Tcl_TransferResult (3) - Linux Manuals

Tcl_TransferResult: manipulate Tcl result


Tcl_SetObjResult, Tcl_GetObjResult, Tcl_SetResult, Tcl_GetStringResult, Tcl_AppendResult, Tcl_AppendResultVA, Tcl_AppendElement, Tcl_ResetResult, Tcl_TransferResult, Tcl_FreeResult - manipulate Tcl result


#include <tcl.h>

Tcl_SetObjResult(interp, objPtr)

Tcl_Obj *

Tcl_SetResult(interp, result, freeProc)

const char *

Tcl_AppendResult(interp, result, result, ... , (char *) NULL)

Tcl_AppendResultVA(interp, argList)


Tcl_TransferResult(sourceInterp, result, targetInterp)

Tcl_AppendElement(interp, element)



Tcl_Interp *interp (out) Interpreter whose result is to be modified or read.
Tcl_Obj *objPtr (in) Tcl value to become result for interp.
char *result (in) String value to become result for interp or to be appended to the existing result.
const char *element (in) String value to append as a list element to the existing result of interp.
Tcl_FreeProc *freeProc (in) Address of procedure to call to release storage at result, or TCL_STATIC, TCL_DYNAMIC, or TCL_VOLATILE.
va_list argList (in) An argument list which must have been initialized using va_start, and cleared using va_end.
Tcl_Interp *sourceInterp (in) Interpreter that the result and error information should be copied from.
Tcl_Interp *targetInterp (in) Interpreter that the result and error information should be copied to.
int result (in) If TCL_OK, only copy the result. If TCL_ERROR, copy the error information as well.


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, Tcl_SetObjResult and Tcl_SetResult set the interpreter result to, respectively, a value and a string. Similarly, Tcl_GetObjResult and Tcl_GetStringResult 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 Tcl_SetObjResult is called to set the result to a value, then Tcl_GetStringResult is called, it will return the value's string representation.

Tcl_SetObjResult arranges for objPtr to be the result for interp, replacing any existing result. The result is left pointing to the value 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 value is decremented and the old result value is freed if no references to it remain.

Tcl_GetObjResult returns the result for interp 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 Tcl_IncrRefCount to increment its reference count in order to keep it from being freed too early or accidentally changed.

Tcl_SetResult arranges for result to be the result for the current Tcl command in interp, replacing any existing result. The freeProc argument specifies how to manage the storage for the result argument; it is discussed in the section THE TCL_FREEPROC ARGUMENT TO TCL_SETRESULT below. If result 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 a string. If the result was set to a value by a Tcl_SetObjResult 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 Tcl_GetObjResult instead.

Tcl_ResetResult clears the result for interp 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. Tcl_ResetResult also clears the error state managed by Tcl_AddErrorInfo, Tcl_AddObjErrorInfo, and Tcl_SetErrorCode.

Tcl_AppendResult makes it easy to build up Tcl results in pieces. It takes each of its result arguments and appends them in order to the current result associated with interp. If the result is in its initialized 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 result 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 associated with managing interp's result, such as allocating a larger result area if necessary. It also manages conversion to and from the result field of the interp so as to handle backward-compatibility with old-style extensions. Any number of result 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_TransferResult moves a result from one interpreter to another, optionally (dependent on the result parameter) including the error information dictionary as well. The interpreters must be in the same thread. The source interpreter will have its result reset by this operation.



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 a value can be significantly more efficient.

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 element argument and it appends that argument to the current 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 element will be extracted as a single element. 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. However 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 does not 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.


It used to be legal for programs to directly read and write interp->result to manipulate the interpreter result. The Tcl headers no longer permit this access by default, and C code still doing this must be updated to use supported routines Tcl_GetObjResult, Tcl_GetStringResult, Tcl_SetObjResult, and Tcl_SetResult. As a migration aid, access can be restored with the compiler directive

but this is meant only to offer life support to otherwise dead code.


Tcl_SetResult's freeProc argument specifies how the Tcl system is to manage the storage for the result 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 result 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 result 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 result 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 is not 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 result passed to Tcl_SetResult.


append, command, element, list, value, result, return value, interpreter


Tcl_AddErrorInfo, Tcl_CreateObjCommand, Tcl_SetErrorCode, Tcl_Interp