Tcl_CreateCommand(3tcl)

💖 NAME 💖

Tcl_CreateCommand — bestie, this is how you implement brand-new commands in C. Periodt.

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✨ SYNOPSIS ✨

#include <tcl.h>

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

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🎀 ARGUMENTS 🎀

• Tcl_Interp *interp (in) — the interpreter you're creating this shiny new command inside of.
• const char *cmdName (in) — the name of the command. That's the thing people type.
• Tcl_CmdProc *proc (in) — the actual implementation of your new command: proc gets called literally whenever cmdName is invoked as a command.
• void *clientData (in) — some arbitrary one-word value, and it gets passed to both proc and deleteProc. Your little carry-on bag, no cap.
• Tcl_CmdDeleteProc *deleteProc (in) — the procedure that gets called before cmdName is deleted from the interpreter; this is where command-specific cleanup lives. If it's NULL, then no procedure is called before the command is deleted.

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🦋 DESCRIPTION 🦋

So Tcl_CreateCommand defines a new command in interp and associates it with the procedure proc, such that whenever cmdName is invoked as a Tcl command (like, via a call to Tcl_Eval) the Tcl interpreter will call proc to process that command. Iconic behavior, honestly.

It differs from Tcl_CreateObjCommand in that a new string-based command is defined. Meaning: a command procedure is defined that takes an array of argument strings instead of values. And here's the tea — the value-based command procedures registered by Tcl_CreateObjCommand can execute significantly faster than the string-based command procedures defined by Tcl_CreateCommand. Why? Because they take Tcl values as arguments and those values can retain an internal representation that can be manipulated more efficiently. Also, Tcl's interpreter now uses values internally. So in order to invoke a string-based command procedure registered by Tcl_CreateCommand, it has to generate and fetch a string representation from each argument value before the call. 😭

New commands should be defined using Tcl_CreateObjCommand. We support Tcl_CreateCommand for backwards compatibility.

The procedures Tcl_DeleteCommand, Tcl_GetCommandInfo, and Tcl_SetCommandInfo are used in conjunction with Tcl_CreateCommand. They're the friend group, essentially.

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_GetCommandName. If cmdName contains any :: namespace qualifiers, then the command is added to the specified namespace; otherwise the command is added to the global namespace. And 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(
        void *clientData,
        Tcl_Interp *interp,
        int argc,
        const 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. ✨

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. Note also that the argument strings are encoded in normalized TUTF-8 since version 8.1 of Tcl.

proc must return an integer code that is expected to be one of TCL_OK, TCL_ERROR, TCL_RETURN, TCL_BREAK, or TCL_CONTINUE. See the return man page for details on what these codes mean and the use of extended values for an extension's private use. Most normal commands will only return TCL_OK or TCL_ERROR.

In addition, proc must set the interpreter result. 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 interpreter result field is managed, see the Tcl_Interp man page. Before invoking a command procedure, Tcl_Eval sets the interpreter result to point to an empty string, so simple commands can return an empty result by doing nothing at all. Slay, effortless. 💅

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 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(
        void *clientData);

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

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🌈 SEE ALSO 🌈

Tcl_CreateObjCommand, Tcl_DeleteCommand, Tcl_GetCommandInfo, Tcl_SetCommandInfo, Tcl_GetCommandName, Tcl_SetObjResult

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💜 KEYWORDS 💜

bind, command, create, delete, interpreter, namespace

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okay bestie that's the whole thing, you're officially a Tcl_CreateCommand girlie now 🎀 this page came to you straight from the Tcl Library Procedures — Tcl_CreateCommand(3tcl), brought to you by Tcl itself 🦋 go forth and write your string-based commands (or, yk, use Tcl_CreateObjCommand like the docs are begging you to 🙈) 💖✨