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Packaging ICU4C


  1. Overview
  2. Making ICU Smaller
    1. Link to ICU statically
    2. Reduce the number of libraries used
    3. Disable ICU features
      1. Using UCONFIG switches with Environment Variables
      2. Using UCONFIG switches by changing uconfig.h
    4. Reduce ICU Data used
  3. ICU Versions
    1. POSIX Library Names
    2. POSIX Library suffix
    3. Windows library names
  4. Packaging ICU4C as Part of the Operating System


This chapter describes, for the advanced user, how to package ICU4C for distribution, whether alone, as part of an application, or as part of the operating system.

Making ICU Smaller

The ICU project is intended to provide everything an application might need in order to process Unicode. However, in doing so, the results may become quite large on disk. A default build of ICU normally results in over 16 MB of data, and a substantial amount of object code. This section describes some techniques to reduce the size of ICU to only the items which are required for your application.

If you add the --enable-static option to the ICU command line build (Makefile or cygwin), ICU will also build a static library version which you can link to only the exact functions your application needs. Users of your ICU must compile with -DU_STATIC_IMPLEMENTATION. Also see How To Use ICU.

Reduce the number of libraries used

ICU consists of a number of different libraries. The library dependency chart in the Design chapter can be used to understand and determine the exact set of libraries needed.

Disable ICU features

Certain features of ICU may be turned on and off through preprocessor defines. These switches are located in the file “uconfig.h”, and disable the code for certain features from being built.

All of these switches are defined to ‘0’ by default, unless overridden by the build environment, or by modifying uconfig.h itself.

Switch Name Library Effect if #defined to ‘1’
UCONFIG_ONLY_COLLATION common & i18n Turn off all other modules named here except collation and legacy conversion
UCONFIG_NO_LEGACY_CONVERSION common Turn off conversion apart from UTF, CESU-8, SCSU, BOCU-1, US-ASCII, and ISO-8859-1. Not possible to turn off legacy conversion on EBCDIC platforms.
UCONFIG_NO_BREAK_ITERATION common Turn off break iteration
UCONFIG_NO_COLLATION i18n Turn off collation and collation-based string search.
UCONFIG_NO_FORMATTING i18n Turn off all formatting (date, time, number, etc), and calendar/timezone services.
UCONFIG_NO_TRANSLITERATION i18n Turn off script-to-script transliteration
UCONFIG_NO_REGULAR_EXPRESSIONS i18n Turn off the regular expression functionality

:point_right: NOTE: These switches do not necessarily disable data generation. For example, disabling formatting does not prevent formatting data from being built into the resource bundles. See the section on ICU data, for information on changing data packaging. However, some ICU data builders will not function with these switches set, such as UCONFIG_NO_FILE_IO or UCONFIG_NO_REGULAR_EXPRESSIONS. If using these switches, it is best to use pre-built ICU data, such as is the default for ICU source downloads, as opposed to data builds “from scratch” out of SVN.

Using UCONFIG switches with Environment Variables

This method involves setting an environment variable when ICU is built. For example, on a POSIX-like platform, settings may be chosen at the point runConfigureICU is run:

  runConfigureICU SOLARISCC ...

:point_right: Note: When end-user code is compiled, it must also have the same CPPFLAGS set, or else calling some functions may result in a link failure.

Using UCONFIG switches by changing uconfig.h

This method involves modifying the source file icu/source/common/unicode/uconfig.h directly, before ICU is built. It has the advantage that the configuration change is propagated to all clients who compile against this build of ICU, however the altered file must be tracked when the next version of ICU is installed.

Modify ‘uconfig.h’ to add the following lines before the first #ifndef UCONFIG_… section


Reduce ICU Data used

There are many ways in which ICU data may be reduced. If only certain locales or converters will be used, others may be removed. Additionally, data may be packaged as individual files or interchangeable archives (.dat files), allowing data to be installed and removed without rebuilding ICU. For details, see the ICU Data chapter.

ICU Versions

(This section assumes the reader is familiar with ICU version numbers (§) as covered in the Design chapter, and filename conventions for libraries in the ReadMe .)

POSIX Library Names

The following table gives an example of the dynamically linked library and symbolic links built by ICU for the common (‘uc’) library, version 5.4.3, for Linux

File Links to Purpose Required for link: Applications compiled with ‘ -licuuc’ will follow this symlink. Required for runtime: This name is what applications actually link against. Actual library Required for runtime and link. Contains the name

:point_right: Note: This discussion gives Linux as an example, but it is typical for most platforms, of which AIX and 390 (zOS) are exceptions.

An application compiled with ‘-licuuc’ will follow the symlink from to, and will actually read the file (fully qualified). This library file has an embedded name (SONAME) of, that is, with only the major and minor number. The linker will write this name into the client application, because Binary compatibility is for versions that share the same major+minor number.

If ICU version 5.4.7 is subsequently installed, the following files may be updated.

File Links to Purpose Required for link: Newly linked applications will follow this link, which should not cause any functional difference at link time. |` Required for runtime: Because it now links to version .7, existing applications linked to version 5.4.3 will follow this link and use the 5.4.7 code. Actual library Required for runtime and link. Contains the name

If ICU version 5.6.3 or 3.2.9 were installed, they would not affect already-linked applications, because the major+minor numbers are different - 56 and 32, respectively, as opposed to 54. They would, however, replace the link, which controls which version of ICU newly-linked applications use.

In summary, what files should an application distribute in order to include a functional runtime copy of ICU 5.4.3? The above application should distribute and the symbolic link (If symbolic links pose difficulty, may be renamed to, and only distributed. This is less informative, but functional.)

POSIX Library suffix

The –with-library-suffix option may be used with runConfigureICU or configure, to distinguish on disk specially modified versions of ICU. For example, the option –with-library-suffix=myapp will produce libraries with names such as, thus preventing another ICU user from using myapp’s custom ICU libraries.

While two or more versions of ICU may be linked into the same application as long as the major and minor numbers are different, changing the library suffix is not sufficient to allow the same version of ICU to be linked. In other words, linking ICU 5.4.3, 5.6.3, and 3.2.9 together is allowed, but 5.4.3 and 5.4.7 may not be linked together, nor may 5.4.3 and 5.4.3-myapp be linked together.

Windows library names

Assuming ICU version 5.4.3, Windows library names will follow this pattern:

File Purpose
icuuc.lib Release Link-time library. Needed for development. Contains icuuc54.dll name internally.
icuuc54.dll Release runtime library. Needed for runtime.
icuucd.lib Debug link-time library (The d suffix indicates debug)
icuuc54d.dll Debug runtime library.

Debug applications must be linked with debug libraries, and release applications with release libraries.

When a new version of ICU is installed, the .lib files will be replaced so as to keep new compiles in sync with the newly installed header files, and the latest DLL. As well, if the new ICU version has the same major+minor version (such as 5.4.7), then DLLs will be replaced, as they are binary compatible. However, if an ICU with a different major+minor version is installed, such as 5.5, then new DLLs will be copied with names such as ‘icuuc55.dll’.

Packaging ICU4C as Part of the Operating System

The services which are now known as ICU were written to provide operating system-level and application environment-level services. Several operating systems include ICU as a standard or optional package. See ICU Binary Compatibility for more details.