---
name: "CrtTrace"
section: "3"
platform: "macos"
package: "tcl"
version: "152"
release: "macOS 26.5"
platforms: ["macos"]
url: "https://new.carta.tech/CrtTrace"
---

# CrtTrace(3)

arrange for command execution to be traced

## NAME

Tcl\_CreateTrace, Tcl\_CreateObjTrace, Tcl\_DeleteTrace - arrange for command execution to be traced

## SYNOPSIS

```
#include <tcl.h>

Tcl_Trace
Tcl_CreateTrace(interp, level, proc, clientData)

Tcl_Trace
Tcl_CreateObjTrace(interp, level, flags, objProc, clientData, deleteProc)

Tcl_DeleteTrace(interp, trace)
```

## ARGUMENTS

Interpreter containing command to be traced or untraced. Only commands at or below this nesting level will be traced unless 0 is specified.  1 means top-level commands only, 2 means top-level commands or those that are invoked as immediate consequences of executing top-level commands (procedure bodies, bracketed commands, etc.) and so on. A value of 0 means that commands at any level are traced. Flags governing the trace execution.  See below for details. Procedure to call for each command that is executed.  See below for details of the calling sequence. Procedure to call for each command that is executed.  See below for details on the calling sequence. Arbitrary one-word value to pass to *objProc* or *proc*. Procedure to call when the trace is deleted.  See below for details of the calling sequence.  A NULL pointer is permissible and results in no callback when the trace is deleted. Token for trace to be removed (return value from previous call to **Tcl\_CreateTrace**).

## DESCRIPTION

**Tcl\_CreateObjTrace** arranges for command tracing.  After it is called, *objProc* will be invoked before the Tcl interpreter calls any command procedure when evaluating commands in *interp*. The return value from **Tcl\_CreateObjTrace** is a token for the trace, which may be passed to **Tcl\_DeleteTrace** to remove the trace. There may be many traces in effect simultaneously for the same interpreter.

*objProc* should have arguments and result that match the type, **Tcl\_CmdObjTraceProc**: typedef int **Tcl\_CmdObjTraceProc**( \
         **ClientData** *clientData*,\
         **Tcl\_Interp**\* *interp*,\
         int *level*,\
         const char \**command*,\
         **Tcl\_Command** *commandToken*,\
         int *objc*,\
         **Tcl\_Obj** \*const *objv*\[\] ); The *clientData* and *interp* parameters are copies of the corresponding arguments given to **Tcl\_CreateTrace**. *ClientData* typically points to an application-specific data structure that describes what to do when *objProc* is invoked.  The *level* parameter gives the nesting level of the command (1 for top-level commands passed to **Tcl\_Eval** by the application, 2 for the next-level commands passed to **Tcl\_Eval** as part of parsing or interpreting level-1 commands, and so on). The *command* parameter points to a string containing the text of the command, before any argument substitution.  The *commandToken* parameter is a Tcl command token that identifies the command to be invoked.  The token may be passed to **Tcl\_GetCommandName**, **Tcl\_GetCommandInfoFromToken**, or **Tcl\_SetCommandInfoFromToken** to manipulate the definition of the command. The *objc* and *objv* parameters designate the final parameter count and parameter vector that will be passed to the command, and have had all substitutions performed.

The *objProc* callback is expected to return a standard Tcl status return code.  If this code is **TCL\_OK** (the normal case), then the Tcl interpreter will invoke the command.  Any other return code is treated as if the command returned that status, and the command is *not* invoked.

The *objProc* callback must not modify *objv* in any way.  It is, however, permissible to change the command by calling **Tcl\_SetCommandTokenInfo** prior to returning.  Any such change takes effect immediately, and the command is invoked with the new information.

Tracing will only occur for commands at nesting level less than or equal to the *level* parameter (i.e. the *level* parameter to *objProc* will always be less than or equal to the *level* parameter to **Tcl\_CreateTrace**).

Tracing has a significant effect on runtime performance because it causes the bytecode compiler to refrain from generating in-line code for Tcl commands such as **if** and **while** in order that they may be traced.  If traces for the built-in commands are not required, the *flags* parameter may be set to the constant value **TCL\_ALLOW\_INLINE\_COMPILATION**.  In this case, traces on built-in commands may or may not result in trace callbacks, depending on the state of the interpreter, but run-time performance will be improved significantly.  (This functionality is desirable, for example, when using **Tcl\_CreateObjTrace** to implement an execution time profiler.)

Calls to *objProc* will be made by the Tcl parser immediately before it calls the command procedure for the command (*cmdProc*).  This occurs after argument parsing and substitution, so tracing for substituted commands occurs before tracing of the commands containing the substitutions.  If there is a syntax error in a command, or if there is no command procedure associated with a command name, then no tracing will occur for that command.  If a string passed to Tcl\_Eval contains multiple commands (bracketed, or on different lines) then multiple calls to *objProc* will occur, one for each command.

**Tcl\_DeleteTrace** removes a trace, so that no future calls will be made to the procedure associated with the trace.  After **Tcl\_DeleteTrace** returns, the caller should never again use the *trace* token.

When **Tcl\_DeleteTrace** is called, the interpreter invokes the *deleteProc* that was passed as a parameter to **Tcl\_CreateObjTrace**.  The *deleteProc* must match the type, **Tcl\_CmdObjTraceDeleteProc**: typedef void **Tcl\_CmdObjTraceDeleteProc**( \
         **ClientData** *clientData*); The *clientData* parameter will be the same as the *clientData* parameter that was originally passed to **Tcl\_CreateObjTrace**.

**Tcl\_CreateTrace** is an alternative interface for command tracing, *not recommended for new applications*.  It is provided for backward compatibility with code that was developed for older versions of the Tcl interpreter.  It is similar to **Tcl\_CreateObjTrace**, except that its *proc* parameter should have arguments and result that match the type **Tcl\_CmdTraceProc**: typedef void Tcl\_CmdTraceProc(\
         ClientData *clientData*,\
         Tcl\_Interp \**interp*,\
         int *level*,\
         char \**command*,\
         Tcl\_CmdProc \**cmdProc*,\
         ClientData *cmdClientData*,\
         int *argc*,\
         const char \**argv*\[\]); The parameters to the *proc* callback are similar to those of the *objProc* callback above. The *commandToken* is replaced with *cmdProc*, a pointer to the (string-based) command procedure that will be invoked; and *cmdClientData*, the client data that will be passed to the procedure.  The *objc* parameter is replaced with an *argv* parameter, that gives the arguments to the command as character strings. *Proc* must not modify the *command* or *argv* strings.

If a trace created with **Tcl\_CreateTrace** is in effect, inline compilation of Tcl commands such as **if** and **while** is always disabled.  There is no notification when a trace created with **Tcl\_CreateTrace** is deleted. There is no way to be notified when the trace created by **Tcl\_CreateTrace** is deleted.  There is no way for the *proc* associated with a call to **Tcl\_CreateTrace** to abort execution of *command*.

## KEYWORDS

command, create, delete, interpreter, trace