all compiler modes except for {$mode delphiunicode}, PChar/Char are also still aliases for PAnsiChar/AnsiChar. Their code page is implicitly CP_ACP and hence will always be equal to the current value of DefaultSystemCodePage.

PWideChar/PUnicodeChar and WideChar/UnicodeChar

These types remain unchanged. WideChar/UnicodeChar can contain a single UTF-16 code unit, while PWideChar/PUnicodeChar point to a single or an array of UTF-16 code units.

In {$mode delphiunicode}, PChar becomes an alias for PWideChar/PUnicodeChar and Char becomes an alias for WideChar/UnicodeChar.


These types behave the same as in previous versions:

  • Widestring is the same as a "COM BSTR" on Windows, and an alias for UnicodeString on all other platforms. Its string data is encoded using UTF-16.
  • UnicodeString is a reference-counted string with a maximum length of high(SizeInt) UTF-16 code units.


AnsiStrings are reference-counted types with a maximum length of high(SizeInt) bytes. Additionally, they now also have code page information associated with them.

The most important thing to understand about the new AnsiString type is that it both has a declared/static/preferred/default code page (called static code page from now on), and a dynamic code page. The static code page tells the compiler that when assigning something to that AnsiString, it should first convert the data to that static code page (except if it is CP_NONE, see #RawByteString below). The dynamic code page is a property of the AnsiString which, similar to the length and the reference count, defines the actual code page of the data currently held by that AnsiString.

Static code page

The static code page of an AnsiString can only be defined by declaring a new type as follows:

  CP866String = type AnsiString(866); // note the extra "type"

The static code page of a variable declared as plain AnsiString is CP_ACP. In effect, the AnsiString type is now semantically defined in the System unit as

  AnsiString = type AnsiString(CP_ACP);

Another predefined AnsiString(X) type in the System unit is UTF8String:

  UTF8String = type AnsiString(CP_UTF8);

Once you have defined such a custom AnsiString(X) type, you can use it to declare variables, parameters, fields etc as usual.

Dynamic code page

If a string with a static code page X1 is assigned to a string with static code page X2 and X1<>X2, the string data will generally first be converted to said code page X2 before assignment, and as a result the dynamic code page of the destination string will be X2. When assigning a string to a plain AnsiString (= AnsiString(CP_ACP)) or ShortString, the string data will however be converted to DefaultSystemCodePage. The dynamic code page of that AnsiString(CP_ACP) will then be the current value of DefaultSystemCodePage (e.g. 1250 for the Windows-1250 code page), even though its static code page is CP_ACP (which is a constant <> 1250). This is one example of how the static code page can differ from the dynamic code page. Subsequent sections will describe more such scenarios.

Note: as mentioned above, whether or not a potential code page conversion happens only depends on the static code pages of the involved strings. This means that if you assign one AnsiString(X) to another AnsiString(X) and the former's dynamic code was different from X, the string data will not be converted to code page X by the assignment.


The RawByteString type is defined as

  RawByteString = type AnsiString(CP_NONE);

As mentioned earlier, the results of conversions from/to the CP_NONE code page are undefined. As it does not make sense to define a type in the RTL whose behaviour is undefined, the behaviour of RawByteString is somewhat different than that of other AnsiString(X) types.

As a first approximation, RawByteString can be thought of as an "untyped AnsiString": assigning an AnsiString(X) to a RawByteString has exactly the same behaviour as assigning that AnsiString(X) to another AnsiString(X) variable with the same value of X: no code page conversion or copying occurs, just the reference count is increased.

Less intuitive is probably that when a RawByteString is assigned to an AnsiString(X), the same happens: no code page conversion or copying, just the reference count is increased. Note that this means that results from functions returning a RawByteString will never be converted to the destination's static code page. This is another way in which the dynamic code page of an AnsiString(X) can become different from its static code page.

This type is mainly used to declare const, constref and value parameters that accept any AnsiString(X) value without converting it to a predefined static code page. Note that if you do this, the routine accepting those parameters should be able to handle strings with any possible dynamic code page.

var and out parameters can also be declared as RawByteString, but in this case the compiler will give an error if an AnsiString(X) whose static code page is different from CP_NONE is passed in. This is consistent with var and out parameters in general: they require an exactly matching type to be passed in. You can add an explicit RawByteString() typecast around an argument to remove this error, but then you must be prepared to deal with the fact that the returned string can have any dynamic code page.

String concatenations

Normally, in Pascal the result type of an expression is independent of how its result is used afterwards. E.g. multiplying two longints on a 32 bit platform and assigning the result to an int64 will still perform the multiplication using 32 bit arithmetic, and only afterwards the result is converted to 64 bit.

Code page-aware strings are the only exception to this rule: concatenating two or more strings always occurs without data loss, although afterwards the resulting string will of course still be converted to the static code page of the destination (which may result in data loss).

Assigning the result of a concatenation to a RawByteString is again special:

  • if all concatenated strings have the same dynamic code page, the result will have this code page too
  • in other cases the result will be converted to CP_ACP (we may add an option in the future to change this RawByteString behaviour, as it is not very practical).

String constants

String constants are parsed by FPC as follows:

  • if a file contains a {$codepage xxx} directive (e.g. {$codepage UTF8}), then string constants are interpreted according to that code page, otherwise
  • if the file starts with an UTF-8 BOM, then string constants are interpreted as UTF-8 strings, otherwise
  • string constants are copied without any translation into an internal buffer and are interpreted as characters using one of the following code pages:
    • the DefaultSystemCodePage of the computer on which the compiler itself is currently running when {$modeswitch systemcodepage} is active (i.e., compiling the source code on a different system may cause string constants to be interpreted differently; this switch is available for Delphi compatibility and is enabled by default in {$mode delphiunicode})
    • CP_ACP in case {$modeswitch systemcodepage} is not active (for backward compatibility with previous FPC versions)

In all but the last case, the actual code page of the source file is known. This knowledge is required when the compiler is forced to convert string constants to a different code page. Therefore, in the last case a default is used in such situations: strings are assumed to be encoded in code page 28591 (ISO 8859-1 Latin 1; Western European). This assumed or actual code page is referred to as the source file code page below.

When a string constant is assigned to an AnsiString(X) either in code or as part of a typed constant or variable initialisation, then

  • if X = CP_NONE (i.e., the target is a RawByteString), the result is the same as if the constant string were assigned to an AnsiString(CP_ACP)
  • if X = CP_ACP and the code page of the string constant is different from CP_ACP, then the string constant is converted, at compile time, to the source file code page. If the source file code page is also CP_ACP, it will be stored in the program unaltered with a code page of CP_ACP and hence its meaning/interpretation will depend on the actual value of DefaultSystemCodePage at run time. This ensures compatibility with older versions of FPC when assigning string constants to AnsiString variables without using a {$codepage xxx} directive or UTF-8 BOM.
  • for other values of X, the string constant is converted, at compile time, to code page X

Similarly, if a string constant is assigned to a UnicodeString, the string constant is converted, at compile time, from the source file code page to UTF-16.

For ShortString and PChar, the same rule as for AnsiString(CP_ACP) is followed.

Note that symbolic string constants will be converted at compile time to the appropriate string type and code page whenever they are used. This means that there is no speed overhead when using a single string constant in multiple code page and string type contexts, only some data size overhead.

From the above it follows that to ensure predictable interpretation of string constants in your source code, it is best to either include an explicit {$codepage xxx} directive (or use the equivalent -Fc command line option), or to save the source code in UTF-8 with a BOM.

String indexing

Nothing changes to string indexing. Every string element of a UnicodeString/WideString is two bytes and every string element of all other strings is one byte. The string indexing mechanism completely ignores code pages and composite code points.

RTL changes

In order to fully guarantee data integrity in the presence of codepage-aware strings, all routines in the RTL and packages that accept AnsiString parameters must be adapted. The reason is that if their parameters remain plain AnsiString, then any string with a different static code page will be converted to DefaultSystemCodePage when it is passed in. This can result in data loss.

Until now, primarily routines dealing with file system access have been updated to preserve all character data. Below is an exhaustive list of all routines that preserve the string encoding in FPC 2.7.1 and later. Unless where explicitly noted otherwise, these routines also all have overloads that accept UnicodeString parameters.

  • System: FExpand, LowerCase, UpperCase, GetDir, MKDir, ChDir, RMDir, Assign, Erase, Rename, standard I/O (Read/Write/Readln/Writeln/Readstr/Writestr), Insert, Copy, Delete, SetString
  • ObjPas (used automatically in Delphi and ObjFPC modes): AssignFile
  • SysUtils: FileCreate, FileOpen, FileExists, DirectoryExists, FileSetDate, FileGetAttr, FileSetAttr, DeleteFile, RenameFile, FileSearch, ExeSearch, FindFirst, FindNext, FindClose, FileIsReadOnly, GetCurrentDir, SetCurrentDir, ChangeFileExt, ExtractFilePath, ExtractFileDrive, ExtractFileName, ExtractFileExt, ExtractFileDir, ExtractShortPathName, ExpandFileName, ExpandFileNameCase, ExpandUNCFileName, ExtractRelativepath, IncludeTrailingPathDelimiter, IncludeTrailingBackslash, ExcludeTrailingBackslash, ExcludeTrailingPathDelimiter, IncludeLeadingPathDelimiter, ExcludeLeadingPathDelimiter, IsPathDelimiter, DoDirSeparators, SetDirSeparators, GetDirs, ConcatPaths, GetEnvironmentVariable
  • Unix: fp*() routines related to file system operations (no UnicodeString overloads), POpen
  • DynLibs: all routines

RTL todos

As the above list is exhaustive, no other RTL routines support arbitrary code pages yet. This section contains a list of gotchas that some people have identified and, if possible, workarounds. Note that routines not mentioned here nor above are equally unsafe as the ones that are explicitly mentioned.

TFormatSettings and DefaultFormatSettings

The type of ThousandSeparator and DecimalSeparator is AnsiChar type. This means that if DefaultSystemCodePage is UTF-8 and the locale's separator is more than one byte long in that encoding, these fields are not large enough. Examples are the French and Russian non-breaking white space character used to represent the ThousandSeparator.

Old/obsolete sections


Warning: These sections are kept for historical reference - please update the sections above with this information if it is still applicable. Since FPC 2.7 (current development version), extensive Unicode support has been implemented.

User visible changes

Full support of code page aware strings is not possible without breaking some existing code. The following list tries to summarize the most important user visible changes.

  • The string header has two new fields: encoding and element size. On 32 Bit platforms this increases the header size by 4 and on 64 bit platforms by 8 bytes.
  • WideCharLenToString, UnicodeCharLenToString, WideCharToString, UnicodeCharToString and OleStrToString return an UnicodeString instead of an Ansistring before.
  • the type of the dest parameter of WideCharLenToString and UnicodeCharLenToString has been changed from Ansistring to Unicodestring
  • UTF8ToAnsi and AnsiToUTF8 take a RawByteString now

See Also