ified in the class command, dbName gives
'\"	the option's name in the option database, and dbClass gives
'\"	the option's class in the option database.
'\"
'\" .UL arg1 arg2
'\"	Print arg1 underlined, then print arg2 normally.
'\"
'\" RCS: @(#) $Id: man.macros,v 1.4 2000/08/25 06:18:32 ericm Exp $
'\"
'\"	# Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
.if t .wh -1.3i ^B
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.AS Tcl_Interp Tcl_CreateInterp in/out
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.RE
.sp
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.de SO
.SH "STANDARD OPTIONS"
.LP
.nf
.ta 5.5c 11c
.ft B
..
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.de SE
.fi
.ft R
.LP
See the \\fBoptions\\fR manual entry for details on the standard options.
..
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.de OP
.LP
.nf
.ta 4c
Command-Line Name:	\\fB\\$1\\fR
Database Name:	\\fB\\$2\\fR
Database Class:	\\fB\\$3\\fR
.fi
.IP
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.de UL
\\$1\l'|0\(ul'\\$2
..
.TH Tcl_OpenFileChannel 3 8.3 Tcl "Tcl Library Procedures"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_OpenFileChannel, Tcl_OpenCommandChannel, Tcl_MakeFileChannel, Tcl_GetChannel, Tcl_GetChannelNames, Tcl_GetChannelNamesEx, Tcl_RegisterChannel, Tcl_UnregisterChannel, Tcl_DetachChannel, Tcl_IsStandardChannel, Tcl_Close, Tcl_ReadChars, Tcl_Read, Tcl_GetsObj, Tcl_Gets, Tcl_WriteObj, Tcl_WriteChars, Tcl_Write, Tcl_Flush, Tcl_Seek, Tcl_Tell, Tcl_GetChannelOption, Tcl_SetChannelOption, Tcl_Eof, Tcl_InputBlocked, Tcl_InputBuffered, Tcl_OutputBuffered, Tcl_Ungets, Tcl_ReadRaw, Tcl_WriteRaw \- buffered I/O facilities using channels
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Channel
\fBTcl_OpenFileChannel\fR(\fIinterp, fileName, mode, permissions\fR)
.sp
Tcl_Channel
\fBTcl_OpenCommandChannel\fR(\fIinterp, argc, argv, flags\fR)
.sp
Tcl_Channel
\fBTcl_MakeFileChannel\fR(\fIhandle, readOrWrite\fR)
.sp
Tcl_Channel
\fBTcl_GetChannel\fR(\fIinterp, channelName, modePtr\fR)
.VS 8.3
.sp
int
\fBTcl_GetChannelNames\fR(\fIinterp\fR)
.sp
int
\fBTcl_GetChannelNamesEx\fR(\fIinterp, pattern\fR)
.VE
.sp
void
\fBTcl_RegisterChannel\fR(\fIinterp, channel\fR)
.sp
int
\fBTcl_UnregisterChannel\fR(\fIinterp, channel\fR)
.sp
int
\fBTcl_DetachChannel\fR(\fIinterp, channel\fR)
.sp
int
\fBTcl_IsStandardChannel\fR(\fIchannel\fR)
.sp
int
\fBTcl_Close\fR(\fIinterp, channel\fR)
.sp
.VS 8.1
int
\fBTcl_ReadChars\fR(\fIchannel, readObjPtr, charsToRead, appendFlag\fR)
.sp
int
\fBTcl_Read\fR(\fIchannel, readBuf, bytesToRead\fR)
.sp
int
\fBTcl_GetsObj\fR(\fIchannel, lineObjPtr\fR)
.sp
int
\fBTcl_Gets\fR(\fIchannel, lineRead\fR)
.sp
int
\fBTcl_Ungets\fR(\fIchannel, input, inputLen, addAtEnd\fR)
.sp
int
\fBTcl_WriteObj\fR(\fIchannel, writeObjPtr\fR)
.sp
int
\fBTcl_WriteChars\fR(\fIchannel, charBuf, bytesToWrite\fR)
.sp
int
\fBTcl_Write\fR(\fIchannel, byteBuf, bytesToWrite\fR)
.VE
.VS 8.3.2
.sp
int
\fBTcl_ReadRaw\fR(\fIchannel, readBuf, bytesToRead\fR)
.sp
int
\fBTcl_WriteRaw\fR(\fIchannel, byteBuf, bytesToWrite\fR)
.VE
.sp
int
\fBTcl_Eof\fR(\fIchannel\fR)
.sp
int
\fBTcl_Flush\fR(\fIchannel\fR)
.sp
int
\fBTcl_InputBlocked\fR(\fIchannel\fR)
.sp
int
\fBTcl_InputBuffered\fR(\fIchannel\fR)
.VS 8.4
.sp
int
\fBTcl_OutputBuffered\fR(\fIchannel\fR)
.VE
.sp
.VS 8.4
Tcl_WideInt
\fBTcl_Seek\fR(\fIchannel, offset, seekMode\fR)
.sp
Tcl_WideInt
\fBTcl_Tell\fR(\fIchannel\fR)
.VE 8.4
.sp
int
\fBTcl_GetChannelOption\fR(\fIinterp, channel, optionName, optionValue\fR)
.sp
int
\fBTcl_SetChannelOption\fR(\fIinterp, channel, optionName, newValue\fR)
.sp
.SH ARGUMENTS
.AS Tcl_ChannelType newClientProcPtr in
.AP Tcl_Interp *interp in
Used for error reporting and to look up a channel registered in it.
.AP "CONST char" *fileName in
The name of a local or network file.
.AP "CONST char" *mode in
Specifies how the file is to be accessed.  May have any of the values
allowed for the \fImode\fR argument to the Tcl \fBopen\fR command.  
.AP int permissions in
POSIX-style permission flags such as 0644.  If a new file is created, these
permissions will be set on the created file.
.AP int argc in
The number of elements in \fIargv\fR.
.AP "CONST char" **argv in
Arguments for constructing a command pipeline.  These values have the same
meaning as the non-switch arguments to the Tcl \fBexec\fR command.
.AP int flags in
Specifies the disposition of the stdio handles in pipeline: OR-ed
combination of \fBTCL_STDIN\fR, \fBTCL_STDOUT\fR, \fBTCL_STDERR\fR, and
\fBTCL_ENFORCE_MODE\fR. If \fBTCL_STDIN\fR is set, stdin for the first child
in the pipe is the pipe channel, otherwise it is the same as the standard
input of the invoking process; likewise for \fBTCL_STDOUT\fR and
\fBTCL_STDERR\fR. If \fBTCL_ENFORCE_MODE\fR is not set, then the pipe can
redirect stdio handles to override the stdio handles for which
\fBTCL_STDIN\fR, \fBTCL_STDOUT\fR and \fBTCL_STDERR\fR have been set.  If it
is set, then such redirections cause an error.
.AP ClientData handle in
Operating system specific handle for I/O to a file. For Unix this is a
file descriptor, for Windows it is a HANDLE.
.AP int readOrWrite in
OR-ed combination of \fBTCL_READABLE\fR and \fBTCL_WRITABLE\fR to indicate
what operations are valid on \fIhandle\fR.
.AP "CONST char" *channelName in
The name of the channel. 
.AP int *modePtr out
Points at an integer variable that will receive an OR-ed combination of
\fBTCL_READABLE\fR and \fBTCL_WRITABLE\fR denoting whether the channel is
open for reading and writing.
.VS 8.3
.AP "CONST char" *pattern in
The pattern to match on, passed to Tcl_StringMatch, or NULL.
.VE
.AP Tcl_Channel channel in
A Tcl channel for input or output.  Must have been the return value
from a procedure such as \fBTcl_OpenFileChannel\fR.
.VS 8.1 br
.AP Tcl_Obj *readObjPtr in/out
A pointer to a Tcl Object in which to store the characters read from the
channel.
.AP int charsToRead in
The number of characters to read from the channel.  If the channel's encoding 
is \fBbinary\fR, this is equivalent to the number of bytes to read from the 
channel.
.AP int appendFlag in
If non-zero, data read from the channel will be appended to the object.
Otherwise, the data will replace the existing contents of the object.
.AP char *readBuf out
A buffer in which to store the bytes read from the channel.
.AP int bytesToRead in
The number of bytes to read from the channel.  The buffer \fIreadBuf\fR must
be large enough to hold this many bytes.
.AP Tcl_Obj *lineObjPtr in/out
A pointer to a Tcl object in which to store the line read from the
channel.  The line read will be appended to the current value of the
object. 
.AP Tcl_DString *lineRead in/out
A pointer to a Tcl dynamic string in which to store the line read from the
channel.  Must have been initialized by the caller.  The line read will be
appended to any data already in the dynamic string.
.VS 8.3
.AP "CONST char" *input in
The input to add to a channel buffer.
.AP int inputLen in
Length of the input
.AP int addAtEnd in
Flag indicating whether the input should be added to the end or
beginning of the channel buffer.
.VE
.AP Tcl_Obj *writeObjPtr in
A pointer to a Tcl Object whose contents will be output to the channel.
.AP "CONST char" *charBuf in
A buffer containing the characters to output to the channel.
.AP "CONST char" *byteBuf in
A buffer containing the bytes to output to the channel.
.AP int bytesToWrite in
The number of bytes to consume from \fIcharBuf\fR or \fIbyteBuf\fR and
output to the channel.
.VE
.AP Tcl_WideInt offset in
How far to move the access point in the channel at which the next input or
output operation will be applied, measured in bytes from the position
given by \fIseekMode\fR.  May be either positive or negative.
.AP int seekMode in
Relative to which point to seek; used with \fIoffset\fR to calculate the new
access point for the channel. Legal values are \fBSEEK_SET\fR,
\fBSEEK_CUR\fR, and \fBSEEK_END\fR.
.AP "CONST char" *optionName in
The name of an option applicable to this channel, such as \fB\-blocking\fR.
May have any of the values accepted by tKc��Lc��Mc��Nc��Oc��Pc��Qc��Rc��Sc��Tc��Uc��Vc��Wc��Xc��Yc��Zc��[c��\c��]c��^c��_c��`c��ac��bc������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������he \fBfconfigure\fR command.
.AP Tcl_DString *optionValue in
Where to store the value of an option or a list of all options and their
values. Must have been initialized by the caller.
.AP "CONST char" *newValue in
New value for the option given by \fIoptionName\fR.
.BE

.SH DESCRIPTION
.PP
The Tcl channel mechanism provides a device-independent and
platform-independent mechanism for performing buffered input
and output operations on a variety of file, socket, and device
types.
The channel mechanism is extensible to new channel types, by
providing a low level channel driver for the new type; the channel driver
interface is described in the manual entry for \fBTcl_CreateChannel\fR. The
channel mechanism provides a buffering scheme modeled after
Unix's standard I/O, and it also allows for nonblocking I/O on
channels.
.PP
The procedures described in this manual entry comprise the C APIs of the
generic layer of the channel architecture. For a description of the channel
driver architecture and how to implement channel drivers for new types of
channels, see the manual entry for \fBTcl_CreateChannel\fR.

.SH TCL_OPENFILECHANNEL
.PP
\fBTcl_OpenFileChannel\fR opens a file specified by \fIfileName\fR and
returns a channel handle that can be used to perform input and output on
the file. This API is modeled after the \fBfopen\fR procedure of
the Unix standard I/O library.
The syntax and meaning of all arguments is similar to those
given in the Tcl \fBopen\fR command when opening a file.
If an error occurs while opening the channel, \fBTcl_OpenFileChannel\fR
returns NULL and records a POSIX error code that can be
retrieved with \fBTcl_GetErrno\fR.
In addition, if \fIinterp\fR is non-NULL, \fBTcl_OpenFileChannel\fR
leaves an error message in \fIinterp\fR's result after any error.  
As of Tcl 8.4, the object-based API \fBTcl_FSOpenFileChannel\fR should 
be used in preference to \fBTcl_OpenFileChannel\fR wherever possible.
.PP

.PP
The newly created channel is not registered in the supplied interpreter; to
register it, use \fBTcl_RegisterChannel\fR, described below.
If one of the standard channels, \fBstdin, stdout\fR or \fBstderr\fR was
previously closed, the act of creating the new channel also assigns it as a
replacement for the standard channel.

.SH TCL_OPENCOMMANDCHANNEL
.PP
\fBTcl_OpenCommandChannel\fR provides a C-level interface to the
functions of the \fBexec\fR and \fBopen\fR commands.
It creates a sequence of subprocesses specified
by the \fIargv\fR and \fIargc\fR arguments and returns a channel that can
be used to communicate with these subprocesses.
The \fIflags\fR argument indicates what sort of communication will
exist with the command pipeline.
.PP
If the \fBTCL_STDIN\fR flag is set then the standard input for the
first subprocess will be tied to the channel: writing to the channel
will provide input to the subprocess.  If \fBTCL_STDIN\fR is not set,
then standard input for the first subprocess will be the same as this
application's standard input.  If \fBTCL_STDOUT\fR is set then
standard output from the last subprocess can be read from the channel;
otherwise it goes to this application's standard output.  If
\fBTCL_STDERR\fR is set, standard error output for all subprocesses is
returned to the channel and results in an error when the channel is
closed; otherwise it goes to this application's standard error.  If
\fBTCL_ENFORCE_MODE\fR is not set, then \fIargc\fR and \fIargv\fR can
redirect the stdio handles to override \fBTCL_STDIN\fR,
\fBTCL_STDOUT\fR, and \fBTCL_STDERR\fR; if it is set, then it is an
error for argc and argv to override stdio channels for which
\fBTCL_STDIN\fR, \fBTCL_STDOUT\fR, and \fBTCL_STDERR\fR have been set.
.PP
If an error occurs while opening the channel, \fBTcl_OpenCommandChannel\fR
returns NULL and records a POSIX error code that can be retrieved with
\fBTcl_GetErrno\fR.
In addition, \fBTcl_OpenCommandChannel\fR leaves an error message in
the interpreter's result if \fIinterp\fR is not NULL.
.PP
The newly created channel is not registered in the supplied interpreter; to
register it, use \fBTcl_RegisterChannel\fR, described below.
If one of the standard channels, \fBstdin, stdout\fR or \fBstderr\fR was
previously closed, the act of creating the new channel also assigns it as a
replacement for the standard channel.

.SH TCL_MAKEFILECHANNEL
.PP
\fBTcl_MakeFileChannel\fR makes a \fBTcl_Channel\fR from an existing,
platform-specific, file handle.
The newly created channel is not registered in the supplied interpreter; to
register it, use \fBTcl_RegisterChannel\fR, described below.
If one of the standard channels, \fBstdin, stdout\fR or \fBstderr\fR was
previously closed, the act of creating the new channel also assigns it as a
replacement for the standard channel.

.SH TCL_GETCHANNEL
.PP
\fBTcl_GetChannel\fR returns a channel given the \fIchannelName\fR used to
create it with \fBTcl_CreateChannel\fR and a pointer to a Tcl interpreter in
\fIinterp\fR. If a channel by that name is not registered in that interpreter,
the procedure returns NULL. If the \fImodePtr\fR argument is not NULL, it
points at an integer variable that will receive an OR-ed combination of
\fBTCL_READABLE\fR and \fBTCL_WRITABLE\fR describing whether the channel is
open for reading and writing.
.PP
\fBTcl_GetChannelNames\fR and \fBTcl_GetChannelNamesEx\fR write the
names of the registered channels to the interpreter's result as a
list object.  \fBTcl_GetChannelNamesEx\fR will filter these names
according to the \fIpattern\fR.  If \fIpattern\fR is NULL, then it
will not do any filtering.  The return value is \fBTCL_OK\fR if no
errors occurred writing to the result, otherwise it is \fBTCL_ERROR\fR,
and the error message is left in the interpreter's result.

.SH TCL_REGISTERCHANNEL
.PP
\fBTcl_RegisterChannel\fR adds a channel to the set of channels accessible
in \fIinterp\fR. After this call, Tcl programs executing in that
interpreter can refer to the channel in input or output operations using
the name given in the call to \fBTcl_CreateChannel\fR.  After this call,
the channel becomes the property of the interpreter, and the caller should
not call \fBTcl_Close\fR for the channel; the channel will be closed
automatically when it is unregistered from the interpreter.
.PP
Code executing outside of any Tcl interpreter can call
\fBTcl_RegisterChannel\fR with \fIinterp\fR as NULL, to indicate that it
wishes to hold a reference to this channel. Subsequently, the channel can
be registered in a Tcl interpreter and it will only be closed when the
matching number of calls to \fBTcl_UnregisterChannel\fR have been made.
This allows code executing outside of any interpreter to safely hold a
reference to a channel that is also registered in a Tcl interpreter.
.PP
This procedure interacts with the code managing the standard
channels. If no standard channels were initialized before the first
call to \fBTcl_RegisterChannel\fR they will get initialized by that
call. See \fBTcl_StandardChannels\fR for a general treatise about
standard channels and the behaviour of the Tcl library with regard to
them.

.SH TCL_UNREGISTERCHANNEL
.PP
\fBTcl_UnregisterChannel\fR removes a channel from the set of channels
accessible in \fIinterp\fR. After this call, Tcl programs will no longer be
able to use the channel's name to refer to the channel in that interpreter.
If this operation removed the last registration of the channel in any
interpreter, the channel is also closed and destroyed.
.PP
Code not associated with a Tcl interpreter can call
\fBTcl_UnregisterChannel\fR with \fIinterp\fR as NULL, to indicate to Tcl
that it no longer holds a reference to that channel. If this is the last
reference to the channel, it will now be closed.  \fBTcl_UnregisterChannel\fR
is very similar to \fBTcl_DetachChannel\fR except that it will also
close the channel if no further references to it exist.

.SH TCL_DETACHCHANNEL
.PP
\fBTcl_DetachChannel\fR removes a channel from the set of channels
accessible in \fIinterp\fR. After this call, Tcl programs will no longer be
able to use the channel's name to refer to the channel in that interpreter.
Beyond that, this command has no further effect.  It cannot be used on
the standard channels (stdout, stderr, stdin), and will return
TCL_ERROR if passed one of those channels.
.PP
Code not associated with a Tcl interpreter can call
\fBTcl_DetachChannel\fR with \fIinterp\fR as NULL, to indicate to Tcl
that it no longer holds a reference to that channel. If this is the last
reference to the channel, unlike \fBTcl_UnregisterChannel\fR, 
it will not be closed.

.SH TCL_ISSTANDARDCHANNEL
.PP
\fBTcl_IsStandardChannel\fR tests whether a channel is one of the
three standard channels, stdin, stdout or stderr.  If so, it returns
1, otherwise 0.
.PP
No attempt is made to check whether the given channel or the standard 
channels are initialized or otherwise valid.

.SH TCL_CLOSE
.PP
\fBTcl_Close\fR destroys the channel \fIchannel\fR, which must denote a
currently open channel. The channel should not be registered in any
interpreter when \fBTcl_Close\fR is called. Buffered output is flushed to
the channel's output device prior to destroying the channel, and any
buffered input is discarded.  If this is a blocking channel, the call does
not return until all buffered data is successfully sent to the channel's
output device.  If this is a nonblocking channel and there is buffered
output that cannot be written without blocking, the call returns
immediately; output is flushed in the background and the channel will be
closed once all of the buffered data has been output.  In this case errors
during flushing are not reported.
.PP
If the channel was closed successfully, \fBTcl_Close\fR returns \fBTCL_OK\fR.
If an error occurs, \fBTcl_Close\fR returns \fBTCL_ERROR\fR and records a
POSIX error code that can be retrieved with \fBTcl_GetErrno\fR.
If the channel is being closed synchronously and an error occurs during
closing of the channel and \fIinterp\fR is not NULL, an error message is
left in the interpreter's result.
.PP
Note: it is not safe to call \fBTcl_Close\fR on a channel that has been
registered using \fBTcl_RegisterChannel\fR; see the documentation for
\fBTcl_RegisterChannel\fR, above, for details. If the channel has ever
been given as the \fBchan\fR argument in a call to
\fBTcl_RegisterChannel\fR, you should instead use
\fBTcl_UnregisterChannel\fR, which will internally call \fBTcl_Close\fR
when all calls to \fBTcl_RegisterChannel\fR have been matched by
corresponding calls to \fBTcl_UnregisterChannel\fR.

.VS 8.1 br
.SH "TCL_READCHARS AND TCL_READ"
.PP
\fBTcl_ReadChars\fR consumes bytes from \fIchannel\fR, converting the bytes
to UTF-8 based on the channel's encoding and storing the produced data in 
\fIreadObjPtr\fR's string representation.  The return value of
\fBTcl_ReadChars\fR is the number of characters, up to \fIcharsToRead\fR,
that were stored in \fIreadObjPtr\fR.  If an error occurs while reading, the
return value is \-1 and \fBTcl_ReadChars\fR records a POSIX error code that
can be retrieved with \fBTcl_GetErrno\fR.
.PP
Setting \fIcharsToRead\fR to \fB-1\fR will cause the command to read
all characters currently available (non-blocking) or everything until
eof (blocking mode).
.PP
The return value may be smaller than the value to read, indicating that less
data than requested was available.  This is called a \fIshort read\fR.  In
blocking mode, this can only happen on an end-of-file.  In nonblocking mode,
a short read can also occur if there is not enough input currently
available:  \fBTcl_ReadChars\fR returns a short count rather than waiting
for more data.
.PP
If the channel is in blocking mode, a return value of zero indicates an
end-of-file condition.  If the channel is in nonblocking mode, a return
value of zero indicates either that no input is currently available or an
end-of-file condition.  Use \fBTcl_Eof\fR and \fBTcl_InputBlocked\fR to tell
which of these conditions actually occurred.
.PP
\fBTcl_ReadChars\fR translates the various end-of-line representations into
the canonical \fB\en\fR internal representation according to the current
end-of-line recognition mode.  End-of-line recognition and the various
platform-specific modes are described in the manual entry for the Tcl
\fBfconfigure\fR command.
.PP
As a performance optimization, when reading from a channel with the encoding
\fBbinary\fR, the bytes are not converted to UTF-8 as they are read.
Instead, they are stored in \fIreadObjPtr\fR's internal representation as a
byte-array object.  The string representation of this object will only be
constructed if it is needed (e.g., because of a call to
\fBTcl_GetStringFromObj\fR).  In this way, byte-oriented data can be read
from a channel, manipulated by calling \fBTcl_GetByteArrayFromObj\fR and
related functions, and then written to a channel without the expense of ever
converting to or from UTF-8.
.PP
\fBTcl_Read\fR is similar to \fBTcl_ReadChars\fR, except that it doesn't do
encoding conversions, regardless of the channel's encoding.  It is deprecated
and exists for backwards compatibility with non-internationalized Tcl
extensions.  It consumes bytes from \fIchannel\fR and stores them in
\fIreadBuf\fR, performing end-of-line translations on the way.  The return value
of \fBTcl_Read\fR is the number of bytes, up to \fIbytesToRead\fR, written in
\fIreadBuf\fR.  The buffer produced by \fBTcl_Read\fR is not null-terminated.
Its contents are valid from the zeroth position up to and excluding the
position indicated by the return value.  
.PP
\fBTcl_ReadRaw\fR is the same as \fBTcl_Read\fR but does not
compensate for stacking. While \fBTcl_Read\fR (and the other functions
in the API) always get their data from the topmost channel in the
stack the supplied channel is part of, \fBTcl_ReadRaw\fR does
not. Thus this function is \fBonly\fR usable for transformational
channel drivers, i.e. drivers used in the middle of a stack of
channels, to move data from the channel below into the transformation.

.SH "TCL_GETSOBJ AND TCL_GETS"
.PP
\fBTcl_GetsObj\fR consumes bytes from \fIchannel\fR, converting the bytes to
UTF-8 based on the channel's encoding, until a full line of input has been
seen.  If the channel's encoding is \fBbinary\fR, each byte read from the
channel is treated as an individual Unicode character.  All of the
characters of the line except for the terminating end-of-line character(s)
are appended to \fIlineObjPtr\fR's string representation.  The end-of-line
character(s) are read and discarded.
.PP
If a line was successfully read, the return value is greater than or equal
to zero and indicates the number of bytes stored in \fIlineObjPtr\fR.  If an
error occurs, \fBTcl_GetsObj\fR returns \-1 and records a POSIX error code
that can be retrieved with \fBTcl_GetErrno\fR.  \fBTcl_GetsObj\fR also
returns \-1 if the end of the file is reached; the \fBTcl_Eof\fR procedure
can be used to distinguish an error from an end-of-file condition.
.PP
If the channel is in nonblocking mode, the return value can also be \-1 if
no data was available or the data that was available did not contain an
end-of-line character.  When \-1 is returned, the \fBTcl_InputBlocked\fR
procedure may be invoked to determine if the channel is blocked because
of input unavailability.
.PP
\fBTcl_Gets\fR is the same as \fBTcl_GetsObj\fR except the resulting
characters are appended to the dynamic string given by
\fIlineRead\fR rather than a Tcl object.

.SH "TCL_UNGETS"
.PP
\fBTcl_Ungets\fR is used to add data to the input queue of a channel,
at either the head or tail of the queue.  The pointer \fIinput\fR points
to the data that is to be added.  The length of the input to add is given
by \fIinputLen\fR.  A non-zero value of \fIaddAtEnd\fR indicates that the
data is to be added at the end of queue; otherwise it will be added at the
head of the queue.  If \fIchannel\fR has a "sticky" EOF set, no data will be
added to the input queue.  \fBTcl_Ungets\fR returns \fIinputLen\fR or
-1 if an error occurs.

.SH "TCL_WRITECHARS, TCL_WRITEOBJ, AND TCL_WRITE"
.PP
\fBTcl_WriteChars\fR accepts \fIbytesToWrite\fR bytes of character data at
\fIcharBuf\fR.  The UTF-8 characters in the buffer are converted to the
channel's encoding and queued for output to \fIchannel\fR.  If
\fIbytesToWrite\fR is negative, \fBTcl_WriteChars\fR expects \fIcharBuf\fR
to be null-terminated and it outputs everything up to the null.
.PP
Data queued for output may not appear on the output device immediately, due
to internal buffering.  If the data should appear immediately, call
\fBTcl_Flush\fR after the call to \fBTcl_WriteChars\fR, or set the 
\fB\-buffering\fR option on the channel to \fBnone\fR.  If you wish the data
to appear as soon as a complete line is accepted for output, set the
\fB\-buffering\fR option on the channel to \fBline\fR mode.
.PP
The return value of \fBTcl_WriteChars\fR is a count of how many bytes were
accepted for output to the channel.  This is either greater than zero to
indicate success or \-1 to indicate that an error occurred.  If an error
occurs, \fBTcl_WriteChars\fR records a POSIX error code that may be
retrieved with \fBTcl_GetErrno\fR.
.PP
Newline characters in the output data are translated to platform-specific
end-of-line sequences according to the \fB\-translation\fR option for the
channel.  This is done even if the channel has no encoding.
.PP
\fBTcl_WriteObj\fR is similar to \fBTcl_WriteChars\fR except it
accepts a Tcl object whose contents will be output to the channel.  The
UTF-8 characters in \fIwriteObjPtr\fR's string representation are converted
to the channel's encoding and queued for output to \fIchannel\fR.  
As a performance optimization, when writing to a channel with the encoding
\fBbinary\fR, UTF-8 characters are not converted as they are written.
Instead, the bytes in \fIwriteObjPtr\fR's internal representation as a
byte-array object are written to the channel.  The byte-array representation
of the object will be constructed if it is needed.  In this way,
byte-oriented data can be read from a channel, manipulated by calling
\fBTcl_GetByteArrayFromObj\fR and related functions, and then written to a
channel without the expense of ever converting to or from UTF-8.
.PP
\fBTcl_Write\fR is similar to \fBTcl_WriteChars\fR except that it doesn't do
encoding conversions, regardless of the channel's encoding.  It is
deprecated and exists for backwards compatibility with non-internationalized
Tcl extensions.  It accepts \fIbytesToWrite\fR bytes of data at
\fIbyteBuf\fR and queues them for output to \fIchannel\fR.  If
\fIbytesToWrite\fR is negative, \fBTcl_Write\fR expects \fIbyteBuf\fR to be
null-terminated and it outputs everything up to the null.
.PP
\fBTcl_WriteRaw\fR is the same as \fBTcl_Write\fR but does not
compensate for stacking. While \fBTcl_Write\fR (and the other
functions in the API) always feed their input to the topmost channel
in the stack the supplied channel is part of, \fBTcl_WriteRaw\fR does
not. Thus this function is \fBonly\fR usable for transformational
channel drivers, i.e. drivers used in the middle of a stack of
channels, to move data from the transformation into the channel below
it.
.VE

.SH TCL_FLUSH
.PP
\fBTcl_Flush\fR causes all of the buffered output data for \fIchannel\fR
to be written to its underlying file or device as soon as possible.
If the channel is in blocking mode, the call does not return until
all the buffered data has been sent to the channel or some error occurred.
The call returns immediately if the channel is nonblocking; it starts
a background flush that will write the buffered data to the channel
eventually, as fast as the channel is able to absorb it.
.PP
The return value is normally \fBTCL_OK\fR.
If an error occurs, \fBTcl_Flush\fR returns \fBTCL_ERROR\fR and
records a POSIX error code that can be retrieved with \fBTcl_GetErrno\fR.

.SH TCL_SEEK
.PP
\fBTcl_Seek\fR moves the access point in \fIchannel\fR where subsequent
data will be read or written. Buffered output is flushed to the channel and
buffered input is discarded, prior to the seek operation.
.PP
\fBTcl_Seek\fR normally returns the new access point.
If an error occurs, \fBTcl_Seek\fR returns \-1 and records a POSIX error
code that can be retrieved with \fBTcl_GetErrno\fR.
After an error, the access point may or may not have been moved.

.SH TCL_TELL
.PP
\fBTcl_Tell\fR returns the current access point for a channel. The returned
value is \-1 if the channel does not support seeking.

.SH TCL_GETCHANNELOPTION
.PP
\fBTcl_GetChannelOption\fR retrieves, in \fIoptionValue\fR, the value of one of
the options currently in effect for a channel, or a list of all options and
their values.  The \fIchannel\fR argument identifies the channel for which
to query an option or retrieve all options and their values.
If \fIoptionName\fR is not NULL, it is the name of the
option to query; the option's value is copied to the Tcl dynamic string
denoted by \fIoptionValue\fR. If
\fIoptionName\fR is NULL, the function stores an alternating list of option
names and their values in \fIoptionValue\fR, using a series of calls to
\fBTcl_DStringAppendElement\fR. The various preexisting options and
their possible values are described in the manual entry for the Tcl
\fBfconfigure\fR command. Other options can be added by each channel type.
These channel type specific options are described in the manual entry for
the Tcl command that creates a channel of that type; for example, the
additional options for TCP based channels are described in the manual entry
for the Tcl \fBsocket\fR command.
The procedure normally returns \fBTCL_OK\fR. If an error occurs, it returns
\fBTCL_ERROR\fR and calls \fBTcl_SetErrno\fR to store an appropriate POSIX
error code.

.SH TCL_SETCHANNELOPTION
.PP
\fBTcl_SetChannelOption\fR sets a new value \fInewValue\fR
for an option \fIoptionName\fR on \fIchannel\fR.
The procedure normally returns \fBTCL_OK\fR.  If an error occurs,
it returns \fBTCL_ERROR\fR;  in addition, if \fIinterp\fR is non-NULL,
\fBTcl_SetChannelOption\fR leaves an error message in the interpreter's result.

.SH TCL_EOF
.PP
\fBTcl_Eof\fR returns a nonzero value if \fIchannel\fR encountered
an end of file during the last input operation.

.SH TCL_INPUTBLOCKED
.PP
\fBTcl_InputBlocked\fR returns a nonzero value if \fIchannel\fR is in
nonblocking mode and the last input operation returned less data than
requested because there was insufficient data available.
The call always returns zero if the channel is in blocking mode.

.SH TCL_INPUTBUFFERED
.PP
\fBTcl_InputBuffered\fR returns the number of bytes of input currently
buffered in the internal buffers for a channel. If the channel is not open
for reading, this function always returns zero.

.SH TCL_OUTPUTBUFFERED
.VS 8.4
\fBTcl_OutputBuffered\fR returns the number of bytes of output
currently buffered in the internal buffers for a channel. If the
channel is not open for writing, this function always returns zero.
.VE

.SH "PLATFORM ISSUES"
.PP
The handles returned from \fBTcl_GetChannelHandle\fR depend on the
platform and the channel type.  On Unix platforms, the handle is
always a Unix file descriptor as returned from the \fBopen\fR system
call.  On Windows platforms, the handle is a file \fBHANDLE\fR when
the channel was created with \fBTcl_OpenFileChannel\fR,
\fBTcl_OpenCommandChannel\fR, or \fBTcl_MakeFileChannel\fR.  Other
channel types may return a different type of handle on Windows
platforms.  On the Macintosh platform, the handle is a file reference
number as returned from \fBHOpenDF\fR.

.SH "SEE ALSO"
DString(3), fconfigure(n), filename(n), fopen(3), Tcl_CreateChannel(3)

.SH KEYWORDS
access point, blocking, buffered I/O, channel, channel driver, end of file,
flush, input, nonblocking, output, read, seek, write
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'\" Copyright (c) 1997-1998 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
'\" RCS: @(#) $Id: Encoding.3,v 1.11.2.1 2003/07/18 16:56:24 dgp Exp $
'\" 
'\" The definitions below are for supplemental macros used in Tcl/Tk
'\" manual entries.
'\"
'\" .AP type name in/out ?indent?
'\"	Start paragraph describing an argument to a library procedure.
'\"	type is type of argument (int, etc.), in/out is either "in", "out",
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'\"	and indent is equivalent to second arg of .IP (shouldn't ever be
'\"	needed;  use .AS below instead)
'\"
'\" .AS ?type? ?name?
'\"	Give maximum sizes of arguments for setting tab stops.  Type and
'\"	name are examples of largest possible arguments that will be passed
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'\" .BS
'\"	Start box enclosure.  From here until next .BE, everything will be
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'\"	Begin an indented unfilled display.
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.LP
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..
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..
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Command-Line Name:	\\fB\\$1\\fR
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.TH Tcl_GetEncoding 3 "8.1" Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_GetEncoding, Tcl_FreeEncoding, Tcl_ExternalToUtfDString, Tcl_ExternalToUtf, Tcl_UtfToExternalDString, Tcl_UtfToExternal, Tcl_WinTCharToUtf, Tcl_WinUtfToTChar, Tcl_GetEncodingName, Tcl_SetSystemEncoding, Tcl_GetEncodingNames, Tcl_CreateEncoding, 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
char *
\fBTcl_ExternalToUtfDString\fR(\fIencoding, src, srcLen, dstPtr\fR)
.sp
int
\fBTcl_ExternalToUtf\fR(\fIinterp, encoding, src, srcLen, flags, statePtr, dst, dstLen, srcReadPtr, dstWrotePtr, 
	dstCharsPtr\fR)
.sp
char * 
\fBTcl_UtfToExternalDString\fR(\fIencoding, src, srcLen, dstPtr\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_SetSystemEncoding\fR(\fIinterp, name\fR)
.sp
void
\fBTcl_GetEncodingNames\fR(\fIinterp\fR)
.sp
Tcl_Encoding
\fBTcl_CreateEncoding\fR(\fItypePtr\fR)
.sp
CONST char *
\fBTcl_GetDefaultEncodingDir\fR(\fIvoid\fR)
.sp
void
\fBTcl_SetDefaultEncodingDir\fR(\fIpath\fR)


.SH ARGUMENTS
.AS Tcl_EncodingState *dstWrotePtr
.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 "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
Various flag bits OR-ed together.  
TCL_ENCODING_START 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.  TCL_ENCODING_END 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.
TCL_ENCODING_STOPONERROR signifies that the conversion routine should
return immediately upon reading a source character that doesn't exist in
the target encoding; otherwise a default fallback character will
automatically be substituted.  
.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 Tcl_EncodingType *typePtr in
Structure that defines a new type of encoding.  
.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 builtin 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 calpc��qc��rc��sc��tc��uc��vc��wc��xc��yc��zc��{c��|c��}c��~c��c��€c��c������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������led 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_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_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 fews 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 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 and TCL_ENCODING_STOPONERROR was specified.  
.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_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.  On Windows 95
(as with the Macintosh and Unix operating systems),
all strings exchanged between Tcl and the operating system are "char"
based.  On Windows NT, some strings exchanged between Tcl and the
operating system are "char" oriented while others are in Unicode.  By
convention, in Windows a TCHAR is a character in the ANSI code page
on Windows 95 and a Unicode character on Windows NT.
.PP
If you planned to use the same "char" based interfaces on both Windows
95 and Windows NT, you could use \fBTcl_UtfToExternal\fR and
\fBTcl_ExternalToUtf\fR (or their \fBTcl_DString\fR equivalents) with an
encoding of NULL (the current system encoding).  On the other hand,
if you planned to use the Unicode interface when running on Windows NT
and the "char" interfaces when running on Windows 95, you would have
to perform the following type of test over and over in your program
(as represented in pseudo-code):
.CS
if (running NT) {
    encoding <- Tcl_GetEncoding("unicode");
    nativeBuffer <- Tcl_UtfToExternal(encoding, utfBuffer);
    Tcl_FreeEncoding(encoding);
} else {
    nativeBuffer <- Tcl_UtfToExternal(NULL, utfBuffer);
.CE
\fBTcl_WinUtfToTChar\fR and \fBTcl_WinTCharToUtf\fR automatically
handle this test and use the proper encoding based on the current
operating system.  \fBTcl_WinUtfToTChar\fR returns a pointer to
a TCHAR string, and \fBTcl_WinTCharToUtf\fR expects a TCHAR string
pointer as the \fIsrc\fR string.  Otherwise, these functions
behave identically to \fBTcl_UtfToExternalDString\fR and
\fBTcl_ExternalToUtfDString\fR.
.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_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, TCL_ERROR 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
TCL_OK.
.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 Tcl_EncodingType {
	CONST char *\fIencodingName\fR;
	Tcl_EncodingConvertProc *\fItoUtfProc\fR;
	Tcl_EncodingConvertProc *\fIfromUtfProc\fR;
	Tcl_EncodingFreeProc *\fIfreeProc\fR;
	ClientData \fIclientData\fR;
	int \fInullSize\fR;
} Tcl_EncodingType;  
.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 Tcl_EncodingConvertProc(
	ClientData \fIclientData\fR,
	CONST char *\fIsrc\fR, 
	int \fIsrcLen\fR, 
	int \fIflags\fR, 
	Tcl_Encoding *\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:
.CS
typedef void Tcl_EncodingFreeProc(
	ClientData \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_GetDefaultEncodingDir\fR and \fBTcl_SetDefaultEncodingDir\fR
access and set the directory to use when locating the default encoding
files.  If this value is not NULL, the \fBTclpInitLibraryPath\fR routine
appends the path to the head of the search path, and uses this path as
the first place to look into when trying to locate the encoding file.

.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 ``#'' 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 a 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):
.CS
# Encoding file: shiftjis, multi-byte
M
003F 0 40
00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0000000000000000000000000000000000000000000000000000000000000000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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