Tcl_AppendElement
Tcl_SetResult(3) Tcl Library Procedures Tcl_SetResult(3)
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NAME
Tcl_SetObjResult, Tcl_GetObjResult, Tcl_SetResult, Tcl_GetStringResult,
Tcl_AppendResult, Tcl_AppendResultVA, Tcl_AppendElement, Tcl_ResetRe-
sult, Tcl_FreeResult - manipulate Tcl result
SYNOPSIS
#include <tcl.h>
Tcl_SetObjResult(interp, objPtr)
Tcl_Obj *
Tcl_GetObjResult(interp)
Tcl_SetResult(interp, string, freeProc)
char *
Tcl_GetStringResult(interp)
Tcl_AppendResult(interp, string, string, ... , (char *) NULL)
Tcl_AppendResultVA(interp, argList)
Tcl_AppendElement(interp, string)
Tcl_ResetResult(interp)
Tcl_FreeResult(interp)
ARGUMENTS
Tcl_Interp *interp (out) Interpreter whose result is to be
modified or read.
Tcl_Obj *objPtr (in) Object value to become result for
interp.
char *string (in) String value to become result for
interp or to be appended to the
existing result.
Tcl_FreeProc *freeProc (in) Address of procedure to call to
release storage at string, or
TCL_STATIC, TCL_DYNAMIC, or
TCL_VOLATILE.
va_list argList (in) An argument list which must have
been initialised using
TCL_VARARGS_START, and cleared
using va_end.
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DESCRIPTION
The procedures described here are utilities for manipulating the result
value in a Tcl interpreter. The interpreter result may be either a Tcl
object or a string. For example, Tcl_SetObjResult and Tcl_SetResult
set the interpreter result to, respectively, an object and a string.
Similarly, Tcl_GetObjResult and Tcl_GetStringResult return the inter-
preter result as an object and as a string. The procedures always keep
the string and object forms of the interpreter result consistent. For
example, if Tcl_SetObjResult is called to set the result to an object,
then Tcl_GetStringResult is called, it will return the object's string
value.
Tcl_SetObjResult arranges for objPtr to be the result for interp,
replacing any existing result. The result is left pointing to the
object referenced by objPtr. objPtr's reference count is incremented
since there is now a new reference to it from interp. The reference
count for any old result object is decremented and the old result
object is freed if no references to it remain.
Tcl_GetObjResult returns the result for interp as an object. The
object's reference count is not incremented; if the caller needs to
retain a long-term pointer to the object they should use Tcl_IncrRef-
Count to increment its reference count in order to keep it from being
freed too early or accidently changed.
Tcl_SetResult arranges for string to be the result for the current Tcl
command in interp, replacing any existing result. The freeProc argu-
ment specifies how to manage the storage for the string argument; it is
discussed in the section THE TCL_FREEPROC ARGUMENT TO TCL_SETRESULT
below. If string is NULL, then freeProc is ignored and Tcl_SetResult
re-initializes interp's result to point to an empty string.
Tcl_GetStringResult returns the result for interp as an string. If the
result was set to an object by a Tcl_SetObjResult call, the object form
will be converted to a string and returned. If the object's string
representation contains null bytes, this conversion will lose informa-
tion. For this reason, programmers are encouraged to write their code
to use the new object API procedures and to call Tcl_GetObjResult
instead.
Tcl_ResetResult clears the result for interp and leaves the result in
its normal empty initialized state. If the result is an object, its
reference count is decremented and the result is left pointing to an
unshared object 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. Tcl_ResetResult also clears the error state
managed by Tcl_AddErrorInfo, Tcl_AddObjErrorInfo, and Tcl_SetErrorCode.
OLD STRING PROCEDURES
Use of the following procedures is deprecated since they manipulate the
Tcl result as a string. Procedures such as Tcl_SetObjResult that
manipulate the result as an object can be significantly more efficient.
Tcl_AppendResult makes it easy to build up Tcl results in pieces. It
takes each of its string arguments and appends them in order to the
current result associated with interp. If the result is in its ini-
tialized empty state (e.g. a command procedure was just invoked or
Tcl_ResetResult was just called), then Tcl_AppendResult sets the result
to the concatenation of its string arguments. Tcl_AppendResult may be
called repeatedly as additional pieces of the result are produced.
Tcl_AppendResult takes care of all the storage management issues asso-
ciated with managing interp's result, such as allocating a larger
result area if necessary. It also converts the current interpreter
result from an object to a string, if necessary, before appending the
argument strings. Any number of string arguments may be passed in a
single call; the last argument in the list must be a NULL pointer.
Tcl_AppendResultVA is the same as Tcl_AppendResult except that instead
of taking a variable number of arguments it takes an argument list.
Tcl_AppendElement is similar to Tcl_AppendResult in that it allows
results to be built up in pieces. However, Tcl_AppendElement takes
only a single string argument and it appends that argument to the cur-
rent result as a proper Tcl list element. Tcl_AppendElement adds
backslashes or braces if necessary to ensure that interp's result can
be parsed as a list and that string will be extracted as a single ele-
ment. Under normal conditions, Tcl_AppendElement will add a space
character to interp's result just before adding the new list element,
so that the list elements in the result are properly separated. How-
ever if the new list element is the first in a list or sub-list (i.e.
interp's current result is empty, or consists of the single character
``{'', or ends in the characters `` {'') then no space is added.
Tcl_FreeResult performs part of the work of Tcl_ResetResult. It frees
up the memory associated with interp's result. It also sets
interp->freeProc to zero, but doesn't change interp->result or clear
error state. Tcl_FreeResult is most commonly used when a procedure is
about to replace one result value with another.
DIRECT ACCESS TO INTERP->RESULT IS DEPRECATED
It used to be legal for programs to directly read and write
interp->result to manipulate the interpreter result. Direct access to
interp->result is now strongly deprecated because it can make the
result's string and object forms inconsistent. Programs should always
read the result using the procedures Tcl_GetObjResult or Tcl_Get-
StringResult, and write the result using Tcl_SetObjResult or Tcl_SetRe-
sult.
THE TCL_FREEPROC ARGUMENT TO TCL_SETRESULT
Tcl_SetResult's freeProc argument specifies how the Tcl system is to
manage the storage for the string argument. If Tcl_SetResult or
Tcl_SetObjResult are called at a time when interp holds a string
result, they do whatever is necessary to dispose of the old string
result (see the Tcl_Interp manual entry for details on this).
If freeProc is TCL_STATIC it means that string refers to an area of
static storage that is guaranteed not to be modified until at least the
next call to Tcl_Eval. If freeProc is TCL_DYNAMIC it means that string
was allocated with a call to Tcl_Alloc and is now the property of the
Tcl system. Tcl_SetResult will arrange for the string's storage to be
released by calling Tcl_Free when it is no longer needed. If freeProc
is TCL_VOLATILE it means that string points to an area of memory that
is likely to be overwritten when Tcl_SetResult returns (e.g. it points
to something in a stack frame). In this case Tcl_SetResult 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.
If freeProc isn't one of the values TCL_STATIC, TCL_DYNAMIC, and
TCL_VOLATILE, 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 freeProc. FreeProc should have arguments and
result that match the type Tcl_FreeProc:
typedef void Tcl_FreeProc(char *blockPtr);
When freeProc is called, its blockPtr will be set to the value of
string passed to Tcl_SetResult.
SEE ALSO
Tcl_AddErrorInfo, Tcl_CreateObjCommand, Tcl_SetErrorCode, Tcl_Interp
KEYWORDS
append, command, element, list, object, result, return value, inter-
preter
Tcl 8.0 Tcl_SetResult(3)
