path: root/doc/Encoding.3

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.TH Tcl_GetEncoding 3 "8.1" Tcl "Tcl Library Procedures"
.so man.macros
.BS
.SH NAME
Tcl_GetEncoding, Tcl_FreeEncoding, Tcl_GetEncodingFromObj, Tcl_ExternalToUtfDString, Tcl_ExternalToUtfDStringEx, Tcl_ExternalToUtf, Tcl_UtfToExternalDString, Tcl_UtfToExternalDStringEx, Tcl_UtfToExternal, Tcl_GetEncodingName, Tcl_SetSystemEncoding, Tcl_GetEncodingNameFromEnvironment, Tcl_GetEncodingNames, Tcl_CreateEncoding, Tcl_GetEncodingSearchPath, Tcl_SetEncodingSearchPath, Tcl_GetDefaultEncodingDir, Tcl_SetDefaultEncodingDir \- procedures for creating and using encodings
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Encoding
\fBTcl_GetEncoding\fR(\fIinterp, name\fR)
.sp
void
\fBTcl_FreeEncoding\fR(\fIencoding\fR)
.sp
int
\fBTcl_GetEncodingFromObj\fR(\fIinterp, objPtr, encodingPtr\fR)
.sp
char *
\fBTcl_ExternalToUtfDString\fR(\fIencoding, src, srcLen, dstPtr\fR)
.sp
int
\fBTcl_ExternalToUtfDStringEx\fR(\fIinterp, encoding, src, srcLen, flags, dstPtr, errorIdxPtr\fR)
.sp
char *
\fBTcl_UtfToExternalDString\fR(\fIencoding, src, srcLen, dstPtr\fR)
.sp
int
\fBTcl_UtfToExternalDStringEx\fR(\fIinterp, encoding, src, srcLen, flags, dstPtr, errorIdxPtr\fR)
.sp
int
\fBTcl_ExternalToUtf\fR(\fIinterp, encoding, src, srcLen, flags, statePtr,
                  dst, dstLen, srcReadPtr, dstWrotePtr, dstCharsPtr\fR)
.sp
int
\fBTcl_UtfToExternal\fR(\fIinterp, encoding, src, srcLen, flags, statePtr,
                  dst, dstLen, srcReadPtr, dstWrotePtr, dstCharsPtr\fR)
.sp
char *
\fBTcl_WinTCharToUtf\fR(\fItsrc, srcLen, dstPtr\fR)
.sp
TCHAR *
\fBTcl_WinUtfToTChar\fR(\fIsrc, srcLen, dstPtr\fR)
.sp
const char *
\fBTcl_GetEncodingName\fR(\fIencoding\fR)
.sp
int
\fBTcl_GetEncodingNulLength\fR(\fIencoding\fR)
.sp
int
\fBTcl_SetSystemEncoding\fR(\fIinterp, name\fR)
.sp
const char *
\fBTcl_GetEncodingNameFromEnvironment\fR(\fIbufPtr\fR)
.sp
void
\fBTcl_GetEncodingNames\fR(\fIinterp\fR)
.sp
Tcl_Encoding
\fBTcl_CreateEncoding\fR(\fItypePtr\fR)
.sp
Tcl_Obj *
\fBTcl_GetEncodingSearchPath\fR()
.sp
int
\fBTcl_SetEncodingSearchPath\fR(\fIsearchPath\fR)
.sp
const char *
\fBTcl_GetDefaultEncodingDir\fR(\fIvoid\fR)
.sp
void
\fBTcl_SetDefaultEncodingDir\fR(\fIpath\fR)
.SH ARGUMENTS
.AS "const Tcl_EncodingType" *dstWrotePtr in/out
.AP Tcl_Interp *interp in
Interpreter to use for error reporting, or NULL if no error reporting is
desired.
.AP "const char" *name in
Name of encoding to load.
.AP Tcl_Encoding encoding in
The encoding to query, free, or use for converting text.  If \fIencoding\fR is
NULL, the current system encoding is used.
.AP Tcl_Obj *objPtr in
Name of encoding to get token for.
.AP Tcl_Encoding *encodingPtr out
Points to storage where encoding token is to be written.
.AP "const char" *src in
For the \fBTcl_ExternalToUtf\fR functions, an array of bytes in the
specified encoding that are to be converted to UTF-8.  For the
\fBTcl_UtfToExternal\fR and \fBTcl_WinUtfToTChar\fR functions, an array of
UTF-8 characters to be converted to the specified encoding.
.AP "const TCHAR" *tsrc in
An array of Windows TCHAR characters to convert to UTF-8.
.AP int srcLen in
Length of \fIsrc\fR or \fItsrc\fR in bytes.  If the length is negative, the
encoding-specific length of the string is used.
.AP Tcl_DString *dstPtr out
Pointer to an uninitialized or free \fBTcl_DString\fR in which the converted
result will be stored.
.AP int flags in
This is a bit mask passed in to control the operation of the encoding functions.
\fBTCL_ENCODING_START\fR signifies that the
source buffer is the first block in a (potentially multi-block) input
stream, telling the conversion routine to reset to an initial state and
perform any initialization that needs to occur before the first byte is
converted. \fBTCL_ENCODING_END\fR signifies that the source buffer is the last
block in a (potentially multi-block) input stream, telling the conversion
routine to perform any finalization that needs to occur after the last
byte is converted and then to reset to an initial state. The
\fBTCL_PROFILE_*\fR bits defined in the \fBPROFILES\fR section below
control the encoding profile to be used for dealing with invalid data or
other errors in the encoding transform.
\fBTCL_ENCODING_STOPONERROR\fR is present for backward compatibility with
Tcl 8.6 and forces the encoding profile to \fBstrict\fR.

Some flags bits may not be usable with some functions as noted in the
function descriptions below.
.AP Tcl_EncodingState *statePtr in/out
Used when converting a (generally long or indefinite length) byte stream
in a piece-by-piece fashion.  The conversion routine stores its current
state in \fI*statePtr\fR after \fIsrc\fR (the buffer containing the
current piece) has been converted; that state information must be passed
back when converting the next piece of the stream so the conversion
routine knows what state it was in when it left off at the end of the
last piece.  May be NULL, in which case the value specified for \fIflags\fR
is ignored and the source buffer is assumed to contain the complete string to
convert.
.AP char *dst out
Buffer in which the converted result will be stored.  No more than
\fIdstLen\fR bytes will be stored in \fIdst\fR.
.AP int dstLen in
The maximum length of the output buffer \fIdst\fR in bytes.
.AP int *srcReadPtr out
Filled with the number of bytes from \fIsrc\fR that were actually
converted.  This may be less than the original source length if there was
a problem converting some source characters.  May be NULL.
.AP int *dstWrotePtr out
Filled with the number of bytes that were actually stored in the output
buffer as a result of the conversion.  May be NULL.
.AP int *dstCharsPtr out
Filled with the number of characters that correspond to the number of bytes
stored in the output buffer.  May be NULL.
.AP int *errorIdxPtr out
Filled with the index of the byte or character that caused the encoding transform
to fail. May be NULL.
.AP Tcl_DString *bufPtr out
Storage for the prescribed system encoding name.
.AP "const Tcl_EncodingType" *typePtr in
Structure that defines a new type of encoding.
.AP Tcl_Obj *searchPath in
List of filesystem directories in which to search for encoding data files.
.AP "const char" *path in
A path to the location of the encoding file.
.BE
.SH INTRODUCTION
.PP
These routines convert between Tcl's internal character representation,
UTF-8, and character representations used by various operating systems or
file systems, such as Unicode, ASCII, or Shift-JIS.  When operating on
strings, such as such as obtaining the names of files or displaying
characters using international fonts, the strings must be translated into
one or possibly multiple formats that the various system calls can use.  For
instance, on a Japanese Unix workstation, a user might obtain a filename
represented in the EUC-JP file encoding and then translate the characters to
the jisx0208 font encoding in order to display the filename in a Tk widget.
The purpose of the encoding package is to help bridge the translation gap.
UTF-8 provides an intermediate staging ground for all the various
encodings.  In the example above, text would be translated into UTF-8 from
whatever file encoding the operating system is using.  Then it would be
translated from UTF-8 into whatever font encoding the display routines
require.
.PP
Some basic encodings are compiled into Tcl.  Others can be defined by the
user or dynamically loaded from encoding files in a
platform-independent manner.
.SH DESCRIPTION
.PP
\fBTcl_GetEncoding\fR finds an encoding given its \fIname\fR.  The name may
refer to a built-in Tcl encoding, a user-defined encoding registered by
calling \fBTcl_CreateEncoding\fR, or a dynamically-loadable encoding
file.  The return value is a token that represents the encoding and can be
used in subsequent calls to procedures such as \fBTcl_GetEncodingName\fR,
\fBTcl_FreeEncoding\fR, and \fBTcl_UtfToExternal\fR.  If the name did not
refer to any known or loadable encoding, NULL is returned and an error
message is returned in \fIinterp\fR.
.PP
The encoding package maintains a database of all encodings currently in use.
The first time \fIname\fR is seen, \fBTcl_GetEncoding\fR returns an
encoding with a reference count of 1.  If the same \fIname\fR is requested
further times, then the reference count for that encoding is incremented
without the overhead of allocating a new encoding and all its associated
data structures.
.PP
When an \fIencoding\fR is no longer needed, \fBTcl_FreeEncoding\fR
should be called to release it.  When an \fIencoding\fR is no longer in use
anywhere (i.e., it has been freed as many times as it has been gotten)
\fBTcl_FreeEncoding\fR will release all storage the encoding was using
and delete it from the database.
.PP
\fBTcl_GetEncodingFromObj\fR treats the string representation of
\fIobjPtr\fR as an encoding name, and finds an encoding with that
name, just as \fBTcl_GetEncoding\fR does. When an encoding is found,
it is cached within the \fBobjPtr\fR value for future reference, the
\fBTcl_Encoding\fR token is written to the storage pointed to by
\fIencodingPtr\fR, and the value \fBTCL_OK\fR is returned. If no such
encoding is found, the value \fBTCL_ERROR\fR is returned, and no
writing to \fB*\fR\fIencodingPtr\fR takes place. Just as with
\fBTcl_GetEncoding\fR, the caller should call \fBTcl_FreeEncoding\fR
on the resulting encoding token when that token will no longer be
used.
.PP
\fBTcl_ExternalToUtfDString\fR converts a source buffer \fIsrc\fR from the
specified \fIencoding\fR into UTF-8.  The converted bytes are stored in
\fIdstPtr\fR, which is then null-terminated.  The caller should eventually
call \fBTcl_DStringFree\fR to free any information stored in \fIdstPtr\fR.
When converting, if any of the characters in the source buffer cannot be
represented in the target encoding, a default fallback character will be
used.  The return value is a pointer to the value stored in the DString.
.PP
\fBTcl_ExternalToUtfDStringEx\fR is a more flexible version of older
\fBTcl_ExternalToUtfDString\fR function. It takes three additional parameters,
\fBinterp\fR, \fBflags\fR and \fBerrorIdxPtr\fR. The \fBflags\fR parameter may
be used to specify the profile to be used for the transform. The
\fBTCL_ENCODING_START\fR and \fBTCL_ENCODING_END\fR bits in \fBflags\fR are
ignored as the function assumes the entire source string to be decoded is passed
into the function. On success, the function returns \fBTCL_OK\fR with the
converted string stored in \fB*dstPtr\fR. For errors \fIother than conversion
errors\fR, such as invalid flags, the function returns \fBTCL_ERROR\fR with an
error message in \fBinterp\fR if it is not NULL.
For conversion errors, \fBTcl_ExternalToUtfDStringEx\fR returns one
of the \fBTCL_CONVERT_*\fR errors listed below for \fBTcl_ExternalToUtf\fR.
When one of these conversion errors is returned, an error message is stored
in \fBinterp\fR only if \fBerrorIdxPtr\fR is NULL. Otherwise, no error message
is stored as the function expects the caller is interested the decoded data
up to that point and not treating this as an immediate error condition.
The index of the error location is stored in \fB*errorIdxPtr\fR.
.PP
The caller must call \fBTcl_DStringFree\fR to free up the \fB*dstPtr\fR resources
irrespective of the return value from the function.
.PP
\fBTcl_ExternalToUtf\fR converts a source buffer \fIsrc\fR from the specified
\fIencoding\fR into UTF-8.  Up to \fIsrcLen\fR bytes are converted from the
source buffer and up to \fIdstLen\fR converted bytes are stored in \fIdst\fR.
In all cases, \fI*srcReadPtr\fR is filled with the number of bytes that were
successfully converted from \fIsrc\fR and \fI*dstWrotePtr\fR is filled with
the corresponding number of bytes that were stored in \fIdst\fR.  The return
value is one of the following:
.RS
.IP \fBTCL_OK\fR 29
All bytes of \fIsrc\fR were converted.
.IP \fBTCL_CONVERT_NOSPACE\fR 29
The destination buffer was not large enough for all of the converted data; as
many characters as could fit were converted though.
.IP \fBTCL_CONVERT_MULTIBYTE\fR 29
The last few bytes in the source buffer were the beginning of a multibyte
sequence, but more bytes were needed to complete this sequence.  A
subsequent call to the conversion routine should pass a buffer containing
the unconverted bytes that remained in \fIsrc\fR plus some further bytes
from the source stream to properly convert the formerly split-up multibyte
sequence.
.IP \fBTCL_CONVERT_SYNTAX\fR 29
The source buffer contained an invalid byte or character sequence.  This may
occur if the input stream has been damaged or if the input encoding method was
misidentified.
.IP \fBTCL_CONVERT_UNKNOWN\fR 29
The source buffer contained a character that could not be represented in
the target encoding.
.RE
.LP
\fBTcl_UtfToExternalDString\fR converts a source buffer \fIsrc\fR from UTF-8
into the specified \fIencoding\fR.  The converted bytes are stored in
\fIdstPtr\fR, which is then terminated with the appropriate encoding-specific
null.  The caller should eventually call \fBTcl_DStringFree\fR to free any
information stored in \fIdstPtr\fR.  When converting, if any of the
characters in the source buffer cannot be represented in the target
encoding, a default fallback character will be used.  The return value is
a pointer to the value stored in the DString.
.PP
\fBTcl_UtfToExternalDStringEx\fR is an enhanced version of
\fBTcl_UtfToExternalDString\fR that transforms UTF-8 encoded source data to a
specified \fIencoding\fR. Except for the direction of the transform, the
parameters and return values are identical to those of
\fBTcl_ExternalToUtfDStringEx\fR. See
that function above for details about the same.
.PP
Irrespective of the return code from the function, the caller must free
resources associated with \fB*dstPtr\fR when the function returns.
.PP
\fBTcl_UtfToExternal\fR converts a source buffer \fIsrc\fR from UTF-8 into
the specified \fIencoding\fR.  Up to \fIsrcLen\fR bytes are converted from
the source buffer and up to \fIdstLen\fR converted bytes are stored in
\fIdst\fR.  In all cases, \fI*srcReadPtr\fR is filled with the number of
bytes that were successfully converted from \fIsrc\fR and \fI*dstWrotePtr\fR
is filled with the corresponding number of bytes that were stored in
\fIdst\fR.  The return values are the same as the return values for
\fBTcl_ExternalToUtf\fR.
.PP
\fBTcl_WinUtfToTChar\fR and \fBTcl_WinTCharToUtf\fR are Windows-only
convenience functions for converting between UTF-8 and Windows strings
based on the TCHAR type which is by convention a Unicode character on
Windows NT. Those functions are deprecated. You can use
\fBTcl_UtfToWCharDString\fR resp. \fBTcl_WCharToUtfDString\fR as replacement.
If you want compatibility with earlier Tcl releases than 8.7, use
\fBTcl_UtfToUniCharDString\fR resp. \fBTcl_UniCharToUtfDString\fR as
replacement, and make sure you compile your extension with -DTCL_UTF_MAX=3.
Beware: Those replacement functions don't initialize their Tcl_DString (you'll
have to do that yourself), and \fBTcl_UniCharToUtfDString\fR from Tcl 8.6
doesn't accept -1 as length parameter.
.PP
\fBTcl_GetEncodingName\fR is roughly the inverse of \fBTcl_GetEncoding\fR.
Given an \fIencoding\fR, the return value is the \fIname\fR argument that
was used to create the encoding.  The string returned by
\fBTcl_GetEncodingName\fR is only guaranteed to persist until the
\fIencoding\fR is deleted.  The caller must not modify this string.
.PP
\fBTcl_GetEncodingNulLength\fR returns the length of the terminating
nul byte sequence for strings in the specified encoding.
.PP
\fBTcl_SetSystemEncoding\fR sets the default encoding that should be used
whenever the user passes a NULL value for the \fIencoding\fR argument to
any of the other encoding functions.  If \fIname\fR is NULL, the system
encoding is reset to the default system encoding, \fBbinary\fR.  If the
name did not refer to any known or loadable encoding, \fBTCL_ERROR\fR is
returned and an error message is left in \fIinterp\fR.  Otherwise, this
procedure increments the reference count of the new system encoding,
decrements the reference count of the old system encoding, and returns
\fBTCL_OK\fR.
.PP
\fBTcl_GetEncodingNameFromEnvironment\fR provides a means for the Tcl
library to report the encoding name it believes to be the correct one
to use as the system encoding, based on system calls and examination of
the environment suitable for the platform.  It accepts \fIbufPtr\fR,
a pointer to an uninitialized or freed \fBTcl_DString\fR and writes
the encoding name to it.  The \fBTcl_DStringValue\fR is returned.
.PP
\fBTcl_GetEncodingNames\fR sets the \fIinterp\fR result to a list
consisting of the names of all the encodings that are currently defined
or can be dynamically loaded, searching the encoding path specified by
\fBTcl_SetDefaultEncodingDir\fR.  This procedure does not ensure that the
dynamically-loadable encoding files contain valid data, but merely that they
exist.
.PP
\fBTcl_CreateEncoding\fR defines a new encoding and registers the C
procedures that are called back to convert between the encoding and
UTF-8.  Encodings created by \fBTcl_CreateEncoding\fR are thereafter
visible in the database used by \fBTcl_GetEncoding\fR.  Just as with the
\fBTcl_GetEncoding\fR procedure, the return value is a token that
represents the encoding and can be used in subsequent calls to other
encoding functions.  \fBTcl_CreateEncoding\fR returns an encoding with a
reference count of 1. If an encoding with the specified \fIname\fR
already exists, then its entry in the database is replaced with the new
encoding; the token for the old encoding will remain valid and continue
to behave as before, but users of the new token will now call the new
encoding procedures.
.PP
The \fItypePtr\fR argument to \fBTcl_CreateEncoding\fR contains information
about the name of the encoding and the procedures that will be called to
convert between this encoding and UTF-8.  It is defined as follows:
.PP
.CS
typedef struct {
    const char *\fIencodingName\fR;
    Tcl_EncodingConvertProc *\fItoUtfProc\fR;
    Tcl_EncodingConvertProc *\fIfromUtfProc\fR;
    Tcl_EncodingFreeProc *\fIfreeProc\fR;
    void *\fIclientData\fR;
    int \fInullSize\fR;
} \fBTcl_EncodingType\fR;
.CE
.PP
The \fIencodingName\fR provides a string name for the encoding, by
which it can be referred in other procedures such as
\fBTcl_GetEncoding\fR.  The \fItoUtfProc\fR refers to a callback
procedure to invoke to convert text from this encoding into UTF-8.
The \fIfromUtfProc\fR refers to a callback procedure to invoke to
convert text from UTF-8 into this encoding.  The \fIfreeProc\fR refers
to a callback procedure to invoke when this encoding is deleted.  The
\fIfreeProc\fR field may be NULL.  The \fIclientData\fR contains an
arbitrary one-word value passed to \fItoUtfProc\fR, \fIfromUtfProc\fR,
and \fIfreeProc\fR whenever they are called.  Typically, this is a
pointer to a data structure containing encoding-specific information
that can be used by the callback procedures.  For instance, two very
similar encodings such as \fBascii\fR and \fBmacRoman\fR may use the
same callback procedure, but use different values of \fIclientData\fR
to control its behavior.  The \fInullSize\fR specifies the number of
zero bytes that signify end-of-string in this encoding.  It must be
\fB1\fR (for single-byte or multi-byte encodings like ASCII or
Shift-JIS) or \fB2\fR (for double-byte encodings like Unicode).
Constant-sized encodings with 3 or more bytes per character (such as
CNS11643) are not accepted.
.PP
The callback procedures \fItoUtfProc\fR and \fIfromUtfProc\fR should match the
type \fBTcl_EncodingConvertProc\fR:
.PP
.CS
typedef int \fBTcl_EncodingConvertProc\fR(
        void *\fIclientData\fR,
        const char *\fIsrc\fR,
        int \fIsrcLen\fR,
        int \fIflags\fR,
        Tcl_EncodingState *\fIstatePtr\fR,
        char *\fIdst\fR,
        int \fIdstLen\fR,
        int *\fIsrcReadPtr\fR,
        int *\fIdstWrotePtr\fR,
        int *\fIdstCharsPtr\fR);
.CE
.PP
The \fItoUtfProc\fR and \fIfromUtfProc\fR procedures are called by the
\fBTcl_ExternalToUtf\fR or \fBTcl_UtfToExternal\fR family of functions to
perform the actual conversion.  The \fIclientData\fR parameter to these
procedures is the same as the \fIclientData\fR field specified to
\fBTcl_CreateEncoding\fR when the encoding was created.  The remaining
arguments to the callback procedures are the same as the arguments,
documented at the top, to \fBTcl_ExternalToUtf\fR or
\fBTcl_UtfToExternal\fR, with the following exceptions.  If the
\fIsrcLen\fR argument to one of those high-level functions is negative,
the value passed to the callback procedure will be the appropriate
encoding-specific string length of \fIsrc\fR.  If any of the \fIsrcReadPtr\fR,
\fIdstWrotePtr\fR, or \fIdstCharsPtr\fR arguments to one of the high-level
functions is NULL, the corresponding value passed to the callback
procedure will be a non-NULL location.
.PP
The callback procedure \fIfreeProc\fR, if non-NULL, should match the type
\fBTcl_EncodingFreeProc\fR:
.PP
.CS
typedef void \fBTcl_EncodingFreeProc\fR(
        void *\fIclientData\fR);
.CE
.PP
This \fIfreeProc\fR function is called when the encoding is deleted.  The
\fIclientData\fR parameter is the same as the \fIclientData\fR field
specified to \fBTcl_CreateEncoding\fR when the encoding was created.
.PP
\fBTcl_GetEncodingSearchPath\fR and \fBTcl_SetEncodingSearchPath\fR
are called to access and set the list of filesystem directories searched
for encoding data files.
.PP
The value returned by \fBTcl_GetEncodingSearchPath\fR
is the value stored by the last successful call to
\fBTcl_SetEncodingSearchPath\fR.  If no calls to
\fBTcl_SetEncodingSearchPath\fR have occurred, Tcl will compute an initial
value based on the environment.  There is one encoding search path for the
entire process, shared by all threads in the process.
.PP
\fBTcl_SetEncodingSearchPath\fR stores \fIsearchPath\fR and returns
\fBTCL_OK\fR, unless \fIsearchPath\fR is not a valid Tcl list, which
causes \fBTCL_ERROR\fR to be returned.  The elements of \fIsearchPath\fR
are not verified as existing readable filesystem directories.  When
searching for encoding data files takes place, and non-existent or
non-readable filesystem directories on the \fIsearchPath\fR are silently
ignored.
.PP
\fBTcl_GetDefaultEncodingDir\fR and \fBTcl_SetDefaultEncodingDir\fR
are obsolete interfaces best replaced with calls to
\fBTcl_GetEncodingSearchPath\fR and \fBTcl_SetEncodingSearchPath\fR.
They are called to access and set the first element of the \fIsearchPath\fR
list.  Since Tcl searches \fIsearchPath\fR for encoding data files in
list order, these routines establish the
.QW default
directory in which to find encoding data files.
.SH "ENCODING FILES"
Space would prohibit precompiling into Tcl every possible encoding
algorithm, so many encodings are stored on disk as dynamically-loadable
encoding files.  This behavior also allows the user to create additional
encoding files that can be loaded using the same mechanism.  These
encoding files contain information about the tables and/or escape
sequences used to map between an external encoding and Unicode.  The
external encoding may consist of single-byte, multi-byte, or double-byte
characters.
.PP
Each dynamically-loadable encoding is represented as a text file.  The
initial line of the file, beginning with a
.QW #
symbol, is a comment
that provides a human-readable description of the file.  The next line
identifies the type of encoding file.  It can be one of the following
letters:
.IP "[1] \fBS\fR"
A single-byte encoding, where one character is always one byte long in the
encoding.  An example is \fBiso8859-1\fR, used by many European languages.
.IP "[2] \fBD\fR"
A double-byte encoding, where one character is always two bytes long in the
encoding.  An example is \fBbig5\fR, used for Chinese text.
.IP "[3] \fBM\fR"
A multi-byte encoding, where one character may be either one or two bytes long.
Certain bytes are lead bytes, indicating that another byte must follow
and that together the two bytes represent one character.  Other bytes are not
lead bytes and represent themselves.  An example is \fBshiftjis\fR, used by
many Japanese computers.
.IP "[4] \fBE\fR"
An escape-sequence encoding, specifying that certain sequences of bytes
do not represent characters, but commands that describe how following bytes
should be interpreted.
.PP
The rest of the lines in the file depend on the type.
.PP
Cases [1], [2], and [3] are collectively referred to as table-based encoding
files.  The lines in a table-based encoding file are in the same
format as this example taken from the \fBshiftjis\fR encoding (this is not
the complete file):
.PP
.CS
# Encoding file: shiftjis, multi-byte
M
003F 0 40
00
0000000100020003000400050006000700080009000A000B000C000D000E000F
0010001100120013001400150016001700180019001A001B001C001D001E001F
0020002100220023002400250026002700280029002A002B002C002D002E002F
0030003100320033003400350036003700380039003A003B003C003D003E003F
0040004100420043004400450046004700480049004A004B004C004D004E004F
0050005100520053005400550056005700580059005A005B005C005D005E005F
0060006100620063006400650066006700680069006A006B006C006D006E006F
0070007100720073007400750076007700780079007A007B007C007D203E007F
0080000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000FF61FF62FF63FF64FF65FF66FF67FF68FF69FF6AFF6BFF6CFF6DFF6EFF6F
FF70FF71FF72FF73FF74FF75FF76FF77FF78FF79FF7AFF7BFF7CFF7DFF7EFF7F
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.CE
.PP
The third line of the file is three numbers.  The first number is the
fallback character (in base 16) to use when converting from UTF-8 to this
encoding.  The second number is a \fB1\fR if this file represents the
encoding for a symbol font, or \fB0\fR otherwise.  The last number (in base
10) is how many pages of data follow.
.PP
Subsequent lines in the example above are pages that describe how to map
from the encoding into 2-byte Unicode.  The first line in a page identifies
the page number.  Following it are 256 double-byte numbers, arranged as 16
rows of 16 numbers.  Given a character in the encoding, the high byte of
that character is used to select which page, and the low byte of that
character is used as an index to select one of the double-byte numbers in
that page \- the value obtained being the corresponding Unicode character.
By examination of the example above, one can see that the characters 0x7E
and 0x8163 in \fBshiftjis\fR map to 203E and 2026 in Unicode, respectively.
.PP
Following the first page will be all the other pages, each in the same
format as the first: one number identifying the page followed by 256
double-byte Unicode characters.  If a character in the encoding maps to the
Unicode character 0000, it means that the character does not actually exist.
If all characters on a page would map to 0000, that page can be omitted.
.PP
Case [4] is the escape-sequence encoding file.  The lines in an this type of
file are in the same format as this example taken from the \fBiso2022-jp\fR
encoding:
.PP
.CS
.ta 1.5i
# Encoding file: iso2022-jp, escape-driven
E
init		{}
final		{}
iso8859-1	\ex1B(B
jis0201		\ex1B(J
jis0208		\ex1B$@
jis0208		\ex1B$B
jis0212		\ex1B$(D
gb2312		\ex1B$A
ksc5601		\ex1B$(C
.CE
.PP
In the file, the first column represents an option and the second column
is the associated value.  \fBinit\fR is a string to emit or expect before
the first character is converted, while \fBfinal\fR is a string to emit
or expect after the last character.  All other options are names of
table-based encodings; the associated value is the escape-sequence that
marks that encoding.  Tcl syntax is used for the values; in the above
example, for instance,
.QW \fB{}\fR
represents the empty string and
.QW \fB\ex1B\fR
represents character 27.
.PP
When \fBTcl_GetEncoding\fR encounters an encoding \fIname\fR that has not
been loaded, it attempts to load an encoding file called \fIname\fB.enc\fR
from the \fBencoding\fR subdirectory of each directory that Tcl searches
for its script library.  If the encoding file exists, but is
malformed, an error message will be left in \fIinterp\fR.
.SH "REFERENCE COUNT MANAGEMENT"
.PP
\fBTcl_GetEncodingFromObj\fR does not modify the reference count of its
\fIobjPtr\fR argument; it only reads. Note however that this function may set
the interpreter result; if that is the only place that is holding a reference
to the object, it will be deleted.
.PP
\fBTcl_GetEncodingSearchPath\fR returns an object with a reference count of at
least 1.
.SH "PROFILES"
Encoding profiles define the manner in which errors in the encoding transforms
are handled by the encoding functions. An application can specify the profile
to be used by OR-ing the \fBflags\fR parameter passed to the function
with at most one of \fBTCL_ENCODING_PROFILE_TCL8\fR,
\fBTCL_ENCODING_PROFILE_STRICT\fR or \fBTCL_ENCODING_PROFILE_REPLACE\fR.
These correspond to the \fBtcl8\fR, \fBstrict\fR and \fBreplace\fR profiles
respectively. If none are specified, a version-dependent default profile is used.
For Tcl 8.7, the default profile is \fBtcl8\fR.
.PP
For details about profiles, see the \fBPROFILES\fR section in
the documentation of the \fBencoding\fR command.
.SH "SEE ALSO"
encoding(n)
.SH KEYWORDS
utf, encoding, convert