Command Writing(TCL) Command Writing(TCL)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 existing editor, communications program, win‐
dow manager, etc. C programming information can also be found in the
*.3 manual pages in the doc directory of the Berkeley distribution, 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("\n");
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 cur‐
rently defined procedures, commands, variables, arrays and the execu‐
tion state of that interpreter. (For more information 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 argu‐
ments 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 point‐
ers works the same as in a C main call; pointers to the arguments to
the function 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 over‐
written 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 SetRe‐
sult(3) manual page in the Tcl distribution.
Sophisticated commands should verify their arguments whenever 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 labor-saving
Tcl_AppendResult routine. See the Tcl manual page, SetResult(3), for
details. Also examine the calls Tcl_AddErrorInfo, Tcl_SetErrorCode and
Tcl_PosixError documented 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 command 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 control and/or looping structures
to Tcl. To see these values 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 function'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 correct 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-highest, 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 manner.
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, &listArgv) != 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_CreateCommand, pass‐
ing 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 imple‐
ments 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, incidentally, comes
from TclX's tclXmath.c in the TclX7.4/src directory):
Tcl_CreateCommand (interp, "max", Tcl_MaxCmd, (ClientData)NULL,
(void (*)())NULL);
In the above example, the max function is added to the specified inter‐
preter. The client data pointer and callback function pointer are
NULL. (For complete information on Tcl_CreateCommand and its companion
routine, Tcl_CommandInfo, please examine the CrtCommand(3) command page
in the Berkeley Tcl distribution.)
DYNAMIC STRINGS
Dynamic strings are an important abstraction that first became avail‐
able with Tcl 7.0. Dynamic strings, or DStrings, provide a way to
build up arbitrarily long strings through a repeated process of append‐
ing information to them. DStrings reduce the amount of allocating and
copying required to add information to a string. Further, they sim‐
plify 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 envi‐
ronment (where a single program has created and is making use of multi‐
ple Tcl interpreters) for the C routines to maintain any permanent data
they need on a per-interpreter basis. Otherwise there would be reen‐
trancy problems. Tcl solves this through the client data mechanism.
When you are about to call Tcl_CreateCommand to add a new command to an
interpreter, if that command needs to keep some read/write data across
invocations, you should allocate the space, preferably 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 argu‐
ment.
Commands that need to share this data with one another can do so by
using the same ClientData pointer when the commands 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 invo‐
cation of Tcl.
THEORY OF HANDLES
Sometimes you need to have a data element that isn't readily repre‐
sentable 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
representations, 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. Han‐
dles 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 handles. 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 handle management rou‐
tines 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 bit‐
map6 than just 6.
TRACKING MEMORY CORRUPTION PROBLEMS
Occasionally you may write code that scribbles past the end of an allo‐
cated 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 routines to be linked into the
tcl and wishx executables, they must be referenced (directly or indi‐
rectly) 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 com‐
mands into, and it should install those commands with Tcl_CreateCommand
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 routine is unable
to initialize, it should use Tcl_AppendResult 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 information relevant
to Extended Tcl, including the application's name, descriptive name,
patch level and version. Your application's startup can set these
variables to application-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 declarations 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 special 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 variables. Once a Tcl
variable has been linked to a C variable 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 var‐
Name in sync with the C variable at the address given by addr.
Whenever the Tcl variable is read the value of the C variable 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 Crt‐
MathFnc(3) manual page in the Berkeley Tcl distribution.
PERFORMING TILDE SUBSTITUTIONS ON FILENAMES
The Tcl_TildeSubst function is available to C extension writers to per‐
form 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 Berkeley 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) man‐
ual 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 exe‐
cuted, 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 middle 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 distribution.
PARSING BACKSLASH SEQUENCES
The Tcl_Backslash function is called to parse Tcl backslash sequences.
These backslash sequences are the usual sort that you see in the C pro‐
gramming language, such as \n for newline, \r for return, and so forth.
Tcl_Backslash 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) manual page in the
Berkeley Tcl distribution. For information 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 vari‐
ables, array elements, 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 ineffi‐
ciency 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 widgets 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
information 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 Crt‐
Trace(3) manual pages in the Berkeley Tcl distribution.
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 bool‐
ean (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 distri‐
bution.
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
format.
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, creates 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 distribu‐
tion.
RECORDING COMMANDS FOR COMMAND HISTORY
Tcl has a history mechanism that is accessed from Tcl through the his‐
tory command. To propagate commands into the command history, your
extension should call Tcl_RecordAndEval. 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 eas‐
ily access, edit and reissue previously issued commands.
For complete information on this function, please examine the RecordE‐
val.3 manual page in the Berkeley Tcl distribution.
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
output string is controlled by the Tcl tcl_precision variable.
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_Cre‐
atePipeline, 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 filehandles 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 Enter‐
File(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
waitpid, 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 mecha‐
nism, can turn over responsibility for detecting the processes' termi‐
nation 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, permitting the processes to be
fully reclaimed by the operating system.
For complete information on these routines, please look at Detach‐
Pids(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 people, 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)
