## Tcl8.6.0/Tk8.6.0 Documentation > [incr Tcl] Package C API, version 4.0.0 > RegisterC

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### NAME

Itcl_RegisterC, Itcl_RegisterObjC, Itcl_FindC — Associate a symbolic name with a C procedure.

### SYNOPSIS

#include <itcl.h>
int
Itcl_RegisterC(interp, cmdName, argProc, clientData, deleteProc)
int
Itcl_RegisterObjC(interp, cmdName, objProc, clientData, deleteProc)
int
Itcl_FindC(interp, cmdName, argProcPtr, objProcPtr, cDataPtr)

### ARGUMENTS

Tcl_Interp *interp (in)
  Interpreter in which to create new command.

CONST char *cmdName (in)
  Name of command.

Tcl_CmdProc *argProc (in)
  Implementation of new command: argProc will be called whenever

Tcl_CmdProc **argProcPtr (in/out)
  The Tcl_CmdProc * to receive the pointer.

Tcl_ObjCmdProc *objProc (in)
  Implementation of the new command: objProc will be called whenever

Tcl_ObjCmdProc **objProcPtr (in/out)
  The Tcl_ObjCmdProc * to receive the pointer.

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

ClientData *cDataPtr (in/out)
  The ClientData to receive the pointer.

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.

### DESCRIPTION

Used to associate a symbolic name with an (argc,argv) C procedure that handles a Tcl command. Procedures that are registered in this manner can be referenced in the body of an [incr Tcl] class definition to specify C procedures to acting as methods/procs. Usually invoked in an initialization routine for an extension, called out in Tcl_AppInit() at the start of an application.

Each symbolic procedure can have an arbitrary client data value associated with it. This value is passed into the command handler whenever it is invoked.

A symbolic procedure name can be used only once for a given style (arg/obj) handler. If the name is defined with an arg-style handler, it can be redefined with an obj-style handler; or if the name is defined with an obj-style handler, it can be redefined with an arg-style handler. In either case, any previous client data is discarded and the new client data is remembered. However, if a name is redefined to a different handler of the same style, this procedure returns an error.

Returns TCL_OK on success, or TCL_ERROR (along with an error message in interp->result) if anything goes wrong.

C procedures can be integrated into an [incr Tcl] class definition to implement methods, procs, and the "config" code for public variables. Any body that starts with "@" is treated as the symbolic name for a C procedure.

Symbolic names are established by registering procedures via Itcl_RegisterC(). This is usually done in the Tcl_AppInit() procedure, which is automatically called when the interpreter starts up. In the following example, the procedure `My_FooCmd()` is registered with the symbolic name "foo". This procedure can be referenced in the body command as "`@foo`".

    int
    Tcl_AppInit(interp)
        Tcl_Interp *interp;     /* Interpreter for application. */
    {
        if (Itcl_Init(interp) == TCL_ERROR) {
            return TCL_ERROR;
        }

        if (Itcl_RegisterC(interp, "foo", My_FooCmd) != TCL_OK) {
            return TCL_ERROR;
        }
    }

C procedures are implemented just like ordinary Tcl commands. See the CrtCommand man page for details. Within the procedure, class data members can be accessed like ordinary variables using Tcl_SetVar(), Tcl_GetVar(), Tcl_TraceVar(), etc. Class methods and procs can be executed like ordinary commands using Tcl_Eval(). [incr Tcl] makes this possible by automatically setting up the context before executing the C procedure.

This scheme provides a natural migration path for code development. Classes can be developed quickly using Tcl code to implement the bodies. An entire application can be built and tested. When necessary, individual bodies can be implemented with C code to improve performance.

See the Archetype class in [incr Tk] for an example of how this C linking method is used.

### SEE ALSO

Tcl_CreateCommand, Tcl_CreateObjCommand

### KEYWORDS

class, object
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