layout: default title: How To Use ICU nav_order: 2 parent: ICU

How To Use ICU

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ICU builds and installs as relatively standard libraries. For details about building, installing and porting see the ICU4C readme and the ICU4J readme. In addition, ICU4C installs several scripts and makefile fragments that help build other code using ICU.

For C++, note that there are Recommended Build Options (both for normal use and for ICU as system-level libraries) which are not default simply for compatibility with older ICU-using code.

Starting with ICU 49, the ICU4C Readme has a short section about User-Configurable Settings.

C++ Makefiles

The recommended way to use ICU in Makefiles is to use the pkg-config files which are installed by ICU upon “make install”. There are files for various libraries and components. This is preferred over the deprecated icu-config script.

This table shows the package names used within pkg-config.

icu-ucCommon (uc) and Data (dt/data) libraries
icu-i18nInternationalization (in/i18n) library
icu-lxParagraph Layout
icu-ioUstdio/iostream library (icuio)

For example, to compile a simple application, you could run the following command. See the pkg-config manpage for more details.

c++ -o test test.c `pkg-config --libs --cflags icu-uc icu-io`

ICU installs the pkg-config (.pc) files in $(prefix)/lib/pkgconfig (where $(prefix) is the installation prefix for ICU). Note that you may need to add $(prefix)/lib/pkgconfig to the PKG_CONFIG_PATH variable.

ICU in a small project

For small projects, it may be convenient to take advantage of ICU's autoconf'ed files. ICU make install writes $(prefix)/lib/icu/ which defines most of the necessary make variables such as $(CXX), $(CXXFLAGS), $(ICULIBS), $(INVOKE), $(ICUPKG), $(datadir), etc.

By itself, is incomplete. It assumes that it will be included into another Makefile which will define $(srcdir), $(DYNAMICCXXFLAGS) and similar values.

In this case, it is probably best to copy ICU's autoconf'ed top-level ./Makefile and/or library-target-style i18n/Makefile and/or binary-target-style tools/icupkg/Makefile. Then modify them as needed.

ICU in a medium-sized project

If you use your own autoconf/CMake/... setup, consider cherry-picking only the definitions needed, for example paths to specific ICU data and tools. This is often preferable to taking the entire and overriding (many) definitions that are different.

For selective ICU definitions, use the installed $(prefix)/bin/icu-config script. Its contents are synchronized with $(prefix)/lib/icu/ For example, use icu-config --invoke=icupkg to invoke the ICU .dat packaging tool.

ICU in a large project

In this case, you probably have your own build system. Just use ICU's public header files, .so files, etc. See the next section, “C++ With Your Own Build System”.

Notes on icu-config

:point_right: Note: icu-config is deprecated, and no longer recommended for production use. Please use pkg-config files or other options.

As of ICU 63.1, icu-config has been deprecated (ICU-10464). icu-config may be disabled by default in the future. As of ICU 63.1, you may enable or disable 63.1 with a configure flag: --enable-icu-config or --disable-icu-config

icu-config is installed (by ICU's make install) into $(prefix)/bin/icu-config. It can be convenient for trivial, single-file programs that use ICU. For example, you could compile and build a small program with this command line:

icu-config --cxx --cxxflags --cppflags --ldflags -o sample sample.cpp

Detailed usage of icu-config script is described in its man page.

C++ With Your Own Build System

If you are not using the standard build system, you will need to construct your own system. Here are a couple of starting points:

  • At least for initial bring-up, use pre-built data files from the ICU download or from a normally-built ICU. Copy the icudtXXx.dat file from icu/source/data/in/ or icu/source/data/out/tmp/ in either of these two locations, into icu/source/data/in/ on your target ICU system. That way, you won‘t need to build ICU’s data-generation tools.
  • Don't compile all files. Look in the files for OBJECTS= clauses which will indicate which source files should be compiled. (Some .c files are #included into others and cannot be compiled by themselves.)
  • ICU does not throw or handle exceptions. Consider turning them off via g++'s -fno-exceptions or equivalent.
  • Each ICU library needs to be compiled with -DU_COMMON_IMPLEMENTATION, -DU_I18N_IMPLEMENTATION etc. as appropriate. See unicode/utypes.h for the set of such macros. If you build one single DLL (shared library) for all of ICU, also use -DU_COMBINED_IMPLEMENTATION. If you build ICU as statically-linked libraries, use -DU_STATIC_IMPLEMENTATION.
  • Use the icu-support mailing list. Ask for help and guidance on your strategy.
  • Up until ICU 4.8, there are one or two header files (platform.h, icucfg.h) that are generated by autoconf/configure and thus differ by platform, sometimes even by target settings on a single platform (e.g., AIX 32-bit vs. 64-bit, Mac OS X universal binaries PowerPC vs. x86). If you do not use autoconf, you probably need to configure-generate these header files for your target platforms and select among them, or merge the generated headers if they are similar, or simulate their generation by other means.
  • Starting with ICU 49, all source code files are fixed (not generated). In particular, there is one single platform.h file which determines platform-specific settings via #if ...

C++ Namespace

ICU C++ APIs are normally defined in a versioned namespace, for example “icu_50”. There is a stable “icu” alias which should be used instead. (Entry point versioning is only to allow for multiple ICU versions linked into one program. It is optional and should be off for system libraries.)

By default, and only for backward compatibility, the ICU headers contain a line using namespace icu_50; which makes all ICU APIs visible in/with the global namespace (and potentially collide with non-ICU APIs there). One of the Recommended Build Options is to turn this off.

To write forward declarations, use

class UnicodeSet;
class UnicodeString;

To qualify an ICU class name, use the “icu” alias:

static myFunction(const icu::UnicodeString &s) {...}

Frequently used ICU classes can be made easier to use in .cpp files with

using icu::UnicodeSet;
using icu::UnicodeString;

Other Notes

Helper Install Utilities

ICU installs $(prefix)/share/icu/$(VERSION)/install-sh and $(prefix)/share/icu/$(VERSION)/mkinstalldirs. These may be used by ICU tools and samples. Their paths are given in the installed (see above).

Data Packaging Settings

The pkgdata tool (see Packaging ICU4C ) makes use of the installed file **$(prefix)/lib/icu/** to set parameters for data packaging operations that require use of platform compilers and linkers ( in static or dll mode). pkgdata uses the icu-config script in order to locate If you are not building ICU using the supplied tools, you may need to modify this file directly to allow static and dll modes to function.

Building and Running Trivial C/C++ Programs with icurun

For building and running trivial (one-compilation-unit) programs with an installed ICU4C, the shell script icurun may be used. For detailed help, see the top of that script. As an example, if ICU is installed to the prefix /opt/local and the current directory contains two sample programs “test1.cpp” and “test2.c”, they may be compiled and run with any of the following commands. The “-i” option specifies either the installed icu-config script, or the directory containing that script, or the path to a ‘bin’ directory.

  • icurun **-i /opt/local** test1.cpp
  • icurun **-i /opt/local/bin** test2.c
  • icurun **-i /opt/local/bin/icu-config** test1.cpp

If “icu-config” is on the PATH, the -i option may be omitted:

  • icurun test1.cpp

Any additional arguments will be passed to the program.

  • icurun test1.cpp *args...*

Please give feedback to the icu-support mailing list, and refer to Ticket #8481.