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Tcl Library Procedures Tcl_TraceVar(3TCL)

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NAME

Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar,

Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - moni-

tor accesses to a variable

SYNOPSIS

#include

int

Tcl_TraceVar(interp, varName, flags, proc, clientData)

int

Tcl_TraceVar2(interp, name1, name2, flags, proc, clientData)

Tcl_UntraceVar(interp, varName, flags, proc, clientData)

Tcl_UntraceVar2(interp, name1, name2, flags, proc, clientData)

ClientData

Tcl_VarTraceInfo(interp, varName, flags, proc, prevClientData)

ClientData

Tcl_VarTraceInfo2(interp, name1, name2, flags, proc, prevClientData)

ARGUMENTS

Tcl_Interp *interp (in) Interpreter

containing variable.

CONST char *varName (in) Name of vari-

able. May refer to a

scalar vari-

able, to an array variable with no index, or to an array variable with a parenthesized index.

int flags (in) OR-ed combina-

tion of the values

TCL_TRACE_READS,

TCL_TRACE_WRITES,

TCL_TRACE_UNSETS,

TCL_TRACE_ARRAY,

TCL_GLOBAL_ONLY,

Tcl Last change: 7.4 1

Tcl Library Procedures Tcl_TraceVar(3TCL)

TCL_NAMESPACE_ONLY,

TCL_TRACE_RESULT_DYNAMIC

and

TCL_TRACE_RESULT_OBJECT.

Not all flags are used by

all pro-

cedures. See below for more information.

Tcl_VarTraceProc *proc (in) Procedure to

invoke when-

ever one of the traced operations occurs. ClientData clientData (in) Arbitrary

one-word value

to pass to proc. CONST char *name1 (in) Name of scalar

or array vari-

able (without array index). CONST char *name2 (in) For a trace on an element of an array, gives the index of the element. For traces on

scalar vari-

ables or on whole arrays, is NULL.

ClientData prevClientData (in) If non-NULL,

gives last value returned by

Tcl_VarTraceInfo

Tcl_VarTraceInfo2,

so this call will return information about next trace. If Tcl Last change: 7.4 2

Tcl Library Procedures Tcl_TraceVar(3TCL)

NULL, this call will

return infor-

mation about first trace.

_________________________________________________________________

DESCRIPTION

Tcl_TraceVar allows a C procedure to monitor and control

access to a Tcl variable, so that the C procedure is invoked whenever the variable is read or written or unset. If the

trace is created successfully then Tcl_TraceVar returns

TCL_OK. If an error occurred (e.g. varName specifies an

element of an array, but the actual variable isn't an array)

then TCL_ERROR is returned and an error message is left in

the interpreter's result.

The flags argument to Tcl_TraceVar indicates when the trace

procedure is to be invoked and provides information for set-

ting up the trace. It consists of an OR-ed combination of

any of the following values:

TCL_GLOBAL_ONLY

Normally, the variable will be looked up at the current level of procedure call; if this bit is set then the variable will be looked up at global level, ignoring any active procedures.

TCL_NAMESPACE_ONLY

Normally, the variable will be looked up at the current level of procedure call; if this bit is set then the variable will be looked up in the current namespace, ignoring any active procedures.

TCL_TRACE_READS

Invoke proc whenever an attempt is made to read the variable.

TCL_TRACE_WRITES

Invoke proc whenever an attempt is made to modify the variable.

TCL_TRACE_UNSETS

Invoke proc whenever the variable is unset. A variable may be unset either explicitly by an unset command, or

implicitly when a procedure returns (its local vari-

ables are automatically unset) or when the interpreter is deleted (all variables are automatically unset).

TCL_TRACE_ARRAY

Invoke proc whenever the array command is invoked. Tcl Last change: 7.4 3

Tcl Library Procedures Tcl_TraceVar(3TCL)

This gives the trace procedure a chance to update the array before array names or array get is called. Note that this is called before an array set, but that will trigger write traces. |

TCL_TRACE_RESULT_DYNAMIC ||

The result of invoking the proc is a dynamically allo- |

cated string that will be released by the Tcl library | via a call to ckfree. Must not be specified at the |

same time as TCL_TRACE_RESULT_OBJECT. |

TCL_TRACE_RESULT_OBJECT ||

The result of invoking the proc is a Tcl_Obj* (cast to |

a char*) with a reference count of at least one. The | ownership of that reference will be transferred to the | Tcl core for release (when the core has finished with |

it) via a call to Tcl_DecrRefCount. Must not be speci- |

fied at the same time as TCL_TRACE_RESULT_DYNAMIC.

Whenever one of the specified operations occurs on the vari-

able, proc will be invoked. It should have arguments and

result that match the type Tcl_VarTraceProc:

typedef char *Tcl_VarTraceProc(

ClientData clientData,

Tcl_Interp *interp,

char *name1, char *name2, int flags); The clientData and interp parameters will have the same

values as those passed to Tcl_TraceVar when the trace was

created. ClientData typically points to an application-

specific data structure that describes what to do when proc is invoked. Name1 and name2 give the name of the traced

variable in the normal two-part form (see the description of

Tcl_TraceVar2 below for details). Flags is an OR-ed combi-

nation of bits providing several pieces of information. One

of the bits TCL_TRACE_READS, TCL_TRACE_WRITES,

TCL_TRACE_ARRAY, or TCL_TRACE_UNSETS will be set in flags to

indicate which operation is being performed on the variable.

The bit TCL_GLOBAL_ONLY will be set whenever the variable

being accessed is a global one not accessible from the current level of procedure call: the trace procedure will

need to pass this flag back to variable-related procedures

like Tcl_GetVar if it attempts to access the variable. The

bit TCL_NAMESPACE_ONLY will be set whenever the variable

being accessed is a namespace one not accessible from the current level of procedure call: the trace procedure will

need to pass this flag back to variable-related procedures

like Tcl_GetVar if it attempts to access the variable. The

bit TCL_TRACE_DESTROYED will be set in flags if the trace is

about to be destroyed; this information may be useful to proc so that it can clean up its own internal data Tcl Last change: 7.4 4

Tcl Library Procedures Tcl_TraceVar(3TCL)

structures (see the section TCL_TRACE_DESTROYED below for

more details). Lastly, the bit TCL_INTERP_DESTROYED will be

set if the entire interpreter is being destroyed. When this bit is set, proc must be especially careful in the things it

does (see the section TCL_INTERP_DESTROYED below). The

trace procedure's return value should normally be NULL; see

ERROR RETURNS below for information on other possibilities.

Tcl_UntraceVar may be used to remove a trace. If the vari-

able specified by interp, varName, and flags has a trace set with flags, proc, and clientData, then the corresponding trace is removed. If no such trace exists, then the call to

Tcl_UntraceVar has no effect. The same bits are valid for

flags as for calls to Tcl_TraceVar.

Tcl_VarTraceInfo may be used to retrieve information about

traces set on a given variable. The return value from

Tcl_VarTraceInfo is the clientData associated with a partic-

ular trace. The trace must be on the variable specified by the interp, varName, and flags arguments (only the

TCL_GLOBAL_ONLY and TCL_NAMESPACE_ONLY bits from flags is

used; other bits are ignored) and its trace procedure must

the same as the proc argument. If the prevClientData argu-

ment is NULL then the return value corresponds to the first (most recently created) matching trace, or NULL if there are no matching traces. If the prevClientData argument isn't NULL, then it should be the return value from a previous

call to Tcl_VarTraceInfo. In this case, the new return

value will correspond to the next matching trace after the one whose clientData matches prevClientData, or NULL if no

trace matches prevClientData or if there are no more match-

ing traces after it. This mechanism makes it possible to step through all of the traces for a given variable that have the same proc.

TWO-PART NAMES

The procedures Tcl_TraceVar2, Tcl_UntraceVar2, and

Tcl_VarTraceInfo2 are identical to Tcl_TraceVar,

Tcl_UntraceVar, and Tcl_VarTraceInfo, respectively, except

that the name of the variable consists of two parts. Name1 gives the name of a scalar variable or array, and name2 gives the name of an element within an array. When name2 is | NULL, name1 may contain both an array and an element name: | if the name contains an open parenthesis and ends with a | close parenthesis, then the value between the parentheses is | treated as an element name (which can have any string value) | and the characters before the first open parenthesis are | treated as the name of an array variable. If name2 is NULL | and name1 does not refer to an array element it means that either the variable is a scalar or the trace is to be set on the entire array rather than an individual element (see Tcl Last change: 7.4 5

Tcl Library Procedures Tcl_TraceVar(3TCL)

WHOLE-ARRAY TRACES below for more information).

ACCESSING VARIABLES DURING TRACES During read, write, and array traces, the trace procedure can read, write, or unset the traced variable using

Tcl_GetVar2, Tcl_SetVar2, and other procedures. While proc

is executing, traces are temporarily disabled for the vari-

able, so that calls to Tcl_GetVar2 and Tcl_SetVar2 will not

cause proc or other trace procedures to be invoked again. Disabling only occurs for the variable whose trace procedure is active; accesses to other variables will still be traced. However, if a variable is unset during a read or write trace then unset traces will be invoked. During unset traces the variable has already been completely expunged. It is possible for the trace procedure to read or write the variable, but this will be a new version of the variable. Traces are not disabled during unset traces as they are for read and write traces, but existing traces have been removed from the variable before any trace procedures

are invoked. If new traces are set by unset trace pro-

cedures, these traces will be invoked on accesses to the variable by the trace procedures. CALLBACK TIMING When read tracing has been specified for a variable, the trace procedure will be invoked whenever the variable's

value is read. This includes set Tcl commands, $-notation

in Tcl commands, and invocations of the Tcl_GetVar and

Tcl_GetVar2 procedures. Proc is invoked just before the

variable's value is returned. It may modify the value of the variable to affect what is returned by the traced access. If it unsets the variable then the access will return an error just as if the variable never existed. When write tracing has been specified for a variable, the trace procedure will be invoked whenever the variable's value is modified. This includes set commands, commands that modify variables as side effects (such as catch and

scan), and calls to the Tcl_SetVar and Tcl_SetVar2 pro-

cedures). Proc will be invoked after the variable's value has been modified, but before the new value of the variable has been returned. It may modify the value of the variable to override the change and to determine the value actually returned by the traced access. If it deletes the variable then the traced access will return an empty string. When array tracing has been specified, the trace procedure will be invoked at the beginning of the array command Tcl Last change: 7.4 6

Tcl Library Procedures Tcl_TraceVar(3TCL)

implementation, before any of the operations like get, set, or names have been invoked. The trace procedure can modify

the array elements with Tcl_SetVar and Tcl_SetVar2.

When unset tracing has been specified, the trace procedure will be invoked whenever the variable is destroyed. The traces will be called after the variable has been completely unset.

WHOLE-ARRAY TRACES

If a call to Tcl_TraceVar or Tcl_TraceVar2 specifies the

name of an array variable without an index into the array, then the trace will be set on the array as a whole. This means that proc will be invoked whenever any element of the array is accessed in the ways specified by flags. When an

array is unset, a whole-array trace will be invoked just

once, with name1 equal to the name of the array and name2 NULL; it will not be invoked once for each element. MULTIPLE TRACES It is possible for multiple traces to exist on the same variable. When this happens, all of the trace procedures

will be invoked on each access, in order from most-

recently-created to least-recently-created. When there

exist whole-array traces for an array as well as traces on

individual elements, the whole-array traces are invoked

before the individual-element traces. If a read or write

trace unsets the variable then all of the unset traces will be invoked but the remainder of the read and write traces will be skipped.

ERROR RETURNS

Under normal conditions trace procedures should return NULL,

indicating successful completion. If proc returns a non-

NULL value it signifies that an error occurred. The return

value must be a pointer to a static character string con-

taining an error message, unless (exactly one of) the |

TCL_TRACE_RESULT_DYNAMIC and TCL_TRACE_RESULT_OBJECT flags |

is set, which specify that the result is either a dynamic |

string (to be released with ckfree) or a Tcl_Obj* (cast to |

char* and to be released with Tcl_DecrRefCount) containing |

the error message. If a trace procedure returns an error, no further traces are invoked for the access and the traced access aborts with the given message. Trace procedures can

use this facility to make variables read-only, for example

(but note that the value of the variable will already have been modified before the trace procedure is called, so the trace procedure will have to restore the correct value). Tcl Last change: 7.4 7

Tcl Library Procedures Tcl_TraceVar(3TCL)

The return value from proc is only used during read and write tracing. During unset traces, the return value is ignored and all relevant trace procedures will always be invoked. RESTRICTIONS A trace procedure can be called at any time, even when there

is a partially-formed result in the interpreter's result

area. If the trace procedure does anything that could dam-

age this result (such as calling Tcl_Eval) then it must save

the original values of the interpreter's result and freeProc fields and restore them before it returns. UNDEFINED VARIABLES It is legal to set a trace on an undefined variable. The variable will still appear to be undefined until the first time its value is set. If an undefined variable is traced and then unset, the unset will fail with an error (``no such variable''), but the trace procedure will still be invoked.

TCL_TRACE_DESTROYED FLAG

In an unset callback to proc, the TCL_TRACE_DESTROYED bit is

set in flags if the trace is being removed as part of the deletion. Traces on a variable are always removed whenever

the variable is deleted; the only time TCL_TRACE_DESTROYED

isn't set is for a whole-array trace invoked when only a

single element of an array is unset.

TCL_INTERP_DESTROYED

When an interpreter is destroyed, unset traces are called

for all of its variables. The TCL_INTERP_DESTROYED bit will

be set in the flags argument passed to the trace procedures. Trace procedures must be extremely careful in what they do

if the TCL_INTERP_DESTROYED bit is set. It is not safe for

the procedures to invoke any Tcl procedures on the inter-

preter, since its state is partially deleted. All that trace procedures should do under these circumstances is to clean up and free their own internal data structures.

BUGS

Tcl doesn't do any error checking to prevent trace pro-

cedures from misusing the interpreter during traces with

TCL_INTERP_DESTROYED set.

Array traces are not yet integrated with the Tcl "info

exists" command, nor is there Tcl-level access to array

traces. Tcl Last change: 7.4 8

Tcl Library Procedures Tcl_TraceVar(3TCL)

KEYWORDS clientData, trace, variable

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes:

_______________________________________

| ATTRIBUTE TYPE | ATTRIBUTE VALUE|

|____________________|__________________|_

| Availability | runtime/tcl-8 |

|____________________|__________________|_

| Interface Stability| Uncommitted |

|____________________|_________________|

NOTES Source for Tcl is available on http://opensolaris.org. Tcl Last change: 7.4 9