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| 181 | \kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c |
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| 219 | See the \\fBoptions\\fR manual entry for details on the standard options. |
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| 226 | Command-Line Name: \\fB\\$1\\fR |
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| 245 | .. |
| 246 | .TH Tcl_TraceVar 3 7.4 Tcl "Tcl Library Procedures" |
| 247 | .BS |
| 248 | .SH NAME |
| 249 | Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 \- monitor accesses to a variable |
| 250 | .SH SYNOPSIS |
| 251 | .nf |
| 252 | \fB#include <tcl.h>\fR |
| 253 | .sp |
| 254 | int |
| 255 | \fBTcl_TraceVar(\fIinterp, varName, flags, proc, clientData\fB)\fR |
| 256 | .sp |
| 257 | int |
| 258 | \fBTcl_TraceVar2(\fIinterp, name1, name2, flags, proc, clientData\fB)\fR |
| 259 | .sp |
| 260 | \fBTcl_UntraceVar(\fIinterp, varName, flags, proc, clientData\fB)\fR |
| 261 | .sp |
| 262 | \fBTcl_UntraceVar2(\fIinterp, name1, name2, flags, proc, clientData\fB)\fR |
| 263 | .sp |
| 264 | ClientData |
| 265 | \fBTcl_VarTraceInfo(\fIinterp, varName, flags, proc, prevClientData\fB)\fR |
| 266 | .sp |
| 267 | ClientData |
| 268 | \fBTcl_VarTraceInfo2(\fIinterp, name1, name2, flags, proc, prevClientData\fB)\fR |
| 269 | .SH ARGUMENTS |
| 270 | .AS Tcl_VarTraceProc prevClientData |
| 271 | .AP Tcl_Interp *interp in |
| 272 | Interpreter containing variable. |
| 273 | .AP "CONST char" *varName in |
| 274 | Name of variable. May refer to a scalar variable, to |
| 275 | an array variable with no index, or to an array variable |
| 276 | with a parenthesized index. |
| 277 | .AP int flags in |
| 278 | OR-ed combination of the values TCL_TRACE_READS, TCL_TRACE_WRITES, |
| 279 | TCL_TRACE_UNSETS, TCL_TRACE_ARRAY, TCL_GLOBAL_ONLY, TCL_NAMESPACE_ONLY, |
| 280 | TCL_TRACE_RESULT_DYNAMIC and TCL_TRACE_RESULT_OBJECT. |
| 281 | Not all flags are used by all |
| 282 | procedures. See below for more information. |
| 283 | .AP Tcl_VarTraceProc *proc in |
| 284 | Procedure to invoke whenever one of the traced operations occurs. |
| 285 | .AP ClientData clientData in |
| 286 | Arbitrary one-word value to pass to \fIproc\fR. |
| 287 | .AP "CONST char" *name1 in |
| 288 | Name of scalar or array variable (without array index). |
| 289 | .AP "CONST char" *name2 in |
| 290 | For a trace on an element of an array, gives the index of the |
| 291 | element. For traces on scalar variables or on whole arrays, |
| 292 | is NULL. |
| 293 | .AP ClientData prevClientData in |
| 294 | If non-NULL, gives last value returned by \fBTcl_VarTraceInfo\fR or |
| 295 | \fBTcl_VarTraceInfo2\fR, so this call will return information about |
| 296 | next trace. If NULL, this call will return information about first |
| 297 | trace. |
| 298 | .BE |
| 299 | |
| 300 | .SH DESCRIPTION |
| 301 | .PP |
| 302 | \fBTcl_TraceVar\fR allows a C procedure to monitor and control |
| 303 | access to a Tcl variable, so that the C procedure is invoked |
| 304 | whenever the variable is read or written or unset. |
| 305 | If the trace is created successfully then \fBTcl_TraceVar\fR returns |
| 306 | TCL_OK. If an error occurred (e.g. \fIvarName\fR specifies an element |
| 307 | of an array, but the actual variable isn't an array) then TCL_ERROR |
| 308 | is returned and an error message is left in the interpreter's result. |
| 309 | .PP |
| 310 | The \fIflags\fR argument to \fBTcl_TraceVar\fR indicates when the |
| 311 | trace procedure is to be invoked and provides information |
| 312 | for setting up the trace. It consists of an OR-ed combination |
| 313 | of any of the following values: |
| 314 | .TP |
| 315 | \fBTCL_GLOBAL_ONLY\fR |
| 316 | Normally, the variable will be looked up at the current level of |
| 317 | procedure call; if this bit is set then the variable will be looked |
| 318 | up at global level, ignoring any active procedures. |
| 319 | .TP |
| 320 | \fBTCL_NAMESPACE_ONLY\fR |
| 321 | Normally, the variable will be looked up at the current level of |
| 322 | procedure call; if this bit is set then the variable will be looked |
| 323 | up in the current namespace, ignoring any active procedures. |
| 324 | .TP |
| 325 | \fBTCL_TRACE_READS\fR |
| 326 | Invoke \fIproc\fR whenever an attempt is made to read the variable. |
| 327 | .TP |
| 328 | \fBTCL_TRACE_WRITES\fR |
| 329 | Invoke \fIproc\fR whenever an attempt is made to modify the variable. |
| 330 | .TP |
| 331 | \fBTCL_TRACE_UNSETS\fR |
| 332 | Invoke \fIproc\fR whenever the variable is unset. |
| 333 | A variable may be unset either explicitly by an \fBunset\fR command, |
| 334 | or implicitly when a procedure returns (its local variables are |
| 335 | automatically unset) or when the interpreter is deleted (all |
| 336 | variables are automatically unset). |
| 337 | .TP |
| 338 | \fBTCL_TRACE_ARRAY\fR |
| 339 | Invoke \fIproc\fR whenever the array command is invoked. |
| 340 | This gives the trace procedure a chance to update the array before |
| 341 | array names or array get is called. Note that this is called |
| 342 | before an array set, but that will trigger write traces. |
| 343 | .VS 8.4 |
| 344 | .TP |
| 345 | \fBTCL_TRACE_RESULT_DYNAMIC\fR |
| 346 | The result of invoking the \fIproc\fR is a dynamically allocated |
| 347 | string that will be released by the Tcl library via a call to |
| 348 | \fBckfree\fR. Must not be specified at the same time as |
| 349 | TCL_TRACE_RESULT_OBJECT. |
| 350 | .TP |
| 351 | \fBTCL_TRACE_RESULT_OBJECT\fR |
| 352 | The result of invoking the \fIproc\fR is a Tcl_Obj* (cast to a char*) |
| 353 | with a reference count of at least one. The ownership of that |
| 354 | reference will be transferred to the Tcl core for release (when the |
| 355 | core has finished with it) via a call to \fBTcl_DecrRefCount\fR. Must |
| 356 | not be specified at the same time as TCL_TRACE_RESULT_DYNAMIC. |
| 357 | .VE 8.4 |
| 358 | .PP |
| 359 | Whenever one of the specified operations occurs on the variable, |
| 360 | \fIproc\fR will be invoked. |
| 361 | It should have arguments and result that match the type |
| 362 | \fBTcl_VarTraceProc\fR: |
| 363 | .CS |
| 364 | typedef char *Tcl_VarTraceProc( |
| 365 | ClientData \fIclientData\fR, |
| 366 | Tcl_Interp *\fIinterp\fR, |
| 367 | char *\fIname1\fR, |
| 368 | char *\fIname2\fR, |
| 369 | int \fIflags\fR); |
| 370 | .CE |
| 371 | The \fIclientData\fR and \fIinterp\fR parameters will |
| 372 | have the same values as those passed to \fBTcl_TraceVar\fR when the |
| 373 | trace was created. |
| 374 | \fIClientData\fR typically points to an application-specific |
| 375 | data structure that describes what to do when \fIproc\fR |
| 376 | is invoked. |
| 377 | \fIName1\fR and \fIname2\fR give the name of the traced variable |
| 378 | in the normal two-part form (see the description of \fBTcl_TraceVar2\fR |
| 379 | below for details). |
| 380 | \fIFlags\fR is an OR-ed combination of bits providing several |
| 381 | pieces of information. |
| 382 | One of the bits TCL_TRACE_READS, TCL_TRACE_WRITES, TCL_TRACE_ARRAY, |
| 383 | or TCL_TRACE_UNSETS |
| 384 | will be set in \fIflags\fR to indicate which operation is being performed |
| 385 | on the variable. |
| 386 | The bit TCL_GLOBAL_ONLY will be set whenever the variable being |
| 387 | accessed is a global one not accessible from the current level of |
| 388 | procedure call: the trace procedure will need to pass this flag |
| 389 | back to variable-related procedures like \fBTcl_GetVar\fR if it |
| 390 | attempts to access the variable. |
| 391 | The bit TCL_NAMESPACE_ONLY will be set whenever the variable being |
| 392 | accessed is a namespace one not accessible from the current level of |
| 393 | procedure call: the trace procedure will need to pass this flag |
| 394 | back to variable-related procedures like \fBTcl_GetVar\fR if it |
| 395 | attempts to access the variable. |
| 396 | The bit TCL_TRACE_DESTROYED will be set in \fIflags\fR if the trace is |
| 397 | about to be destroyed; this information may be useful to \fIproc\fR |
| 398 | so that it can clean up its own internal data structures (see |
| 399 | the section TCL_TRACE_DESTROYED below for more details). |
| 400 | Lastly, the bit TCL_INTERP_DESTROYED will be set if the entire |
| 401 | interpreter is being destroyed. |
| 402 | When this bit is set, \fIproc\fR must be especially careful in |
| 403 | the things it does (see the section TCL_INTERP_DESTROYED below). |
| 404 | The trace procedure's return value should normally be NULL; see |
| 405 | ERROR RETURNS below for information on other possibilities. |
| 406 | .PP |
| 407 | \fBTcl_UntraceVar\fR may be used to remove a trace. |
| 408 | If the variable specified by \fIinterp\fR, \fIvarName\fR, and \fIflags\fR |
| 409 | has a trace set with \fIflags\fR, \fIproc\fR, and |
| 410 | \fIclientData\fR, then the corresponding trace is removed. |
| 411 | If no such trace exists, then the call to \fBTcl_UntraceVar\fR |
| 412 | has no effect. |
| 413 | The same bits are valid for \fIflags\fR as for calls to \fBTcl_TraceVar\fR. |
| 414 | .PP |
| 415 | \fBTcl_VarTraceInfo\fR may be used to retrieve information about |
| 416 | traces set on a given variable. |
| 417 | The return value from \fBTcl_VarTraceInfo\fR is the \fIclientData\fR |
| 418 | associated with a particular trace. |
| 419 | The trace must be on the variable specified by the \fIinterp\fR, |
| 420 | \fIvarName\fR, and \fIflags\fR arguments (only the TCL_GLOBAL_ONLY and |
| 421 | TCL_NAMESPACE_ONLY bits from \fIflags\fR is used; other bits are |
| 422 | ignored) and its trace procedure must the same as the \fIproc\fR |
| 423 | argument. |
| 424 | If the \fIprevClientData\fR argument is NULL then the return |
| 425 | value corresponds to the first (most recently created) matching |
| 426 | trace, or NULL if there are no matching traces. |
| 427 | If the \fIprevClientData\fR argument isn't NULL, then it should |
| 428 | be the return value from a previous call to \fBTcl_VarTraceInfo\fR. |
| 429 | In this case, the new return value will correspond to the next |
| 430 | matching trace after the one whose \fIclientData\fR matches |
| 431 | \fIprevClientData\fR, or NULL if no trace matches \fIprevClientData\fR |
| 432 | or if there are no more matching traces after it. |
| 433 | This mechanism makes it possible to step through all of the |
| 434 | traces for a given variable that have the same \fIproc\fR. |
| 435 | |
| 436 | .SH "TWO-PART NAMES" |
| 437 | .PP |
| 438 | The procedures \fBTcl_TraceVar2\fR, \fBTcl_UntraceVar2\fR, and |
| 439 | \fBTcl_VarTraceInfo2\fR are identical to \fBTcl_TraceVar\fR, |
| 440 | \fBTcl_UntraceVar\fR, and \fBTcl_VarTraceInfo\fR, respectively, |
| 441 | except that the name of the variable consists of two parts. |
| 442 | \fIName1\fR gives the name of a scalar variable or array, |
| 443 | and \fIname2\fR gives the name of an element within an array. |
| 444 | .VS 8.1 |
| 445 | When \fIname2\fR is NULL, |
| 446 | \fIname1\fR may contain both an array and an element name: |
| 447 | if the name contains an open parenthesis and ends with a |
| 448 | close parenthesis, then the value between the parentheses is |
| 449 | treated as an element name (which can have any string value) and |
| 450 | the characters before the first open |
| 451 | parenthesis are treated as the name of an array variable. |
| 452 | If \fIname2\fR is NULL and \fIname1\fR does not refer |
| 453 | to an array element |
| 454 | .VE |
| 455 | it means that either the variable is |
| 456 | a scalar or the trace is to be set on the entire array rather |
| 457 | than an individual element (see WHOLE-ARRAY TRACES below for |
| 458 | more information). |
| 459 | |
| 460 | |
| 461 | .SH "ACCESSING VARIABLES DURING TRACES" |
| 462 | .PP |
| 463 | During read, write, and array traces, the |
| 464 | trace procedure can read, write, or unset the traced |
| 465 | variable using \fBTcl_GetVar2\fR, \fBTcl_SetVar2\fR, and |
| 466 | other procedures. |
| 467 | While \fIproc\fR is executing, traces are temporarily disabled |
| 468 | for the variable, so that calls to \fBTcl_GetVar2\fR and |
| 469 | \fBTcl_SetVar2\fR will not cause \fIproc\fR or other trace procedures |
| 470 | to be invoked again. |
| 471 | Disabling only occurs for the variable whose trace procedure |
| 472 | is active; accesses to other variables will still be traced. |
| 473 | However, if a variable is unset during a read or write trace then unset |
| 474 | traces will be invoked. |
| 475 | .PP |
| 476 | During unset traces the variable has already been completely |
| 477 | expunged. |
| 478 | It is possible for the trace procedure to read or write the |
| 479 | variable, but this will be a new version of the variable. |
| 480 | Traces are not disabled during unset traces as they are for |
| 481 | read and write traces, but existing traces have been removed |
| 482 | from the variable before any trace procedures are invoked. |
| 483 | If new traces are set by unset trace procedures, these traces |
| 484 | will be invoked on accesses to the variable by the trace |
| 485 | procedures. |
| 486 | |
| 487 | .SH "CALLBACK TIMING" |
| 488 | .PP |
| 489 | When read tracing has been specified for a variable, the trace |
| 490 | procedure will be invoked whenever the variable's value is |
| 491 | read. This includes \fBset\fR Tcl commands, \fB$\fR-notation |
| 492 | in Tcl commands, and invocations of the \fBTcl_GetVar\fR |
| 493 | and \fBTcl_GetVar2\fR procedures. |
| 494 | \fIProc\fR is invoked just before the variable's value is |
| 495 | returned. |
| 496 | It may modify the value of the variable to affect what |
| 497 | is returned by the traced access. |
| 498 | If it unsets the variable then the access will return an error |
| 499 | just as if the variable never existed. |
| 500 | .PP |
| 501 | When write tracing has been specified for a variable, the |
| 502 | trace procedure will be invoked whenever the variable's value |
| 503 | is modified. This includes \fBset\fR commands, |
| 504 | commands that modify variables as side effects (such as |
| 505 | \fBcatch\fR and \fBscan\fR), and calls to the \fBTcl_SetVar\fR |
| 506 | and \fBTcl_SetVar2\fR procedures). |
| 507 | \fIProc\fR will be invoked after the variable's value has been |
| 508 | modified, but before the new value of the variable has been |
| 509 | returned. |
| 510 | It may modify the value of the variable to override the change |
| 511 | and to determine the value actually returned by the traced |
| 512 | access. |
| 513 | If it deletes the variable then the traced access will return |
| 514 | an empty string. |
| 515 | .PP |
| 516 | When array tracing has been specified, the trace procedure |
| 517 | will be invoked at the beginning of the array command implementation, |
| 518 | before any of the operations like get, set, or names have been invoked. |
| 519 | The trace procedure can modify the array elements with \fBTcl_SetVar\fR |
| 520 | and \fBTcl_SetVar2\fR. |
| 521 | .PP |
| 522 | When unset tracing has been specified, the trace procedure |
| 523 | will be invoked whenever the variable is destroyed. |
| 524 | The traces will be called after the variable has been |
| 525 | completely unset. |
| 526 | |
| 527 | .SH "WHOLE-ARRAY TRACES" |
| 528 | .PP |
| 529 | If a call to \fBTcl_TraceVar\fR or \fBTcl_TraceVar2\fR specifies |
| 530 | the name of an array variable without an index into the array, |
| 531 | then the trace will be set on the array as a whole. |
| 532 | This means that \fIproc\fR will be invoked whenever any |
| 533 | element of the array is accessed in the ways specified by |
| 534 | \fIflags\fR. |
| 535 | When an array is unset, a whole-array trace will be invoked |
| 536 | just once, with \fIname1\fR equal to the name of the array |
| 537 | and \fIname2\fR NULL; it will not be invoked once for each |
| 538 | element. |
| 539 | |
| 540 | .SH "MULTIPLE TRACES" |
| 541 | .PP |
| 542 | It is possible for multiple traces to exist on the same variable. |
| 543 | When this happens, all of the trace procedures will be invoked on each |
| 544 | access, in order from most-recently-created to least-recently-created. |
| 545 | When there exist whole-array traces for an array as well as |
| 546 | traces on individual elements, the whole-array traces are invoked |
| 547 | before the individual-element traces. |
| 548 | If a read or write trace unsets the variable then all of the unset |
| 549 | traces will be invoked but the remainder of the read and write traces |
| 550 | will be skipped. |
| 551 | |
| 552 | .SH "ERROR RETURNS" |
| 553 | .PP |
| 554 | Under normal conditions trace procedures should return NULL, indicating |
| 555 | successful completion. |
| 556 | If \fIproc\fR returns a non-NULL value it signifies that an |
| 557 | error occurred. |
| 558 | The return value must be a pointer to a static character string |
| 559 | containing an error message, |
| 560 | .VS 8.4 |
| 561 | unless (\fIexactly\fR one of) the TCL_TRACE_RESULT_DYNAMIC and |
| 562 | TCL_TRACE_RESULT_OBJECT flags is set, which specify that the result is |
| 563 | either a dynamic string (to be released with \fBckfree\fR) or a |
| 564 | Tcl_Obj* (cast to char* and to be released with |
| 565 | \fBTcl_DecrRefCount\fR) containing the error message. |
| 566 | .VE 8.4 |
| 567 | If a trace procedure returns an error, no further traces are |
| 568 | invoked for the access and the traced access aborts with the |
| 569 | given message. |
| 570 | Trace procedures can use this facility to make variables |
| 571 | read-only, for example (but note that the value of the variable |
| 572 | will already have been modified before the trace procedure is |
| 573 | called, so the trace procedure will have to restore the correct |
| 574 | value). |
| 575 | .PP |
| 576 | The return value from \fIproc\fR is only used during read and |
| 577 | write tracing. |
| 578 | During unset traces, the return value is ignored and all relevant |
| 579 | trace procedures will always be invoked. |
| 580 | |
| 581 | .SH "RESTRICTIONS" |
| 582 | .PP |
| 583 | A trace procedure can be called at any time, even when there |
| 584 | is a partially-formed result in the interpreter's result area. If |
| 585 | the trace procedure does anything that could damage this result (such |
| 586 | as calling \fBTcl_Eval\fR) then it must save the original values of |
| 587 | the interpreter's \fBresult\fR and \fBfreeProc\fR fields and restore |
| 588 | them before it returns. |
| 589 | |
| 590 | .SH "UNDEFINED VARIABLES" |
| 591 | .PP |
| 592 | It is legal to set a trace on an undefined variable. |
| 593 | The variable will still appear to be undefined until the |
| 594 | first time its value is set. |
| 595 | If an undefined variable is traced and then unset, the unset will fail |
| 596 | with an error (``no such variable''), but the trace |
| 597 | procedure will still be invoked. |
| 598 | |
| 599 | .SH "TCL_TRACE_DESTROYED FLAG" |
| 600 | .PP |
| 601 | In an unset callback to \fIproc\fR, the TCL_TRACE_DESTROYED bit |
| 602 | is set in \fIflags\fR if the trace is being removed as part |
| 603 | of the deletion. |
| 604 | Traces on a variable are always removed whenever the variable |
| 605 | is deleted; the only time TCL_TRACE_DESTROYED isn't set is for |
| 606 | a whole-array trace invoked when only a single element of an |
| 607 | array is unset. |
| 608 | |
| 609 | .SH "TCL_INTERP_DESTROYED" |
| 610 | .PP |
| 611 | When an interpreter is destroyed, unset traces are called for |
| 612 | all of its variables. |
| 613 | The TCL_INTERP_DESTROYED bit will be set in the \fIflags\fR |
| 614 | argument passed to the trace procedures. |
| 615 | Trace procedures must be extremely careful in what they do if |
| 616 | the TCL_INTERP_DESTROYED bit is set. |
| 617 | It is not safe for the procedures to invoke any Tcl procedures |
| 618 | on the interpreter, since its state is partially deleted. |
| 619 | All that trace procedures should do under these circumstances is |
| 620 | to clean up and free their own internal data structures. |
| 621 | |
| 622 | .SH BUGS |
| 623 | .PP |
| 624 | Tcl doesn't do any error checking to prevent trace procedures |
| 625 | from misusing the interpreter during traces with TCL_INTERP_DESTROYED |
| 626 | set. |
| 627 | .PP |
| 628 | Array traces are not yet integrated with the Tcl "info exists" command, |
| 629 | nor is there Tcl-level access to array traces. |
| 630 | |
| 631 | .SH KEYWORDS |
| 632 | clientData, trace, variable |