command writing(3)

NAME

TclCommandWriting - Writing C language extensions to Tcl.

OVERVIEW

This document is intended to help the programmer who
wishes to extend Tcl with C language routines. It should
also be useful to someone wishing to add Tcl to an exist
ing editor, communications program, window manager, etc.
C programming information can also be found in the *.3
manual pages in the doc directory of the Berkeley distri
bution, and in the *.3 manpages in the man directory of Extended Tcl.

WRITING TCL EXTENSIONS IN C

All C-based Tcl commands are called with four arguments: a
client data pointer, an interpreter pointer, an argument
count and a pointer to an array of pointers to character
strings containing the Tcl arguments to the command.

A simple Tcl extension in C is now presented, and
described below:

#include "tcl.h"

int App_EchoCmd(clientData, interp, argc, argv)

void *clientData;
Tcl_Interp *interp;
int argc;
char **argv;

{

int i;

for (i = 1; i < argc; i++) {

printf("%s",argv[i]);

if (i < argc - 1) printf(" ");

}
printf("0);
return TCL_OK;

}

The client data pointer will be described later.

The interpreter pointer is the ``key'' to an interpreter.
It is returned by Tcl_CreateInterp and is used extensively within Tcl, and will be by your C extensions. The data
structure pointed to by the interpreter pointer, and all
of the subordinate structures that branch off of it, make
up a Tcl interpreter, which includes all of the currently
defined procedures, commands, variables, arrays and the
execution state of that interpreter. (For more informa
tion on creating and deleting interpreters, please examine
the CrtInterp(3) manpage in the Berkeley Tcl distribution. For information on creating interpreters that include the
commands provided by Extended Tcl, check out the
TclX_Init(3) manpage of Extended Tcl. For a manual page describing the user-visible fields of a Tcl interpreter,
please look at Interp(3) in Berkeley Tcl.)

The argument count and pointer to an array of pointers to
textual arguments is handled by your C code in the same
manner that you would use in writing a C main function -the argument count and array of pointers works the same as
in a C main call; pointers to the arguments to the func
tion are contained in the argv array. Similar to a C
main, the first argument (argv[0]) is the name the routine was called as (in a main, the name the program was invoked
as).

In the above example, all of the arguments are output with
a space between each one by looping through argv from one
to the argument count, argc, and a newline is output to
terminate the line -- an ``echo'' command.

All arguments from a Tcl call to a Tcl C extension are
passed as strings. If your C routine expects certain
numeric arguments, your routine must first convert them
using the Tcl_GetInt or Tcl_GetDouble function, Extended Tcl's Tcl_GetLong or Tcl_GetUnsigned, or some other method of your own devising. Likewise for converting boolean
values, Tcl_GetBoolean should be used. These routines automatically leave an appropriate error message in the
Tcl interpreter's result buffer and return TCL_ERROR if a conversion error occurs. (For more information on these
routines, please look at the GetInt(3) manpage in the Berkeley Tcl distribution.)

Likewise, if you program produces a numeric result, it
should return a string equivalent to that numeric value.
A common way of doing this is something like...

sprintf(interp->result, "%ld", result);

Writing results directly into the interpreter's result
buffer is only good for relatively short results. Tcl has
a function, Tcl_SetResult, which provides the ability for your C extensions to return very large strings to Tcl,
with the ability to tell the interpreter whether it
``owns'' the string (meaning that Tcl should delete the
string when it's done with it), that the string is likely
to be changed or overwritten soon (meaning that Tcl should
make a copy of the string right away), or that the string
won't change (so Tcl can use the string as is and not
worry about it). Understanding how results are passed
back to Tcl is essential to the C extension writer.
Please study the SetResult(3) manual page in the Tcl dis tribution.

Sophisticated commands should verify their arguments when
ever possible, both by examining the argument count, by
verifying that numeric fields are really numeric, that
values are in range (when their ranges are known), and so
forth.

Tcl is designed to be as bullet-proof as possible, in the
sense that no Tcl program should be able to cause Tcl to
dump core. Please carry this notion forward with your C
extensions by validating arguments as above.

ANOTHER C EXTENSION - THE MAX COMMAND

In the command below, two or more arguments are compared
and the one with the maximum value is returned, if all
goes well. It is an error if there are fewer than three
arguments (the pointer to the ``max'' command text itself,
argv[0], and pointers to at least two arguments to compare the values of).

This routine also shows the use of the programmer laborsaving Tcl_AppendResult routine. See the Tcl manual page, SetResult(3), for details. Also examine the calls Tcl_AddErrorInfo, Tcl_SetErrorCode and Tcl_PosixError doc umented in the Tcl manual page AddErrInfo(3).

int
Tcl_MaxCmd (clientData, interp, argc, argv)

char *clientData;
Tcl_Interp *interp;
int argc;
char **argv;

{

int maxVal = MININT;
int maxIdx = 1;
int value, idx;

if (argc < 3) {

Tcl_AppendResult (interp, "bad # arg: ",

argv[0],

" num1 num2 [..numN]", (char

*)NULL);

return TCL_ERROR;

}

for (idx = 1; idx < argc; idx++) {

if (Tcl_GetInt (argv[idx], 10, &Value) !=

TCL_OK)

return TCL_ERROR;

if (value > maxVal) {

maxVal = value;
maxIdx = idx;

}

}
Tcl_SetResult (interp, argv [maxIdx],

TCL_VOLATILE);
return TCL_OK;

}

When Tcl-callable functions complete, they should normally
return TCL_OK or TCL_ERROR. TCL_OK is returned when the command succeeded and TCL_ERROR is returned when the com mand has failed in some abnormal way. TCL_ERROR should be returned for all syntax errors, non-numeric values (when
numeric ones were expected), and so forth. Less clear in
some cases is whether Tcl errors should be returned or
whether a function should just return a status value. For
example, end-of-file during a gets returns a status, but
open returns an error if the open fails. Errors can be
caught from Tcl programs using the catch command. (See
Tcl's catch(n) and error(n) manual pages.)

Less common return values are TCL_RETURN, TCL_BREAK and TCL_CONTINUE. These are used if you are adding new con trol and/or looping structures to Tcl. To see these val
ues in action, examine the source code to Tcl's while, for and if, and Extended Tcl's loop commands.

Note the call to Tcl_SetResult in the above command to set the return value to Tcl. TCL_VOLATILE is used because the memory containing the result will be freed upon the func
tion's return.

ANOTHER C EXTENSION - THE LREVERSE COMMAND

In the command below, one list is passed as an argument,
and a list containing all of the elements of the list in
reverse order is returned. It is an error if anything
other than two arguments are passed (the pointer to the
``lreverse'' command text itself, argv[0], and a pointer to the list to reverse.

Once lreverse has determined that it has received the cor rect number of arguments, Tcl_SplitList is called to break the list into an argc and argv array of pointers.

lreverse then operates on the array of pointers, swapping them from lowest to highest, second-lowest to second-high
est, and so forth.

Finally Tcl_Merge is calleds to create a single new string containing the reversed list and it is set as the result
via Tcl_SetResult. Note that TCL_DYNAMIC is used to tell Tcl_SetResult that it now owns the string and it is up to Tcl to free the string when it is done with it.

Note that it is safe to play around with the argv list like this, and that a single call to ckfree can be made to free all the data returned by Tcl_SplitList in this man ner.

int
Tcl_LreverseCmd(notUsed, interp, argc, argv)

ClientData notUsed; /* Not used. */
Tcl_Interp *interp; /* Current interpreter.

*/
int argc; /* Number of arguments. */
char **argv; /* Argument strings. */

{

int listArgc, lowListIndex, hiListIndex;
char **listArgv;
char *temp, *resultList;

if (argc != 2) {

Tcl_AppendResult(interp, "wrong # args: should be

" list

return TCL_ERROR;

}

if (Tcl_SplitList(interp, argv[1], &listArgc, &lis

tArgv) != TCL_OK) {

return TCL_ERROR;

}
for (lowListIndex = 0, hiListIndex = listArgc;

--hiListIndex > lowListIndex; lowListIndex++) {

temp = listArgv[lowListIndex];
listArgv[lowListIndex] = listArgv[hiListIndex];
listArgv[hiListIndex] = temp;

}
resultList = Tcl_Merge (listArgc, listArgv);
ckfree (listArgv);
Tcl_SetResult (interp, resultList, TCL_DYNAMIC);
return TCL_OK;

}

INSTALLING YOUR COMMAND

To install your command into Tcl you must call Tcl_Create Command, passing it the pointer to the interpreter you want to install the command into, the name of the command,
a pointer to the C function that implements the command, a
client data pointer, and a pointer to an optional callback
routine.

The client data pointer and the callback routine will be
described later.

For example, for the max function above (which, inciden
tally, comes from TclX's tclXmath.c in the TclX7.4/src directory):

Tcl_CreateCommand (interp, "max", Tcl_MaxCmd, (Client

Data)NULL,

(void (*)())NULL);

In the above example, the max function is added to the
specified interpreter. The client data pointer and call
back function pointer are NULL. (For complete information
on Tcl_CreateCommand and its companion routine, Tcl_Com mandInfo, please examine the CrtCommand(3) command page in the Berkeley Tcl distribution.)

DYNAMIC STRINGS

Dynamic strings are an important abstraction that first became available with Tcl 7.0. Dynamic strings, or
DStrings, provide a way to build up arbitrarily long strings through a repeated process of appending informa
tion to them. DStrings reduce the amount of allocating
and copying required to add information to a string. Fur
ther, they simplify the process of doing so. For complete
information on dynamic strings, please examine the
DString(3) manual page in the Berkeley Tcl distribution.

CLIENT DATA

The client data pointer provides a means for Tcl commands
to have data associated with them that is not global to
the C program nor included in the Tcl core. Client data
is essential in a multi-interpreter environment (where a
single program has created and is making use of multiple
Tcl interpreters) for the C routines to maintain any per
manent data they need on a per-interpreter basis. Other
wise there would be reentrancy problems. Tcl solves this
through the client data mechanism. When you are about to
call Tcl_CreateCommand to add a new command to an inter preter, if that command needs to keep some read/write data
across invocations, you should allocate the space, prefer
ably using ckalloc, then pass the address of that space as the ClientData pointer to Tcl_CreateCommand.

When your command is called from Tcl, the ClientData
pointer you gave to Tcl_CreateCommand when you added the command to that interpreter is passed to your C routine
through the ClientData pointer calling argument.

Commands that need to share this data with one another can
do so by using the same ClientData pointer when the com
mands are added.

It is important to note that the Tcl extensions in the
tclX7.4/src directory have had all of their data set up in this way. Since release 6.2, Extended Tcl has supported
multiple interpreters within one invocation of Tcl.

THEORY OF HANDLES

Sometimes you need to have a data element that isn't read
ily representable as a string within Tcl, for example a
pointer to a complex C data structure. It is not a good
idea to try to pass pointers around within Tcl as strings
by converting them to and from hex or integer representa
tions, for example. It is too easy to mess one up, and
the likely outcome of doing that is a core dump.

Instead we have developed and made use of the concept of
handles. Handles are identifiers a C extension can pass to, and accept from, Tcl to make the transition between
what your C code knows something as and what name Tcl
knows it by to be as safe and painless as possible. For
example, the stdio package included in Tcl uses file han
dles. When you open a file from Tcl, a handle is returned
of the form filen where n is a file number. When you pass the file handle back to puts, gets, seek, flush and so forth, they validate the file handle by checking the the
file text is present, then converting the file number to
an integer that they use to look into a data structure of
pointers to Tcl open file structures, which contain a Unix
file descriptor, flags indicating whether or not the file
is currently open, whether the file is a file or a pipe
and so forth.

Handles have proven so useful that, as of release 6.1a,
general support has been added for them. If you need a
similar capability, it would be best to use the handle
routines, documented in Handles(3) in Extended Tcl. We recommend that you use a unique-to-your-package textual
handle coupled with a specific identifier and let the han
dle management routines validate it when it's passed back.
It is much easier to track down a bug with an implicated
handle named something like file4 or bitmap6 than just 6.

TRACKING MEMORY CORRUPTION PROBLEMS

Occasionally you may write code that scribbles past the
end of an allocated piece of memory. The memory debugging
routines included in Tcl can help find these problems.
See Memory(TCL) for details.

INSTALLING YOUR EXTENSIONS INTO EXTENDED TCL

To add your extensions to Extended Tcl, you must compile
them and cause them to be linked with TclX. For the rou
tines to be linked into the tcl and wishx executables, they must be referenced (directly or indirectly) from
TclX. For these extensions to be visible as Tcl commands,
they must be installed into Tcl with Tcl_CreateCommand.

Application-specific startup is accomplished by creating
or editing the Tcl_AppInit function. In Tcl_AppInit you should add a call to an application-specific init function
which you create. This function should take the address
of the interpreter it should install its commands into,
and it should install those commands with Tcl_CreateCom mand and do any other application-specific startup that is
necessary.

The naming convention for application startup routines is
App_Init, where App is the name of your application. For example, to add an application named cute one would create
a Cute_Init routine that expected a Tcl_Interp pointer as an argument, and add the following code to Tcl_AppInit:

if (Cute_Init (interp) == TCL_ERROR) {

return TCL_ERROR;

}

As you can guess from the above example, if your init rou
tine is unable to initialize, it should use Tcl_Appen dResult to provide some kind of useful error message back to TclX, then return TCL_ERROR to indicate that an error occurred. If the routine executed successfully, it should
return TCL_OK.

When you examine Tcl_AppInit, note that there is one call already there to install an application -- the call to
TclX_Init installs Extended Tcl into the Tcl core.

MAKING APPLICATION INFORMATION VISIBLE FROM EXTENDED TCL

TclX's infox command can return several pieces of informa
tion relevant to Extended Tcl, including the application's
name, descriptive name, patch level and version. Your
application's startup can set these variables to applica
tion-specific values. If it doesn't, they are given
default values for Extended Tcl.

To set these values, first be sure that you include either
tclExtend.h or tclExtdInt.h from the source file that defines your init routine. This will create external dec
larations for the variables. Then, set the variables in
your init route, for example:

tclAppName = "cute";
tclAppLongName = "Call Unix/Tcl Environment";
tclAppVersion = "2.1";

Note that the default values are set by TclX_Init, so if you wish to override them, you must call your init routine
in Tcl_AppInit after its call to TclX_Init.

EXTENDED TCL EXIT

When Extended Tcl exits, Tcl_DeleteInterp may be called to free memory used by Tcl -- normally, this is only called
if TCL_MEM_DEBUG was defined, since Unix will return all of the allocated memory back to the system, anyway. If
TCL_MEM_DEBUG was defined, it is called so that any memory that was allocated without ever being freed can be
detected. This greatly reduces the amount of work to
detect and track down memory leaks, a situation where some
piece of your code allocates memory repeatedly without
ever freeing it, or without always freeing it.

It is often necessary for an application to perform spe
cial cleanup functions upon the deletion of an interpreter
as well. To facilitate this activity, Tcl provides the
ability to perform a function callback when an interpreter
is deleted. To arrange for a C function to be called when
the interpreter is deleted, call Tcl_CallWhenDeleted from your application initialization routine. For details on
how to use this function, read the CallDel(3) manual page that ships with Berkeley Tcl.

EXECUTING TCL CODE FROM YOUR C EXTENSION

Suppose you are in the middle of coding a C extension and
you realize that you need some operation performed, one
that would be simple from Tcl but possibly excruciating to
do directly in C. Tcl provides the Tcl_Eval, Tcl_VarEval, Tcl_EvalFile and Tcl_GlobalEval functions for the purpose of executing Tcl code from within a C extension. The
results of the call will be in interp->result. For more information please consult the Eval(3) manual page within
the Tcl distribution.

ACCESSING TCL VARIABLES AND ARRAYS FROM YOUR C EXTENSIONS

Tcl variables and arrays can be read from a C extension
through the Tcl_GetVar and Tcl_GetVar2 functions, and set from C extensions through the Tcl_SetVar and Tcl_SetVar2 functions. They can also be unset via the Tcl_UnsetVar and Tcl_UnsetVar2 functions. For complete information on these functions, please refer to the SetVar(3) manual page in the doc directory of the Berkeley Tcl distribution.

LINKING TCL VARIABLES TO C VARIABLES

Tcl_LinkVar and Tcl_UnlinkVar can be used to automatically keep Tcl variables synchronized with corresponding C vari
ables. Once a Tcl variable has been linked to a C vari
able with Tcl_LinkVar, anytime the Tcl variable is read the value of the C variable will be returned, and when the
Tcl variable is written, the C variable will be updated
with the new value.

Tcl_LinkVar uses variable traces to keep the Tcl variable named by varName in sync with the C variable at the address given by addr.

Whenever the Tcl variable is read the value of the C vari
able will be returned, and whenever the Tcl variable is
written the C variable will be updated to have the same
value.

Int, double, boolean and char * variables are supported. For more information, please examine the LinkVar(3) manual page in the Berkeley Tcl distribution.

ADDING NEW MATH FUNCTIONS TO TCL

As of Tcl version 7.0, math functions such as sin, cos, etc, are directly supported within Tcl expressions. These
obsolete the Extended Tcl commands that provided explicit
calls for these functions for many releases.

New math functions can be added to Tcl, or existing math
functions can be replaced, by calling Tcl_CreateMathFunc.

For more information on adding math functions, please
examine the CrtMathFnc(3) manual page in the Berkeley Tcl distribution.

PERFORMING TILDE SUBSTITUTIONS ON FILENAMES

The Tcl_TildeSubst function is available to C extension writers to perform tilde substitutions on filenames. If
the name starts with a ``~'' character, the function
returns a new string where the name is replaced with the
home directory of the given user. For more information
please consult the TildeSubst(3) manual page in the Berke ley Tcl distribution.

SETTING THE RECURSION LIMIT

Tcl has a preset recursion limit that limits the maximum
allowable nesting depth of calls within an interpreter.
This is useful for detecting infinite recursions before
other limits such as the process memory limit or, worse,
available swap space on the system, are exceeded.

The default limit is just a guess, however, and
applications that make heavy use of recursion may need to
call Tcl_SetRecursionLimit to raise this limit. For more information, please consult the SetRecLmt(3) manual page in the Berkeley Tcl distribution.

HANDLING SIGNALS FROM TCL EXTENSIONS

If an event such as a signal occurs while a Tcl script is
being executed, it isn't safe to do much in the signal
handling routine -- the Tcl environment cannot be safely
manipulated at this point because it could be in the mid
dle of some operation, such as updating pointers, leaving
the interpreter in an unreliable state.

The only safe approach is to set a flag indicating that
the event occurred, then handle the event later when the
interpreter has returned to a safe state, such as after
the current Tcl command completes.

The Tcl_AsyncCreate, Tcl_AsyncMark, Tcl_AsyncInvoke, and Tcl_AsyncDelete functions provide a safe mechanism for dealing with signals and other asynchronous events. For
more information on how to use this capability, please
refer to the Async(3) manual page in the Berkeley Tcl dis
tribution.

PARSING BACKSLASH SEQUENCES

The Tcl_Backslash function is called to parse Tcl back slash sequences. These backslash sequences are the usual
sort that roforereturn,eandprsoraforth.lanTclg_Backch as

n for newline,

slash parses a single backslash sequence and returns a
single character corresponding to the backslash sequence.

For more info on this call, look at the Backslash(3) man ual page in the Berkeley Tcl distribution. For informa
tion on the valid backslash sequences, consult the summary
of Tcl language syntax, Tcl(n) in the same distribution.

HASH TABLES

Hash tables provide Tcl with a high-performance facility for looking up and managing key-value pairs located and
maintained in memory. Tcl uses hash tables internally to
locate procedure definitions, Tcl variables, array ele
ments, file handles and so forth. Tcl makes the hash
table functions accessible to C extension writers as well.

Hash tables grow automatically to maintain efficiency,
rather than exposing the table size to the programmer at
allocation time, which would needlessly add complexity to
Tcl and would be prone to inefficiency due to the need to
guess the number of items that will go into the table, and
the seemingly inevitable growth in amount of data pro
cessed per run over the life of the program.

For more information on hash tables, please consult the
Hash(3) manual page in the Berkeley Tcl distribution.

TRACING VARIABLE ACCESSES

The C extension writer can arrange to have a C routine
called whenever a Tcl variable is read, written, or unset.
Variable traces are the mechanism by which Tk toolkit wid
gets such as radio and checkbuttons, messages and so forth
update without Tcl programmer intervention when their data
variables are changed. They are also used by the routine
that links Tcl and C variables, Tcl_LinkVar, described above.

Tcl_TraceVar is called to establish a variable trace. Entire arrays and individual array elements can be traced
as well. If the programmer already has an array name in
one string and a variable name in another, Tcl_TraceVar2 can be called. Calls are also available to request infor
mation about traces and to delete them.

For more information on variable traces, consult the
TraceVar(3) manual page in the Berkeley Tcl distribution.

TRACING EXECUTION

Tcl has the ability to call C routines for every command
it executes, up to a specified depth of nesting levels.
The command Tcl_CreateTrace creates an execution trace;
Tcl_DeleteTrace deletes it.

Command tracing is used in Extended Tcl to implement the
cmdtrace Tcl command, a useful command for debugging Tcl applications.

For complete information on execution tracing, please look
at the CrtTrace(3) manual pages in the Berkeley Tcl dis tribution.

EVALUATING TCL EXPRESSIONS FROM C

Tcl_ExprLong, Tcl_ExprDouble, Tcl_ExprBool, and Tcl_ExprString can be called to evaluate Tcl expressions from within a C routine. Depending on the routine called,
the result is either a C long, a double, a boolean (int with a value of 0 or 1), or a char * (pointed to by interp->result).

For complete information on evaluating Tcl expressions
from C, you are invited to examine the ExprLong(3) manpage in the Berkeley Tcl distribution.

PATTERN MATCHING

The Tcl_StringMatch function can be called to see if a string matches a specified pattern. Tcl_StringMatch is called by the Tcl string match command, so the format for patterns is identical. The pattern format is similar to
the one used by the C-shell; string(n) describes this for
mat.

More information about Tcl_StringMatch is available in the StrMatch(3) manpage in the Berkeley Tcl distribution.

REGULAR EXPRESSION PATTERN MATCHING

Tcl_RegExpMatch can be called to determine whether a string matches a regular expression. Tcl_RegExpMatch is used internally by the regexp Tcl command.

For more information on this function, please consult the
RegExp(3) manpage in the Berkeley Tcl distribution.

MANIPULATING TCL LISTS FROM C EXTENSIONS

The C extension writer often needs to create, manipulate
and decompose Tcl lists. Tcl_SplitList parses a list into an argv and argc like to the way command-line arguments are passed to a Tcl extension. Tcl_Merge, likewise, cre ates a single string (pointer to a char *) from an argv and argc.

Two routines, Tcl_ScanElement and Tcl_ConvertElement, do most of the work of Tcl_Merge, and may also be of use to the C programmer.

For more information on these commands, please consult the
SplitList(3) manual page in the Berkeley Tcl distribution.

CONCATENATING STRINGS

Tcl_Concat concatenates zero or more strings into a single string. The strings are space-separated. Tcl_Concat works like Tcl_Merge, except that Tcl_Concat does not attempt to make the resulting string into a valid Tcl
list.

Tcl_Concat is documented in the Concat(3) manpage in the Berkeley Tcl distribution.

DETECTING WHETHER OR NOT YOU HAVE A COMPLETE COMMAND

C routines that collect data to form a command to be
passed to Tcl_Eval often need a way to tell whether they have a complete command already or whether they need more
data. (Programs that read typed-in Tcl input such as Tcl
shells need this capability.) Tcl_CommandComplete can be used to tell whether or not you have a complete command.

For more information examine CmdCmplt(3) in the Berkeley Tcl distribution.

RECORDING COMMANDS FOR COMMAND HISTORY

Tcl has a history mechanism that is accessed from Tcl
through the history command. To propagate commands into the command history, your extension should call Tcl_Recor_ dAndEval. This command works just like Tcl_Eval, except that it records the command as well as executing it.

Tcl_RecordAndEval should only be called with user-entered top-level commands, since the history mechanism exists to
allow the user to easily access, edit and reissue previ
ously issued commands.

For complete information on this function, please examine
the RecordEval.3 manual page in the Berkeley Tcl distribu tion.

CONVERTING FLOATING POINT VALUES TO STRINGS

Tcl_PrintDouble converts a C double into an ASCII string. It ensures that the string output will continue to be
interpreted as a floating point number, rather than an
integer, by always putting a ``.'' or ``e'' into the
string representing the number. The precision of the out
put string is controlled by the Tcl tcl_precision vari able.

For complete information on Tcl_PrintDouble, examine PrintDbl(3) in the Berkeley Tcl distribution.

CREATING CHILD PROCESSES AND PIPELINES FROM C

Tcl_CreatePipeline is a useful procedure for spawning child processes. The child (or pipeline of children) can
have its standard input, output and error redirected from
files, variables or pipes. To understand the meaning of
the redirection symbols understood by this function, look
at the exec(n) Tcl command. For complete information on
Tcl_CreatePipeline, please examine CrtPipelin(3).

ACCESSING TCL FILEHANDLES FROM C

Files opened from your C code can be made visible to Tcl
code via the Tcl_EnterFile function. Likewise, Tcl file handles passed to your C extension can be translated to a
Posix FILE * structure using the Tcl_GetOpenFile function.

For complete explanations of these commands, please look
at EnterFile(3) in the Berkeley Tcl distribution.

MANAGING BACKGROUND PROCESS TERMINATION AND CLEANUP

When a Posix system does a fork to create a new process,
the process ID of the child is returned to the caller.
After the child process exits, its process table entry
(and some other data associated with the process) cannot
be reclaimed by the operating system until a call to wait_
pid, or one of a couple of other, similar system calls,
has been made by the parent process.

The C extension writer who has created a subprocess, by
whatever mechanism, can turn over responsibility for
detecting the processes' termination and calling waitpid to obtain its exit status by calling Tcl_DetachPids.

Tcl_ReapDetachedProcs is the C routine that will detect the termination of any processes turned over to Tcl, per
mitting the processes to be fully reclaimed by the operat
ing system.

For complete information on these routines, please look at
DetachPids(3) in the Berkeley Tcl distribution.

FOR MORE INFORMATION

In addition to the documentation referenced above, you can
learn a lot by studying the source code of the commands
added by Tcl, Tk and Extended Tcl. The comp.lang.tcl Usenet newsgroup is read by tens of thousands of Tcl peo
ple, and is a good place to ask questions. Finally, if
you have interactive Internet access, you can ftp to
ftp.aud.alcatel.com, the site for contributed Tcl sources. This site contains quite a few extensions, applications,
and so forth, including several object-oriented extension
packages.

AUTHORS

Extended Tcl was created by Karl Lehenbauer
(karl@neosoft.com) and Mark Diekhans (markd@grizzly.com).

Tcl Command Writing(TCL)