# Tcl_CreateCommand, Tcl_DeleteCommand, Tcl_GetCommandInfo, Tcl_SetCommandInfo - implement new commands in C

## SYNOPSIS

    #include <tcl.h>

    Tcl_Command
    Tcl_CreateCommand(interp, cmdName, proc, clientData, deleteProc)

    int
    Tcl_DeleteCommand(interp, cmdName)

    int
    Tcl_GetCommandInfo(interp, cmdName, infoPtr)

    int
    Tcl_SetCommandInfo(interp, cmdName, infoPtr)

    char *
    Tcl_GetCommandName(interp, token)

## ARGUMENTS

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 command.

ClientData clientData (in)
  Arbitrary one-word value to pass to proc and deleteProc.

Tcl_CmdDeleteProc *deleteProc (in)
  Procedure to call before cmdName is deleted from the interpreter; allows for command-specific cleanup. If NULL, then no procedure is called before the command is deleted.

Tcl_CmdInfo *infoPtr (in/out)
  Pointer to structure containing various information about a Tcl command.

Tcl_Command token (in)
  Token for command, returned by previous call to Tcl_CreateCommand. The command must not have been deleted.

## DESCRIPTION

Tcl_CreateCommand defines a new command in interp and associates it with procedure proc such that whenever cmdName is invoked as a Tcl command (via a call to Tcl_Eval) the Tcl interpreter will call proc to process the command. If there is already a command cmdName associated with the interpreter, it is deleted. Tcl_CreateCommand returns a token that may be used to refer to the command in subsequent calls to Tcl_GetCommandName. Proc should have arguments and result that match the type Tcl_CmdProc:

    typedef int Tcl_CmdProc(

    ClientData clientData,
    Tcl_Interp *interp,
    int argc,
    char *argv[]);

When proc is invoked the clientData and interp parameters will be copies of the clientData and interp arguments given to Tcl_CreateCommand. 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.

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 interp->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 interp->result field is managed, see the Tcl_Interp man page. Before invoking a command procedure, Tcl_Eval sets interp->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 interp->result to point anywhere within the argv values. Call Tcl_SetResult with status TCL_VOLATILE if you want to return something 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 Tcl_CmdDeleteProc:

    typedef void Tcl_CmdDeleteProc(ClientData clientData);

The clientData argument will be the same as the clientData argument passed to Tcl_CreateCommand.

Tcl_DeleteCommand deletes a command from a command interpreter. Once the call completes, attempts to invoke cmdName in interp will result in errors. If cmdName isn't bound as a command in interp then Tcl_DeleteCommand does nothing and returns -1; otherwise it returns 0. There are no restrictions on cmdName: it may refer to a built-in command, an application-specific command, or a Tcl procedure.

Tcl_GetCommandInfo checks to see whether its cmdName argument exists as a command in interp. If not then it returns 0. Otherwise it places information about the command in the structure pointed to by infoPtr and returns 1. Tcl_CmdInfo structures have fields named proc, clientData, and deleteProc, which have the same meaning as the corresponding arguments to Tcl_CreateCommand. There is also a field deleteData, which is the ClientData value to pass to deleteProc; it is normally the same as clientData but may be set independently using the Tcl_SetCommandInfo procedure.

Tcl_SetCommandInfo is used to modify the procedures and ClientData values associated with a command. Its cmdName argument is the name of a command in interp. If this command does not exist then Tcl_SetCommandInfo returns 0. Otherwise, it copies the information from *infoPtr to Tcl's internal structure for the command and returns 1. Note that this procedure allows the ClientData for a command's deletion procedure to be given a different value than the ClientData for its command procedure.

Tcl_GetCommandName provides a mechanism for tracking commands that have been renamed. Given a token returned by Tcl_CreateCommand when the command was created, Tcl_GetCommandName returns the string name of the command. If the command has been renamed since it was created, then Tcl_GetCommandName returns the current name. The command corresponding to token must not have been deleted. The string returned by Tcl_GetCommandName is in dynamic memory owned by Tcl and is only guaranteed to retain its value as long as the command isn't deleted or renamed; callers should copy the string if they need to keep it for a long time.

## KEYWORDS

bind, command, create, delete, interpreter