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Introduction to ICU

What is ICU?

ICU is a cross-platform Unicode based globalization library. It includes support for locale-sensitive string comparison, date/time/number/currency/message formatting, text boundary detection, character set conversion and so on.

Where can I get ICU?

You can get ICU4C and ICU4J from

Why don't you build binaries for my platform?

There are many versions of compilers on so many platforms that we cannot build them all and guarantee compatibility between them all even on the same platform. Due to these restrictions, we only distribute a limited number of binary versions of ICU, but we will assist in building other versions from source.

Why don't you provide project files for my MSVC version (MSVC 2008, etc)?

You can use the Cygwin build environment to build ICU from source against the MSVC compiler. See the ICU4C Readme.

How do I install the binary versions of ICU?

  • Windows:
    • The DLLs you may need for your application are located in bin\icuXX##.dll, where “XX” are two letters (such as “uc” for the “common” library, “in” for the “i18n” library, etc.) and ## is the major and the minor version number (such as 42 for 4.2 / or 4.2.4 ).
    • Either place the DLLs in the same directory as your application's .EXE files, or set the PATH variable to point to the directory containing the ICU DLLs.
    • For compiling applications, add the “include” direcotry (the parent of the “unicode” and “layout” directories) to the include search path.
    • For linking applications, add the “lib” directory to the appropriate path.
  • Other Platforms:
    • For other platforms, the .tgz file unpacks to a “/usr/local” type hierarchy. For system-wide installation, you can unpack all of the files into /usr/local/bin, /usr/local/include, etc.
    • The configuration script /usr/local/bin/icu-config or the similar Makefile include fragment /usr/local/lib/icu/current/ can be used in building applications.

Can you help me build ICU4C for ...

We can try ... make sure you read the latest “readme” and also the ICU Data section. You might also searching the icu-support archives, and then posting a question there. Additionally, sites such as StackOverflow may have helpful tips for your topic.

What is the ICU binary compatibility policy?

Please see the section on binary compatibility in the design chapter.

How is ICU licensed?

The ICU license is intended to allow ICU to be included both in free software projects and in proprietary or commercial products.

Since ICU 58, ICU is covered by the Unicode license which is very similar to the previous ICU license.

ICU 1.8.1–ICU 57 and ICU4J 1.3.1–ICU4J 57 are covered by the ICU license, a simple, permissive non-copyleft free software license, compatible with the GNU GPL. The ICU license is identical to the version of the X license that was formerly available at . (This site no longer exists, but can still be retrieved through internet archive services.)

Can I use ICU from other languages besides C/C++ and Java?

There are a number of wrappers available, please see the Related Projects page.

How do I upgrade to a new version of ICU? Should I be concerned about API changes, a new Unicode version or a new CLDR version)?

Our goal is for ICU upgrades to go smoothly. Here are some steps you can take to prepare for an upgrade, or to make sure that your usage of ICU is upgrade-friendly.

  • API: ensure that you are not using draft APIs which may have changed in a future release. See the section on API compatibility in the design chapter.
  • Unicode: See the release notes for particular versions of Unicode to ensure that your code is not affected by property changes or other specification changes.
  • CLDR: If your application has test cases which depend on specific translations, these assumptions may become invalid if the translation of an item changes, new support is added, or if a country changes its currency. Try not to depend on specific translations, or be prepared to change test cases. Also, a newer version may support additional translations, currencies, types of calendars
  • Building/Deploying your Application (ICU4C): ICU4C usually builds with symbol renaming (See: binary compatibility in the design chapter). Be sure that you build your application with the updated ICU header files, so that it will link against the current ICU. Also, don't hard-code the names of ICU libraries in your build scripts and projects. Where possible, link against just the ‘base name’ such as or icuuc.lib rather than a name containing the version number such as**46** or icuuc**46**.dll.

Building and Testing ICU

How do I build ICU?

See the readme.html that is included with ICU.

How do I get 32- or 64-bit versions of the ICU libraries?

From ICU version 4.2 on, the configure script will build with the default bit width of your platform. You can request 64 or 32 bits with the --with-library-bits= option, (e.g. runConfigureICU Linux **--with-library-bits=64** or runConfigureICU MacOSX **--with-library-bits=32**). (For the behavior of attempting 64 bits if possible, use --with-library-bits=64else32).

How do I build an optimized, non debug ICU?

On Win32, choose the ‘Release’ configuration from the drop down menu. On other platforms, use the runConfigureICU script, which uses the configure script. The runConfigureICU script uses the safest level of optimization for the ICU libraries. If your platform is not specified, set the following environment variables before running configure or runConfigureICU: CFLAGS=-O CXXFLAGS=-O

Why am I getting so many test failures when I use “gmake check”?

Please view the readme that is included with ICU. It has all the details on how to build and test ICU, and it usually answers most problems.

If you are using a compiler that hasn't been tested with ICU before, you may have encountered an optimization bug with the compiler. On Unix platforms you can specify --disable-release when you are using runConfigureICU (e.g. runConfigureICU --disable-release LinuxRedHat). If this fixes your problem, it is recommended that you report the optimization bug to the compiler manufacturer.

If neither of these fix your problem, please send an e-mail to the ICU4C Support List .

How can I reduce the size of the ICU data library?

Use the Data Customizer or see Customizing ICU's Data Library in the ICU Data Management chapter of this User's Guide.

Why am I seeing a small ( only a few K ) instead of a large ( several megabytes ) data shared library (icudt)?

Opening ICU services fails with U_MISSING_RESOURCE_ERROR and u_init() returns failure.

ICU libraries always must link with the ICU data library. However, so that ICU can bootstrap itself, it first builds a ‘stub’ data library, in icu\source\stubdata, so that the tools can function. You should only use this in production if you are NOT using DLL-mode data access, in which case you are accessing ICU data as individual files, as an archive (.dat) file, or some other means. Normally, you should be using the larger library built from icu\source\data. If you see this issue after ICU has completed building, re-run ‘make’ in icu\source\data, or the ‘makedata’ project in Visual Studio.

Can I add or remove a converter from ICU?

Yes. Please see Customizing ICU's Data Library in the ICU Data Management of this User's Guide. You can also get extra converters from or use the ICU Data Customizer tool.

Why don't the makefiles work?

You need GNU's make program version 3.8 or later, and you need to run the runConfigureICU script, which is located in the icu/source directory. You may be using a platform that ICU does not support. If the first two answers do not apply to you, then you should send an e-mail to the ICU4C Support List.

Here are some places you can find gmake:

  1. GNU:

  2. Sun® Source/Binaries:

  3. z/OS (OS/390) Source/Binaries:

  4. IBM i (OS/400) Source/Binaries:

Due to differences in every platform's make program, we will not support other versions of our make files.

What version of the C iostream is used in ICU4C?

ICU4C uses the latest available version of the iostream on the target platform. Only the io library uses iostream.

I only want to use the C APIs, do I need a C++ compiler?

Large portions of ICU4C were always implemented in C++, and over time we are moving more into that direction. We continue to support and add C APIs, in order to provide binary-compatible APIs. For the implementation, C++ is much better: It is generally easier to work with, which reduces bugs and maintenance. It is closer to Java, which is important for porting between ICU4C and ICU4J. We use RAII (e.g., LocalPointer) to reduce opportunities for memory leaks, we use inline functions and type-safe constants instead of #define, etc. However, we do not use exceptions, and we do not use the Standard Template Library (STL), so ICU4C's dependencies on the C++ library are minimal. See the new dependencies.txt and search for “group: cplusplus”.

As ICU does not use exceptions, the GCC option -fno-exceptions will reduce or remove the dependencies on the standard C++ library. In GCC 4.5 there is an option -static-libstdc++ which will remove C++ library dependencies. Visual Studio has the /MT option, and other compilers may have similar options. See the How To Use ICU page for related information on this topic.

Features of ICU

What computer languages does ICU support?

ICU4C (ICU) is written in C and C++, and ICU4J is written in Java™.

How are the APIs documented for deprecation?

Please read the ICU API compatibility section in the ICU Design chapter.

What version of Unicode standard does ICU support?

ICU versions 65 supports Unicode version 12.

The Unicode versions for older versions of ICU are listed on the ICU download page,

Does ICU support UTF-16 surrogates and Unicode supplementary characters?


Does Java support UTF-16 surrogates and Unicode supplementary characters?

Java 5 introduced support for Unicode supplementary characters. Java 1.4 and earlier do not directly support them.

How does ICU relate to Java's java.text.* package?

The International Components for Unicode are available both as a C/C++ library and a Java class library. ICU provides internationalization utilities for writing global applications in C, C++ or Java programming languages. ICU was originally developed by the Unicode group at the IBM Globalization Center of Competency in Cupertino, and ICU was contributed to Sun for inclusion into the JDK 1.1. ICU4J includes enhanced versions of some of these contributed classes plus additional classes that complement the classes in the JDK.

ICU4C started as a C++ port of the original Java Internationalization classes. These classes are now partially implemented in C, with largely parallel C and C++ APIs. ICU4C and ICU4J continue to leapfrog each other with features and bug fixes. Over time, features from ICU4J get added to the JDK as well.

Both versions of ICU have a goal to implement the latest Unicode standard, maintain a single portable source code base, and to make it easier for software developers to create global applications.

Using ICU

Can I use any of the features of ICU without Unicode strings?

No. In order to use the collation, text boundary analysis, formatting or other ICU APIs, you must use Unicode strings. In order to get Unicode strings from your native codepage, you can use the conversion API.

How do I declare a Unicode string in ICU?

Use the U_STRING_DECL and U_STRING_INIT macros or use the UnicodeString class for C++. Strings are represented as UChar \* as the base string type.

Even though most platforms declare wide strings as wchar_t \* or L"" as the base string type, that declaration is not portable because the sizeof(wchar_t) can be 1, 2 or 4, and the encoding may not even be Unicode. On the platforms where sizeof(wchar_t) is 2 bytes, UChar is defined as wchar_t. In that case you can use ICU's strings with 3rd party legacy functions; however, we do not suggest using Unicode strings without the U_STRING_DECL and U_STRING_INIT macros or UnicodeString class because they are platform independent implementations.

How is a Unicode string represented in ICU4C?

A Unicode string is currently represented as UTF-16. The endianess of UTF-16 is platform dependent. You can guarantee the endianess of UTF-16 by using a converter. UTF-16 strings can be converted to other Unicode forms by using a converter or with the UTF conversion macros.

ICU does not use UCS-2. UCS-2 is a subset of UTF-16. UCS-2 does not support surrogates, and UTF-16 does support surrogates. This means that UCS-2 only supports UTF-16's Base Multilingual Plane (BMP). The notion of UCS-2 is deprecated and dead. Unicode 2.0 in 1996 changed its default encoding to UTF-16.

If you need to do a quick and easy conversion between UTF-16 and UTF-8, UTF-32 or an encoding in wchar_t, you should take a look at unicode/ustring.h. In that header file you will find u_strToWCS, u_strFromWCS, u_strToUTF8, u_strFromUTF8, u_strToUTF32 and u_strFromUTF32 functions. These functions are provided for your convenience instead of using the ucnv_\* API.

You can also take a look at the UTF_\*, UTF8_\*, UTF16_\* and UTF32_\* macros, which are defined in unicode/utf.h, unicode/utf8.h, unicode/utf16.h and unicode/utf32.h. These macros are helpful for programmers that need to manipulate and process Unicode strings.

How do I index into a UTF-16 string?

Typically, indexes and offsets in strings count string units, not characters (although in C and Java they have a char type).

For example, in old-fashioned MBCS strings, you would count indexes and offsets by bytes, not by the variable-width character count. In UTF-16, you do the same, just count 16-bit units (in ICU: UChar).

What is the performance difference between UTF-8 and UTF-16?

Most of the time, the memory throughput of the hard drive and RAM is the main performance constraint. UTF-8 is 50% smaller than UTF-16 for US-ASCII, but UTF-8 is 50% larger than UTF-16 for East and South Asian scripts. There is no memory difference for Latin extensions, Greek, Cyrillic, Hebrew, and Arabic.

For processing Unicode data, UTF-16 is much easier to handle. You get a choice between either one or two units per character, not a choice among four lengths. UTF-16 also does not have illegal 16-bit unit values, while you might want to check for illegal bytes in UTF-8. Incomplete character sequences in UTF-16 are less important and more benign. If you want to quickly convert small strings between the different UTF encodings or get a UChar32 value, you can use the macros provided in utf.h and its siblings utf8.h and utf16.h. For larger or partial strings, please use the conversion API.

How do the converters work?

The converters act like a data stream. This means that the state of the last character is saved in the converter after each call to the ucnv_fromUnicode() and ucnv_toUnicode() functions. So if the source buffer ends with part of a surrogate Unicode character pair, the next call to ucnv_toUnicode() will write out the equivalent character to the destination buffer. Please see the Conversion chapter of the User's Guide for details.

What does a locale look like in ICU?

ICU locales are lightweight, and they are represented by just a string. Lightweight means that there is just a string to represent a locale and nothing more. Many platforms have numbers and other data structures to represent a locale, but ICU has one simple platform independent string to represent a locale.

ICU locales usually contain an ISO-639 language name (2-3 characters), an ISO-3166 country name (2-3 characters), and a variant name which is user specified. When a language or country is not represented by these standards, ICU uses 3 characters to represent that part of the locale. All three parts are separated by an underscore “_”. For example, US English is “en_US”, and German in Germany with the Euro symbol is represented as “de_DE_EURO”. Traditionally the language part of the locale is lowercase, the country is uppercase and the variant is uppercase. More details are available from the Locale Chapter of this User's Guide.

How is ICU versioned?

Please read the ICU Design chapter of the User's Guide.

What is the relationship between ICU locale data and system locale data?

There is no relationship. ICU is not dependent on the operating system for the locale data.

This also means that uloc_setDefault() does not affect the operating system. The function uloc_setDefault() only sets ICU's default locale. Normally the default locale for ICU is whatever the operating system says is the default locale.

How are errors handled in ICU?

Since not all compilers can handle exceptions, we return an error from functions with a UErrorCode parameter. The UErrorCode parameter of a function will return any errors that occurred while it was executing. It's usually a good idea to check for errors after calling a function by using the U_SUCCESS and U_FAILURE macros. U_SUCCESS returns true when the function did run properly, and U_FAILURE returns true when the function did NOT run properly. You may handle specific errors from a function by checking the exact value of error. The possible values of UErrorCode are located in utypes.h of the common project. Before any function is called with a UErrorCode, it must be initialized to U_ZERO_ERROR.

Here is an example of UErrorCode being used.

UErrorCode err = U_ZERO_ERROR;
if (U_FAILURE(err)) {
puts("callMyFunction() Failed!");

Please see the ICU Design chapter for details.

With calendar classes, why are months 0-based?

“I have been using ICU for its calendar classes, and have found it to be excellent. That said, I am wondering why the decision was made to keep months 0-based while almost all the other calendrical units (years, weeks of year, weeks of month, date, days of year, days of week, days of week in month) are 1-based? This has been the source of several bugs whenever the mind is slightly less than razor sharp.” --Contributor

This was not our choice. We inherited it from the Java Calendar API, unfortunately.

Is there a guideline for COBOL programs that want to use ICU?

There is a COBOL/ICU guideline available since ICU 2.2. For more details, please refer to the COBOL section of this User's Guide.

Where can I get more information about using ICU?

Please send an e-mail to the ICU4C Support List .