path: root/doc/SetResult.3

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.TH Tcl_SetResult 3 8.7 Tcl "Tcl Library Procedures"
.so man.macros
.BS
.SH NAME
Tcl_SetObjResult, Tcl_GetObjResult, Tcl_SetResult, Tcl_GetStringResult, Tcl_AppendResult, Tcl_AppendResultVA, Tcl_AppendElement, Tcl_ResetResult, Tcl_TransferResult, Tcl_FreeResult \- manipulate Tcl result
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_SetObjResult\fR(\fIinterp, objPtr\fR)
.sp
Tcl_Obj *
\fBTcl_GetObjResult\fR(\fIinterp\fR)
.sp
\fBTcl_SetResult\fR(\fIinterp, result, freeProc\fR)
.sp
const char *
\fBTcl_GetStringResult\fR(\fIinterp\fR)
.sp
\fBTcl_AppendResult\fR(\fIinterp, result, result, ... , \fB(char *) NULL\fR)
.sp
\fBTcl_AppendResultVA\fR(\fIinterp, argList\fR)
.sp
\fBTcl_ResetResult\fR(\fIinterp\fR)
.sp
\fBTcl_TransferResult\fR(\fIsourceInterp, code, targetInterp\fR)
.sp
\fBTcl_AppendElement\fR(\fIinterp, element\fR)
.sp
\fBTcl_FreeResult\fR(\fIinterp\fR)
.SH ARGUMENTS
.AS Tcl_FreeProc sourceInterp out
.AP Tcl_Interp *interp out
Interpreter whose result is to be modified or read.
.AP Tcl_Obj *objPtr in
Tcl value to become result for \fIinterp\fR.
.AP char *result in
String value to become result for \fIinterp\fR or to be
appended to the existing result.
.AP "const char" *element in
String value to append as a list element
to the existing result of \fIinterp\fR.
.AP Tcl_FreeProc *freeProc in
Address of procedure to call to release storage at
\fIresult\fR, or \fBTCL_STATIC\fR, \fBTCL_DYNAMIC\fR, or
\fBTCL_VOLATILE\fR.
.AP va_list argList in
An argument list which must have been initialized using
\fBva_start\fR, and cleared using \fBva_end\fR.
.AP Tcl_Interp *sourceInterp in
Interpreter that the result and return options should be transferred from.
.AP Tcl_Interp *targetInterp in
Interpreter that the result and return options should be transferred to.
.AP int code in
Return code value that controls transfer of return options.
.BE
.SH DESCRIPTION
.PP
The procedures described here are utilities for manipulating the
result value in a Tcl interpreter.
The interpreter result may be either a Tcl value or a string.
For example, \fBTcl_SetObjResult\fR and \fBTcl_SetResult\fR
set the interpreter result to, respectively, a value and a string.
Similarly, \fBTcl_GetObjResult\fR and \fBTcl_GetStringResult\fR
return the interpreter result as a value and as a string.
The procedures always keep the string and value forms
of the interpreter result consistent.
For example, if \fBTcl_SetObjResult\fR is called to set
the result to a value,
then \fBTcl_GetStringResult\fR is called,
it will return the value's string representation.
.PP
\fBTcl_SetObjResult\fR
arranges for \fIobjPtr\fR to be the result for \fIinterp\fR,
replacing any existing result.
The result is left pointing to the value
referenced by \fIobjPtr\fR.
\fIobjPtr\fR's reference count is incremented
since there is now a new reference to it from \fIinterp\fR.
The reference count for any old result value
is decremented and the old result value is freed if no
references to it remain.
.PP
\fBTcl_GetObjResult\fR returns the result for \fIinterp\fR as a value.
The value's reference count is not incremented;
if the caller needs to retain a long-term pointer to the value
they should use \fBTcl_IncrRefCount\fR to increment its reference count
in order to keep it from being freed too early or accidentally changed.
.PP
\fBTcl_SetResult\fR
arranges for \fIresult\fR to be the result for the current Tcl
command in \fIinterp\fR, replacing any existing result.
The \fIfreeProc\fR argument specifies how to manage the storage
for the \fIresult\fR argument;
it is discussed in the section
\fBTHE TCL_FREEPROC ARGUMENT TO TCL_SETRESULT\fR below.
If \fIresult\fR is \fBNULL\fR, then \fIfreeProc\fR is ignored
and \fBTcl_SetResult\fR
re-initializes \fIinterp\fR's result to point to an empty string.
.PP
\fBTcl_GetStringResult\fR returns the result for \fIinterp\fR as a string.
If the result was set to a value by a \fBTcl_SetObjResult\fR call,
the value form will be converted to a string and returned.
If the value's string representation contains null bytes,
this conversion will lose information.
For this reason, programmers are encouraged to
write their code to use the new value API procedures
and to call \fBTcl_GetObjResult\fR instead.
.PP
\fBTcl_ResetResult\fR clears the result for \fIinterp\fR
and leaves the result in its normal empty initialized state.
If the result is a value,
its reference count is decremented and the result is left
pointing to an unshared value representing an empty string.
If the result is a dynamically allocated string, its memory is free*d
and the result is left as a empty string.
\fBTcl_ResetResult\fR also clears the error state managed by
\fBTcl_AddErrorInfo\fR, \fBTcl_AddObjErrorInfo\fR,
and \fBTcl_SetErrorCode\fR.
.PP
\fBTcl_AppendResult\fR makes it easy to build up Tcl results in pieces.
It takes each of its \fIresult\fR arguments and appends them in order
to the current result associated with \fIinterp\fR.
If the result is in its initialized empty state (e.g. a command procedure
was just invoked or \fBTcl_ResetResult\fR was just called),
then \fBTcl_AppendResult\fR sets the result to the concatenation of
its \fIresult\fR arguments.
\fBTcl_AppendResult\fR may be called repeatedly as additional pieces
of the result are produced.
\fBTcl_AppendResult\fR takes care of all the
storage management issues associated with managing \fIinterp\fR's
result, such as allocating a larger result area if necessary.
It also manages conversion to and from the \fIresult\fR field of the
\fIinterp\fR so as to handle backward-compatibility with old-style
extensions.
Any number of \fIresult\fR arguments may be passed in a single
call; the last argument in the list must be a NULL pointer.
.PP
\fBTcl_AppendResultVA\fR is the same as \fBTcl_AppendResult\fR except that
instead of taking a variable number of arguments it takes an argument list.
Interfaces using argument lists have been found to be nonportable in practice.
This function is deprecated and will be removed in Tcl 9.0.
.PP
\fBTcl_TransferResult\fR transfers interpreter state from \fIsourceInterp\fR
to \fItargetInterp\fR. The two interpreters must have been created in the
same thread.  If \fIsourceInterp\fR and \fItargetInterp\fR are the same,
nothing is done. Otherwise, \fBTcl_TransferResult\fR moves the result
from \fIsourceInterp\fR to \fItargetInterp\fR, and resets the result
in \fIsourceInterp\fR. It also moves the return options dictionary as
controlled by the return code value \fIcode\fR in the same manner
as \fBTcl_GetReturnOptions\fR.
.SH "DEPRECATED INTERFACES"
.SS "OLD STRING PROCEDURES"
.PP
Use of the following procedures is deprecated
since they manipulate the Tcl result as a string.
Procedures such as \fBTcl_SetObjResult\fR
that manipulate the result as a value
can be significantly more efficient.
.PP
\fBTcl_AppendElement\fR is similar to \fBTcl_AppendResult\fR in
that it allows results to be built up in pieces.
However, \fBTcl_AppendElement\fR takes only a single \fIelement\fR
argument and it appends that argument to the current result
as a proper Tcl list element.
\fBTcl_AppendElement\fR adds backslashes or braces if necessary
to ensure that \fIinterp\fR's result can be parsed as a list and that
\fIelement\fR will be extracted as a single element.
Under normal conditions, \fBTcl_AppendElement\fR will add a space
character to \fIinterp\fR's result just before adding the new
list element, so that the list elements in the result are properly
separated.
However if the new list element is the first in a list or sub-list
(i.e. \fIinterp\fR's current result is empty, or consists of the
single character
.QW { ,
or ends in the characters
.QW " {" )
then no space is added.
.PP
\fBTcl_FreeResult\fR performs part of the work
of \fBTcl_ResetResult\fR.
It frees up the memory associated with \fIinterp\fR's result.
It also sets \fIinterp->freeProc\fR to zero, but does not
change \fIinterp->result\fR or clear error state.
\fBTcl_FreeResult\fR is most commonly used when a procedure
is about to replace one result value with another.
.SS "DIRECT ACCESS TO INTERP->RESULT"
.PP
It used to be legal for programs to
directly read and write \fIinterp->result\fR
to manipulate the interpreter result.  The Tcl headers no longer
permit this access. C code still doing this must
be updated to use supported routines \fBTcl_GetObjResult\fR,
\fBTcl_GetStringResult\fR, \fBTcl_SetObjResult\fR, and \fBTcl_SetResult\fR.
.SH "THE TCL_FREEPROC ARGUMENT TO TCL_SETRESULT"
.PP
\fBTcl_SetResult\fR's \fIfreeProc\fR argument specifies how
the Tcl system is to manage the storage for the \fIresult\fR argument.
If \fBTcl_SetResult\fR or \fBTcl_SetObjResult\fR are called
at a time when \fIinterp\fR holds a string result,
they do whatever is necessary to dispose of the old string result
(see the \fBTcl_Interp\fR manual entry for details on this).
.PP
If \fIfreeProc\fR is \fBTCL_STATIC\fR it means that \fIresult\fR
refers to an area of static storage that is guaranteed not to be
modified until at least the next call to \fBTcl_Eval\fR.
If \fIfreeProc\fR
is \fBTCL_DYNAMIC\fR it means that \fIresult\fR was allocated with a call
to \fBTcl_Alloc\fR and is now the property of the Tcl system.
\fBTcl_SetResult\fR will arrange for the string's storage to be
released by calling \fBTcl_Free\fR when it is no longer needed.
If \fIfreeProc\fR is \fBTCL_VOLATILE\fR it means that \fIresult\fR
points to an area of memory that is likely to be overwritten when
\fBTcl_SetResult\fR returns (e.g. it points to something in a stack frame).
In this case \fBTcl_SetResult\fR will make a copy of the string in
dynamically allocated storage and arrange for the copy to be the
result for the current Tcl command.
.PP
If \fIfreeProc\fR is not one of the values \fBTCL_STATIC\fR,
\fBTCL_DYNAMIC\fR, and \fBTCL_VOLATILE\fR, then it is the address
of a procedure that Tcl should call to free the string.
This allows applications to use non-standard storage allocators.
When Tcl no longer needs the storage for the string, it will
call \fIfreeProc\fR. \fIFreeProc\fR should have arguments and
result that match the type \fBTcl_FreeProc\fR:
.PP
.CS
typedef void \fBTcl_FreeProc\fR(
        char *\fIblockPtr\fR);
.CE
.PP
When \fIfreeProc\fR is called, its \fIblockPtr\fR will be set to
the value of \fIresult\fR passed to \fBTcl_SetResult\fR.

.SH "REFERENCE COUNT MANAGEMENT"
.PP
The interpreter result is one of the main places that owns references to
values, along with the bytecode execution stack, argument lists, variables,
and the list and dictionary collection values.
.PP
\fBTcl_SetObjResult\fR takes a value with an arbitrary reference count
\fI(specifically including zero)\fR and guarantees to increment the reference
count. If code wishes to continue using the value after setting it as the
result, it should add its own reference to it with \fBTcl_IncrRefCount\fR.
.PP
\fBTcl_GetObjResult\fR returns the current interpreter result value. This will
have a reference count of at least 1. If the caller wishes to keep the
interpreter result value, it should increment its reference count.
.PP
\fBTcl_GetStringResult\fR does not manipulate reference counts, but the string
it returns is owned by (and has a lifetime controlled by) the current
interpreter result value; it should be copied instead of being relied upon to
persist after the next Tcl API call, as most Tcl operations can modify the
interpreter result.
.PP
\fBTcl_SetResult\fR, \fBTcl_AppendResult\fR, \fBTcl_AppendResultVA\fR,
\fBTcl_AppendElement\fR, and \fBTcl_ResetResult\fR all modify the interpreter
result. They may cause the old interpreter result to have its reference count
decremented and a new interpreter result to be allocated. After they have been
called, the reference count of the interpreter result is guaranteed to be 1.
.SH "SEE ALSO"
Tcl_AddErrorInfo, Tcl_CreateObjCommand, Tcl_SetErrorCode, Tcl_Interp,
Tcl_GetReturnOptions
.SH KEYWORDS
append, command, element, list, value, result, return value, interpreter