Tcl_CreateCommand(3) Tcl Library Procedures Tcl_CreateCommand(3)______________________________________________________________________________NAME
Tcl_CreateCommand - implement new commands in C
Tcl_CreateCommand(interp, cmdName, proc, clientData, deleteProc)
Tcl_Interp *interp (in) Interpreter in which to
create new command.
char *cmdName (in) Name of command.
Tcl_CmdProc *proc (in) Implementation of new
command: proc will be
called whenever cmdName
is invoked as a com‐
ClientData clientData (in) Arbitrary one-word
value to pass to proc
Tcl_CmdDeleteProc *deleteProc (in) Procedure to call
before cmdName is
deleted from the inter‐
preter; allows for com‐
If NULL, then no proce‐
dure is called before
the command is deleted.
Tcl_CreateCommand defines a new command in interp and associates it
with procedure proc such that whenever cmdName is invoked as a Tcl com‐
mand (via a call to Tcl_Eval) the Tcl interpreter will call proc to
process the command. It differs from Tcl_CreateObjCommand in that a
new string-based command is defined; that is, a command procedure is
defined that takes an array of argument strings instead of objects.
The object-based command procedures registered by Tcl_CreateObjCommand
can execute significantly faster than the string-based command proce‐
dures defined by Tcl_CreateCommand. This is because they take Tcl
objects as arguments and those objects can retain an internal represen‐
tation that can be manipulated more efficiently. Also, Tcl's inter‐
preter now uses objects internally. In order to invoke a string-based
command procedure registered by Tcl_CreateCommand, it must generate and
fetch a string representation from each argument object before the call
and create a new Tcl object to hold the string result returned by the
string-based command procedure. New commands should be defined using
Tcl_CreateObjCommand. We support Tcl_CreateCommand for backwards com‐
The procedures Tcl_DeleteCommand, Tcl_GetCommandInfo, and Tcl_SetCom‐
mandInfo are used in conjunction with Tcl_CreateCommand.
Tcl_CreateCommand will delete an existing command cmdName, if one is
already associated with the interpreter. It returns a token that may
be used to refer to the command in subsequent calls to Tcl_GetCommand‐
Name. If cmdName contains any :: namespace qualifiers, then the com‐
mand is added to the specified namespace; otherwise the command is
added to the global namespace. If Tcl_CreateCommand is called for an
interpreter that is in the process of being deleted, then it does not
create a new command and it returns NULL. Proc should have arguments
and result that match the type Tcl_CmdProc:
typedef int Tcl_CmdProc(
When proc is invoked the clientData and interp parameters will be
copies of the clientData and interp arguments given to Tcl_CreateCom‐
mand. Typically, clientData points to an application-specific data
structure that describes what to do when the command procedure is
invoked. Argc and argv describe the arguments to the command, argc
giving the number of arguments (including the command name) and argv
giving the values of the arguments as strings. The argv array will
contain argc+1 values; the first argc values point to the argument
strings, and the last value is NULL. Note that the argument strings │
should not be modified as they may point to constant strings or may be │
shared with other parts of the interpreter.
Proc must return an integer code that is either TCL_OK, TCL_ERROR,
TCL_RETURN, TCL_BREAK, or TCL_CONTINUE. See the Tcl overview man page
for details on what these codes mean. Most normal commands will only
return TCL_OK or TCL_ERROR. In addition, proc must set the interpreter
result to point to a string value; in the case of a TCL_OK return code
this gives the result of the command, and in the case of TCL_ERROR it
gives an error message. The Tcl_SetResult procedure provides an easy
interface for setting the return value; for complete details on how
the the interpreter result field is managed, see the Tcl_Interp man
page. Before invoking a command procedure, Tcl_Eval sets the inter‐
preter result to point to an empty string, so simple commands can
return an empty result by doing nothing at all.
The contents of the argv array belong to Tcl and are not guaranteed to
persist once proc returns: proc should not modify them, nor should it
set the interpreter result to point anywhere within the argv values.
Call Tcl_SetResult with status TCL_VOLATILE if you want to return some‐
thing from the argv array.
DeleteProc will be invoked when (if) cmdName is deleted. This can
occur through a call to Tcl_DeleteCommand or Tcl_DeleteInterp, or by
replacing cmdName in another call to Tcl_CreateCommand. DeleteProc is
invoked before the command is deleted, and gives the application an
opportunity to release any structures associated with the command.
DeleteProc should have arguments and result that match the type
typedef void Tcl_CmdDeleteProc(ClientData clientData);
The clientData argument will be the same as the clientData argument
passed to Tcl_CreateCommand.
Tcl_CreateObjCommand, Tcl_DeleteCommand, Tcl_GetCommandInfo, Tcl_Set‐
CommandInfo, Tcl_GetCommandName, Tcl_SetObjResult
bind, command, create, delete, interpreter, namespace