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Using Converters


When designing applications around Unicode characters, it is sometimes required to convert between Unicode encodings or between Unicode and legacy text data. The vast majority of modern Operating Systems support Unicode to some degree, but sometimes the legacy text data from older systems need to be converted to and from Unicode. This conversion process can be done with an ICU converter.

ICU converters

ICU provides comprehensive character set conversion services, mapping tables, and implementations for many encodings. Since ICU uses Unicode (UTF-16) internally, all converters convert between UTF-16 (with the endianness according to the current platform) and another encoding. This includes Unicode encodings. In other words, internal text is 16-bit Unicode, while "external text" used as source or target for a conversion is always treated as a byte stream.

ICU converters are available for a wide range of encoding schemes. Most of them are based on mapping table data that is handled by few generic implementations. Some encodings are implemented algorithmically in addition to (or instead of) using mapping tables, especially Unicode encodings. The partly or entirely table-based encoding schemes include: All ICU converters map only single Unicode character code points to and from single codepage character code points. ICU converters do not deal directly with combining characters, bidirectional reordering, or Arabic shaping, for example. Such processes, if required, must be handled separately. For example, while in Unicode, the ICU BiDi APIs can be used for bidirectional reordering after a conversion to Unicode or before a conversion from Unicode.

ICU converters are not designed to perform any encoding autodetection. This means that the converters do not autodetect "endianness", the 6 Unicode encoding signatures, or the Shift-JIS vs. EUC-JP, etc. There are two exceptions: The UTF-16 and UTF-32 converters work according to Unicode's specification of their Character Encoding Schemes, that is, they read the BOM to figure out the actual "endianness".

The ICU mapping tables mostly come from an IBM® codepage repository. For non-IBM codepages, there is typically an equivalent codepage registered with this repository. However, the textual data format (.ucm files) is generic, and data for other codepage mapping tables can also be added.

Using the Default Codepage

ICU has code to determine the default codepage of the system or process. This default codepage can be used to convert char * strings to and from Unicode.

Depending on system design, setup and APIs, it may not always be possible to find a default codepage that fully works as expected. For example,

  1. On Windows there are three encodings in use at the same time. Unicode (UTF-16) is always used inside of Windows, while for char * encodings there are two classes, called "ANSI" and "OEM" codepages. ICU will use the ANSI codepage. Note that the OEM codepage is used by default for console window output.

  2. On some UNIX-type systems, non-standard names are used for encodings, or non-standard encodings are used altogether. Although ICU supports over 200 encodings in its standard build and many more aliases for them, it will not be able to recognize such non-standard names.

  3. Some systems do not have a notion of a system or process codepage, and may not have APIs for that.

If you have means of detecting a default codepage name that are more appropriate for your application, then you should set that name with ucnv_setDefaultName() as the first ICU function call. This makes sure that the internally cached default converter will be instantiated from your preferred name.

Starting in ICU 2.0, when a converter for the default codepage cannot be opened, a fallback default codepage name and converter will be used. On most platforms, this will be US-ASCII. For z/OS (OS/390), ibm-1047,swaplfnl is the default fallback codepage. For AS/400 (iSeries), ibm-37 is the default fallback codepage. This default fallback codepage is used when the operating system is using a non-standard name for a default codepage, or the converter was not packaged with ICU. The feature allows ICU to run in unusual computing environments without completely failing.

Usage Model

A "Converter" refers to the C structure "UConverter". Converters are cheap to create. Any data that is shared between converters of the same kind (such as the mappings, the name and the properties) are automatically cached and shared in memory.

Converter Names

Codepages with encoding schemes have been given many names by various vendors and platforms over the years. Vendors have different ways specify which codepage and encoding are being used. IBM uses a CCSID (Coded Character Set IDentifier). Windows uses a CPID (CodePage IDentifier). Macintosh has a TextEncoding. Many Unix vendors use IANA character set names. Many of these names are aliases to converters within ICU.

In order to help identify which names are recognized by certain platforms, ICU provides several converter alias functions. The complete description of these functions can be found in the ICU API Reference .

Function NamesShort Description
ucnv_countAvailable ucnv_getAvailableName Get a list of available converter names that can be opened.
ucnv_openAllNames Get a list of all known converter names.
ucnv_getName Get the name of an open converter.
ucnv_countAliases ucnv_getAlias Get the list of aliases for the specified converter.
ucnv_countStandards ucnv_getStandard Get the list of known standards.
ucnv_openStandardNames Get a filtered list of aliases for a converter that is known by the specified standard.
ucnv_getStandardName Get the preferred alias name specified by a given standard.
ucnv_getCanonicalName Get the converter name from the alias that is recognized by the specified standard.
ucnv_getDefaultName Get the default converter name that is currently used by ICU and the operating system.
ucnv_setDefaultName Use this function to override the default converter name.

Even though IANA specifies a list of aliases, it usually does not specify the mappings or the actual character set for the aliases. Sometimes vendors will map similar glyph variants to different Unicode code points or sometimes they will assign completely different glyphs for the same codepage code point. Because of these ambiguities, you can sometimes get U_AMBIGUOUS_ALIAS_WARNING for the returned UErrorCode when more than one converter uses the requested alias. This is only a warning, and the results can still be used. This UErrorCode value is just a reminder that you may not get what you expected. The above functions can help you to determine which converter you actually wanted.

EBCDIC based converters do have the option to swap the newline and linefeed character mappings. This can be useful when transferring EBCDIC documents between z/OS (MVS, os/390 and the rest of the zSeries family) and another EBCDIC machine like OS/400 on iSeries. The ",swaplnlf" or UCNV_SWAP_LFNL_OPTION_STRING from ucnv.h can be appended to a converter alias in order to achieve this behavior. You can view other available options in ucnv.h.

You can always skip many of these aliasing and mapping problems by just using Unicode.

Creating a Converter

There are four ways to create a converter:

  1. By name: Converters can be created using different types of names. No distinction is made when the converter is created, as to which name is being employed. There are many types of aliases possible. Among these are IANA ("shift_jis", "koi8-r", or "iso-8859-3"), host specific names ("cp1252" which is the name for a Microsoft® Windows™ or a similar IBM® codepage). Finally, ICU's own internal canonical names for a converter can be used. These include "UTF-8" or "ISO-8859-1" for built-in conversion types, and names such as "ibm-949_P110-2000" (Shift-JIS with '\' <-> '¥' mapping) or "ibm-949_P11A-2000" (Shift-JIS with '\' <-> '\' mapping) for data-file based conversions.

    UConverter *conv = ucnv_open("shift_jis", &myError);

    As a convenience, converter names can be passed in as Unicode. (for example, if a user passed in the string from a Unicode-based user interface). However, the actual names are restricted to an invariant ASCII/EBCDIC subset.
    UChar *name = ...; UConverter *conv = ucnv_openU(name, &myError);

    Converter names are case-insensitive. In addition, beginning with ICU 3.6, leading zeroes are ignored in sequences of digits (if further digits follow), and all non-alphanumeric characters are ignored. Thus the strings "UTF-8", "utf_8", "u*T@f08" and "Utf 8" are equivalent. (Before ICU 3.6, leading zeroes were not ignored, and only spaces, dashes and underscores were ignored.) The ucnv_compareNames() function provides such string comparisons.

    Unlike the names of resources or other types of ICU data, converter names can not be qualified with a path that indicates the directory or common data file containing the corresponding converter data. The requested converter's data must be present either in the main ICU data library or as a separate file located in the ICU data directory. However, you can always create a package of converters with pkgdata and open a converter from the package with ucnv_openPackage()

       UConverter *conv = ucnv_openPackage("./myPackage.dat",
                             "customConverter", &myError);

  2. By number: The design of the ICU is to accommodate codepages provided by different vendors. For example, the IBM CDRA (Character Data Representation Architecture which is an IBM architecture that defines a set of identifiers) has an ID type called the CCSID (Coded Character Set Identifier). The ICU API for opening a codepage by number must be given a vendor along with the number. Currently, only IBM (UCNV_IBM) is supported. For example, the US EBCDIC codepage (IBM #37) can be opened with the following code:

       ucnv_openCCSID( 37, UCNV_IBM, &myErr);

  3. By iteration: An application might not know ahead of time which codepage to use, and thus might need to query ICU to determine the entire list of installed converters. The ICU returns a list of its canonical (internal) names. From each names, the standard IANA name can be determined, and also a list of aliases which point to that name can be determined. For example, ICU might return among the canonical names "ibm-367". That name itself may or may not provide the application or its users with the information needed. (367 is actually the decimal form of a number that is calculated by appending certain hex digits together.) However, the IANA name can be requested from this canonical name, which should return something like "us-ascii". The alias list for ibm-367 can be iterated over as well, which returns additional names like "ascii", "646", "ansi_x3.4-1968" etc. If this is not sufficient information, once a converter is opened, it can be queried for its type, min and max char size, etc. This information is not available without actually opening the converter (a fairly lightweight process.)

       /* Returns count of the number of available names */
      int count = ucnv_countAvailable();

      /* get the canonical name of the 36th available converter */
      const char *convName1 = ucnv_getAvailableName(36);

      /* get the 3rd alias for a given codepage. */
      const char *asciiAlias = ucnv_getAlias("ibm-367", 3, &myError);

      /* Get the IANA name of the converter */
      const char *ascii = ucnv_getStandardName("ibm-367", "IANA");

      /* Get the one of the non preferred IANA name of the converter. */
      UEnumeration *asciiEnum =
           ucnv_openStandardNames("ibm-367", "IANA", &myError);
      uenum_next(asciiEnum, &myError); /* skip preferred IANA alias */
    /* get one of the non-preferred IANA aliases */
      const char *ascii2 = uenum_next(asciiEnum, &myError);

  4. By using the default converter: The default converter can be opened by passing a NULL as the name of the converter.

       ucnv_open(NULL, &myErr);

NoteICU chooses this converter based on the best information available to it. The purpose of this converter is to interface with the OS using a codepage (i.e. char*). Do not use it as a way of determining the best overall converter to use. Usually any Unicode encoding form is the best way to store and send text data so that important data does not get lost in the conversion.
Also, if the OS supports Unicode-based API's (such as Win32), it is better to use only those Unicode API's. As an example, the new Windows 2000 locales (such as Hindi) do not define the default codepage to something that supports Hindi. The default converter is used in expressions such as: UnicodeString text("abc"); .. to convert 'abc', and in the u_uastrcpy() C functions.
Code operating at the OS level MAY choose to change the default converter with ucnv_setDefaultName(). However, be aware that this change has inconsistent results if it is done after ICU components are initialized.

Closing a Converter

Closing a converter frees memory occupied by that instance of the converter. However it does not release the larger shared data tables the converter might use. OS-level code may call ucnv_flushCache() to explicitly free memory occupied by unused tables .


Converter Life Cycle

Note that a Converter is created with a certain type (for instance, ISO-8859-3) which does not change over the life of that object . Converters should be allocated one per thread. They are cheap to create, as the shared data doesn't need to be reallocated.

This is the typical life cycle of a converter, as shown step-by-step:

  1. First, open up the converter with a specified name [or alias name].

        UConverter *conv = ucnv_open("shift_jis", &status);

  2. Target here is the char s[] to write into, and targetSize is how big the target buffer is. Source is the UChars that are being converted.

        int32_t len = ucnv_fromUChars(conv, target, targetSize, source,
                                      u_strlen(source), &status);

  3. Clean up the converter.


Sharing Converters Between Threads

A converter cannot be shared between threads at the same time. However, if it is reset it can be used for unrelated chunks of data. For example, use the same converter for converting data from Unicode to ISO-8859-3, and then reset it. Use the same converter for converting data from ISO-8859-3 back into Unicode.

Converting Large Quantities of Data

If it is necessary to convert a large quantity of data in smaller buffers, use the same converter to convert each buffer. This will make sure any state is preserved from one chunk to the next. Doing this conversion is known as streaming or buffering, and is mentioned Buffered Conversion section (§) later in this chapter.

Cloning a Converter

Cloning a converter returns a clone of the converter object along with any internal state that the converter might be storing. Cloning routines must be used with extreme care when using converters for stateful or multibyte encodings. If the converter object is carrying an internal state, and the newly-created clone is used to convert a new chunk of text, the converter produces incorrect results. Also note that the caller owns the cloned object and has to call ucnv_close() to dispose of the object. Calling ucnv_reset() before cloning will reset the converter to its original state.

UConverter* newCnv = ucnv_safeClone(oldCnv, 0, &bufferSize, &err)

Converter Behavior


  1. The converters always consume the source buffer as far as possible, and advance the source pointer.

  2. The converters write to the target all converted output as far as possible, and then write any remaining output to the internal services buffer. When the conversion routines are called again, the internal buffer is flushed out and written to the target buffer before proceeding with any further conversion.

  3. In conversions to Unicode from Multi-byte encodings or conversions from Unicode involving surrogates, if a) only a partial byte sequence is retrieved from the source buffer, b) the "flush" parameter is set to "TRUE" and c) the end of source is reached, then the callback is called with U_TRUNCATED_CHAR_FOUND.


Converters can be reset explicitly or implicitly. Explicit reset is done by calling:

  1. ucnv_reset(): Resets the converter to initial state in both directions.

  2. ucnv_resetToUnicode(): Resets the converter to initial state to Unicode direction.

  3. ucnv_resetFromUnicode(): Resets the converter to initial state from Unicode direction.

The converters are reset implicitly when the conversion functions are called with the "flush" parameter set to "TRUE" and the source is consumed.


Conversion from Unicode

Not all characters can be converted from Unicode to other codepages. In most cases, Unicode is a superset of the characters supported by any given codepage.

The default behavior of ICU in this case is to substitute the illegal or unmappable sequence, with the appropriate substitution sequence for that codepage. For example, ISO-8859-1, along with most ASCII-based codepages, has the character 0x1A (Control-Z) as the substitution sequence. When converting from Unicode to ISO-8859-1, any characters which cannot be converted would be replaced by 0x1A's.

SubChar1 is sometimes used as substitution character in MBCS conversions. For more information on SubChar1 please see the Conversion Data chapter.

In stateful converters like ISO-2022-JP, if a substitution character has to be written to the target, then an escape/shift sequence to change the state to single byte mode followed by a substitution character is written to the target.

The substitution character can be changed by calling the ucnv_setSubstChars() function with the desired codepage byte sequence. However, this has some limitations: It only allows setting a single character (although the character can consist of multiple bytes), and it may not work properly for some stateful converters (like HZ or ISO 2022 variants) when setting a multi-byte substitution character. (It will work for EBCDIC_STATEFUL ones.) Moreover, for setting a particular character, the caller needs to know the correct byte sequence for that character in the converter's codepage. (For example, a space [U+0020] is encoded as 0x20 in ASCII-based codepages, 0x40 in EBCDIC-based ones, 0x00 0x20 or 0x20 0x00 in UTF-16 depending on the stream's endianness, etc.)

The ucnv_setSubstString() function (new in ICU 3.6) lifts these limitations. It takes a Unicode string and verifies that it can be converted to the codepage without error and that it is not too long (32 bytes as of ICU 3.6). The string can contain zero, one or more characters. An empty string has the effect of using the skip callback. See the Error Callbacks below. Stateful converters are fully supported. The same Unicode string will give equivalent results with all converters that support its conversion.

Internally, ucnv_setSubstString() stores the byte sequence from the test conversion if the converter is stateless, or the Unicode string itself if the converter is stateful. If the Unicode string is stored, then it is converted on the fly during substitution, handling all state transitions.

The function ucnv_getSubstChars() can be used to retrieve the substitution byte sequence if it is the default one, set by ucnv_setSubstChars(), or if ucnv_setSubstString() stored the byte sequence for a stateless converter. The Unicode string set for a stateful converter cannot be retrieved.

Conversion to Unicode

In conversion to Unicode, errors are normally due to ill-formed byte sequences: Unused byte values, or lead bytes not followed by trail bytes according to the encoding scheme. Well-formed but unmappable sequences are unusual but possible.

The ICU default behavior is to emit an U+FFFD REPLACEMENT CHARACTER per offending sequence.

If the conversion table .ucm file contains a <subchar1> entry (such as in the ibm-943 table), a U+001A C0 control ("SUB") is emitted for single-byte illegal/unmappable input rather than U+FFFD REPLACEMENT CHARACTER. For details on this behavior look for "001A" in the Conversion Data chapter.

  • This behavior originates from mainframes with dedicated single-byte-to-single-byte and double-to-double conversions.
  • Emitting U+001A for single-byte errors can be avoided by (a) removing the <subchar1> mapping or (b) using a similar conversion table that does not have this mapping (e.g., windows-932 instead of ibm-943) or (c) writing a custom callback function.

Error Codes

Here are some of the error codes which have significant meaning for conversion:

U_INDEX_OUTOFBOUNDS_ERROR in getNextUChar() - all source data has been consumed without producing a Unicode character
U_INVALID_CHAR_FOUND No mapping was found from the source to the target encoding. For example, U+0398 [Capital Theta] has no mapping into ISO-8859-1, and so U_INVALID_CHAR_FOUND will result.
U_TRUNCATED_CHAR_FOUND All of the source data was read, and a character sequence was incomplete. For example, only half of a double-byte sequence may have been encountered. When converting FROM Unicode, this error would occur when a conversion ends with a low surrogate (U+D800) at the end of the source, with no corresponding high surrogate.
U_ILLEGAL_CHAR_FOUND A character sequence was found in the source which is disallowed in the source encoding scheme. For example, many MBCS encodings have only certain byte sequences which are allowed as lead bytes. When converting from Unicode, if a low surrogate is NOT followed immediately by a high surrogate, or a high surrogate without its preceding low surrogate, an illegal sequence results.
Note: Most, but not all, converters forbid surrogate code points or unpaired surrogate code units. (Lead surrogate without trail, or trail without lead.) Some converters permit surrogate code points/unpaired surrogates because their charset specification permits it. For example, LMBCS, SCSU and BOCU-1.
U_INVALID_TABLE_FORMAT An error occurred trying to read the backing data for the converter. The data could be corrupt, or the wrong version.
U_BUFFER_OVERFLOW_ERROR More output (target) characters were produced than fit in the target buffer. If in to/fromUnicode() , then process the target buffer and call the function again to retrieve the overflowed characters.

Error Callbacks

What actually happens is that an "error callback function" is called at the point where the conversion failure occurred. The function can deal with the failed characters as it sees fit. Possible options at the callback's disposal include ignoring the bad sequence, converting it to a different sequence, and returning an error to the caller. The callback can also consume any data past where the error occurred, whether or not that data would have caused an error. Only one callback is installed at a time, per direction (to or from unicode).

A number of canned functions are provided by ICU, and an application can write new ones. The "callbacks" are either From Unicode (to codepage), or To Unicode (from codepage). Here is a list of the canned callbacks in ICU:

  1. UCNV_FROM_U_CALLBACK_SUBSTITUTE: This callback is installed by default. It will write the codepage's substitute sequence or a user-set substitute sequence, or convert a user-set substitute UnicodeString to the codepage. See "Error / Conversion from Unicode" above.

  2. UCNV_TO_U_CALLBACK_SUBSTITUTE: This callback is installed by default. It will write U+FFFD or sometimes U+001A. See "Error / Conversion to Unicode" above.

  3. UCNV_FROM_U_CALLBACK_SKIP, UCNV_TO_U_CALLBACK_SKIP: Simply ignores any invalid characters in the input, no error is returned.
  4. UCNV_FROM_U_CALLBACK_STOP, UCNV_TO_U_CALLBACK_STOP: Stop at the error. Return the error to the caller. (When using the 'BUFFER' mode of conversion, the source and target pointers returned can be examined to determine where the error occurred. ucnv_getInvalidUChars() and ucnv_getInvalidChars() return the actual text which failed).

  5. UCNV_FROM_U_CALLBACK_ESCAPE, UCNV_TO_U_CALLBACK_ESCAPE: This callback is especially useful for debugging. Missing codepage characters are replaced by strings such as '%U094D' with the Unicode value, and missing Unicode chars are replaced with text of the form '%X0A' where the codepage had the unconvertible byte hex 0A.

    When a callback is set, a "context" pointer is also provided. How this pointer is created depends on the specific callback. There is usually a createContext() function for that specific callback, where the caller can set certain options for the callback. Consult the documentation for the specific callback you are using. For ICU's canned callbacks, this pointer may be set to NULL. The functions for setting a different callback also return the old callback, and the old context pointer. These may be stored so that the old callback is re-installed when an operation is finished.

    Additionally the following options can be passed as the context parameter to UCNV_FROM_U_CALLBACK_ESCAPE callback function to produce different outputs.

    UCNV_ESCAPE_ICU      %U12345
    UCNV_ESCAPE_JAVA     \u1234
    UCNV_ESCAPE_C        \udbc9\udd36 for Plane 1 and
                         \u1234        for Plane 0 codepoints.
    UCNV_ESCAPE_XML_DEC &#4460;       number expressed in Decimal
    UCNV_ESCAPE_XML_HEX &#x1234;      number expressed in Hexadecimal.

Here are some examples of how to use callbacks.

UConverter              *u;
void                    *oldContext, *newContext;
UConverterFromUCallback oldAction, newAction;
u = ucnv_open("shift_jis", &myError);

... /* do some conversion with u from unicode.. */

    u, MY_FROMU_CALLBACK, newContext, &oldAction, &oldContext, &myError);

... /* do some other conversion from unicode */

/* Now, set the callback back */
    u, oldAction, oldContext, &newAction, &newContext, &myError);

Custom Callbacks

Writing a callback is somewhat involved, and will be covered more completely in a future version of this document. One might look at the source to the provided callbacks as a starting point, and address any further questions to the mailing list.

Basically, callback, unlike other ICU functions which expect to be called with U_ZERO_ERROR as the input, is called in an exceptional error condition. The callback is a kind of 'last ditch effort' to rectify the error which occurred, before it is returned back to the caller. This is why the implementation of STOP is very simple:


The error code such as U_INVALID_CHAR_FOUND is returned to the user. If the callback determines that no error should be returned to the user, then the callback must set the error code to U_ZERO_ERROR. Note that this is a departure from most ICU functions, which are supposed to check the error code and return immediately if it is set.

NoteSee the functions ucnv_cb_write...() for functions which a callback may use to perform its task.

Ignore Default_Ignorable_Code_Point

Unicode has a number of characters that are not by themselves meaningful but assist with line breaking (e.g., U+00AD Soft Hyphen & U+200B Zero Width Space), bi-directional text layout (U+200E Left-To-Right Mark), collation and other algorithms (U+034F Combining Grapheme Joiner), or indicate a preference for a particular glyph variant (U+FE0F Variation Selector 16). These characters are "invisible" by default, that is, they should normally not be shown with a glyph of their own, except in special circumstances. Examples include showing a hyphen for when a Soft Hyphen was used for a line break, or  modifying the glyph of a character preceding a Variation Selector.

Unicode has a character property to identify such characters, as well as currently-unassigned code points that are intended to be used for similar purposes: Default_Ignorable_Code_Point, or "DI" for short:[:DI:]

Most charsets do not have most or any of these characters.

ICU 54 and above by default skip default-ignorable code points if they are unmappable. (Ticket #10551)

Older versions of ICU replaced unmappable default-ignorable code points like any other unmappable code points, by a question mark or whatever substitution character is defined for the charset.

For best results, a custom from-Unicode callback can be used to ignore Default_Ignorable_Code_Point characters that cannot be converted, so that they are removed from the charset output rather than replaced by a visible character.

This is a code snippet for use in a custom from-Unicode callback:

#include "unicode/uchar.h"
(from-Unicode callback)
    switch(reason) {
        if(u_hasBinaryProperty(codePoint, UCHAR_DEFAULT_IGNORABLE_CODE_POINT)) {
            // Ignore/drop default ignorable code points that cannot be converted,
            // rather than treating them like errors/writing a substitution character etc.
            // For example, U+200B Zero Width Space,
            // U+200E Left-To-Right Mark, U+FE0F Variation Selector 16.
            *pErrorCode = U_ZERO_ERROR;
        } else {

Modes of Conversion

When a converter is instantiated, it can be used to convert both in the Unicode to Codepage direction, and also in the Codepage to Unicode direction. There are three ways to use the converters, as well as a convenience function which does not require the instantiation of a converter.

  1. Single-String: Simplest type of conversion to or from Unicode. The data is entirely contained within a single string.

  2. Character: Converting from the codepage to a single Unicode codepoint, one at a time.

  3. Buffer: Convert data which may not fit entirely within a single buffer. Usually the most efficient and flexible.

  4. Convenience: Convert a single buffer from one codepage to another through Unicode, without requiring the instantiation of a converter.

1. Single-String

Data must be contained entirely within a single string or buffer.

conv = ucnv_open("shift_jis", &status);

/* Convert from Unicode to Shift JIS */
len = ucnv_fromUChars(conv, target, targetLen, source, sourceLen, &status);

conv = ucnv_open("iso-8859-3", &status);
/* Convert from ISO-8859-3 to Unicode */
len = ucnv_toUChars(conv, target, targetSize, source, sourceLen, &status);

2. Character

In this type, the input data is in the specified codepage. With each function call, only the next Unicode codepoint is converted at a time. This might be the most efficient way to scan for a certain character, or other processing of a single character at a time, because converters are stateful. This works even for multibyte charsets, and for stateful ones such as iso-2022-jp.

conv = ucnv_open("Big-5", &status);
UChar32 target;
while(source < sourceLimit) {
    target = ucnv_getNextUChar(conv, &source, sourceLimit, &status);

3. Buffered or Streamed

This is used in situations where a large document may be read in off of disk and processed. Also, many codepages take multiple bytes to encode a character, or have state. These factors make it impossible to convert arbitrary chunks of data without maintaining state across chunks. Even conversion from Unicode may encounter a leading surrogate at the end of one buffer, which needs to be paired with the trailing surrogate in the next buffer.

A basic API principle of the ICU to/from Unicode functions is that they will ALWAYS attempt to consume all of the input (source) data, unless the output buffer is full or some other error occurs. In other words, there is no need to ever test whether all of the source data has been consumed.

The basic loop that is used with the ICU buffer conversion routines is the same in the to and from Unicode directions. In the following pseudocode, either 'source' (for fromUnicode) or 'target' (for toUnicode) are UTF-16 UChars.

UErrorCode err = U_ZERO_ERROR;

while (... /*input data available*/ ) {
    ... /* read input data into buffer */
    source = ... /* beginning of read data */;
    sourceLimit = source + readLength; // end + 1

    UBool flush = (further input data still available) // (i.e. feof())

    /* loop until all source has been processed */
    do {
        /* set up target pointers */
        target = ... /* beginning of output buffer */;
        targetLimit = target + sizeOfOutput;

        err = U_ZERO_ERROR; /* so that the to/from does not fail */

        ucnv_to/fromUnicode(converter, &target, targetLimit,
                    &source, sourceLimit, NULL, flush, &err);

        ... /* write (target-beginningOfOutputBuffer) items
               starting at beginning of output buffer */
    } while (err == U_BUFFER_OVERFLOW_ERROR);
    if(U_FAILURE(error)) {
        ... /* process error */
        break; /* out of the 'while' loop that reads source data */
/* loop to read input data */
if(U_FAILURE(error)) {
    ... /* process error further */

The above code optimizes for processing entire chunks of input data. An efficient size for the output buffer can be calculated as follows. (in bytes):

ucnv_getMinCharSize() * inputBufferSize * sizeof(UChar)
ucnv_getMaxCharSize() * inputBufferSize

There are two loops used, an outer and an inner. The outer loop fetches input data to keep the source buffer full, and the inner loop 'writes' out data to keep the output buffer empty.

Note that while this efficiently handles data on the input side, there are some cases where the size of the output buffer is fixed. For instance, in network applications it is sometimes desirable to fill every output packet completely (not including the last packet in the sequence). The above loop does not ensure that every output buffer is completely full. For example, if a 4 UChar input buffer was used, and a 3 byte output buffer with fromUnicode(), the loop would typically write 3 bytes, then 1, then 3, and so on. If, instead of efficient use of the input data, the goal is filling output buffers, a slightly different loop can be used.

In such a scenario, the inner write does not occur unless a buffer overflow occurs OR 'flush' is true. So, the 'write' and resetting of the target and targetLimit pointers would only happen if(err == U_BUFFER_OVERFLOW_ERROR || flush == TRUE)

The flush parameter on each conversion call should be set to FALSE, until the conversion call is called for the last time for the buffer. This is because the conversion is stateful. On the last conversion call, the flush parameter should be set to TRUE. More details are mentioned in the API reference in ucnv.h .

4. Pre-flighting

Preflighting is the process of asking the conversion API for the size of target buffer required. (For a more general discussion, see the Preflighting section (§) in the Strings chapter.)

This is accomplished by calling the ucnv_fromUChars and ucnv_toUChars functions.

UChar uchar2;
char input_char_buffer = "This is some text";

targetsize = ucnv_toUChars(myConverter, NULL, targetcapacity,
                           input_char_buffer, sizeof(input_char_buffer), &err);
    uchar2=(UChar*)malloc((targetsize) * sizeof(UChar));
    targetsize = ucnv_toUChars(myConverter, uchar2, targetsize,
                               input_char_buffer, sizeof(input_char_buffer), &err);
    if(U_FAILURE(err)) {
        printf("ucnv_toUChars() FAILED %s\n", myErrorName(err));
    else {
        printf("ucnv_toUChars() o.k.\n");
NoteThis is inefficient since the conversion is performed twice, once for finding the size of target and once for writing to the target.

5. Convenience

ICU provides some convenience functions for conversions:

ucnv_toUChars(myConverter, target_uchars, targetsize,
                           input_char_buffer, sizeof(input_char_buffer), &err);
ucnv_fromUChars(cnv, cTarget, (cTargetLimit-cTarget),
                     uSource, (uSourceLimit-uSource), &errorCode);

char target[100];
UnicodeString str("ABCDEF", "iso-8859-1");
int32_t targetsize = str.extract(0, str.length(), target, sizeof(target), "SJIS");
target[targetsize] = 0; /* NULL termination */

Conversion Examples

See the ICU Conversion Examples for more information.