Tcl_CreateTrace(3) Tcl Library Procedures Tcl_CreateTrace(3)
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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
Tcl_Interp *interp (in) Interpreter containing command to be traced or
untraced.
int level (in) 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 immedi-ate immediate
ate consequences of executing top-level com-mands commands
mands (procedure bodies, bracketed commands,
etc.) and so on. A value of 0 means that com-mands commands
mands at any level are traced.
int flags (in) Flags governing the trace execution. See
below for details.
Tcl_CmdObjTraceProc *objProc (in) Procedure to call for each command that's exe-cuted. executed.
cuted. See below for details of the calling
sequence.
Tcl_CmdTraceProc *proc (in) Procedure to call for each command that's exe-cuted. executed.
cuted. See below for details on the calling
sequence.
ClientData clientData (in) Arbitrary one-word value to pass to objProc or
proc.
Tcl_CmdObjTraceDeleteProc *deleteProc 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.
Tcl_Trace trace (in) Token for trace to be removed (return value
from previous call to Tcl_CreateTrace).
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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_Create-Trace. Tcl_CreateTrace.
Trace. 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 toplevel
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 com-mandToken commandToken
mandToken parameter is a Tcl command token that identifies the command to be invoked. The token may
be passed to Tcl_GetCommandName, Tcl_GetCommandTokenInfo, or Tcl_SetCommandTokenInfo 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 com-mand command
mand by calling Tcl_SetCommandTokenInfo prior to returning. Any such change takes effect immedi-ately, immediately,
ately, 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_Cre-ateTrace). Tcl_CreateTrace).
ateTrace).
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_Cre-ateObjTrace Tcl_CreateObjTrace
ateObjTrace 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 substi-tuted substituted
tuted 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 applica-tions. applications.
tions. 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 command-Token commandToken
Token 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
Tcl Tcl_CreateTrace(3)
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