The Tcl_CreateInterp procedure returns a pointer to a Tcl_Interp
structure. This pointer is then passed into other Tcl procedures
to process commands in the interpreter and perform other operations
on the interpreter. Interpreter structures contain many fields
that are used by Tcl, but only three that may be accessed by
clients: result, freeProc, and errorLine.
Note that access to all three fields, result, freeProc and
errorLine is deprecated. Use Tcl_SetResult,
Tcl_GetResult, and Tcl_GetReturnOptions instead.
The result and freeProc fields are used to return
results or error messages from commands.
This information is returned by command procedures back to Tcl_Eval,
and by Tcl_Eval back to its callers.
The result field points to the string that represents the
result or error message, and the freeProc field tells how
to dispose of the storage for the string when it is not needed anymore.
The easiest way for command procedures to manipulate these
fields is to call procedures like Tcl_SetResult
or Tcl_AppendResult; they
will hide all the details of managing the fields.
The description below is for those procedures that manipulate the
Whenever a command procedure returns, it must ensure
that the result field of its interpreter points to the string
being returned by the command.
The result field must always point to a valid string.
If a command wishes to return no result then interp->result
should point to an empty string.
Normally, results are assumed to be statically allocated,
which means that the contents will not change before the next time
Tcl_Eval is called or some other command procedure is invoked.
In this case, the freeProc field must be zero.
Alternatively, a command procedure may dynamically
allocate its return value (e.g. using Tcl_Alloc)
and store a pointer to it in interp->result.
In this case, the command procedure must also set interp->freeProc
to the address of a procedure that can free the value, or TCL_DYNAMIC
if the storage was allocated directly by Tcl or by a call to
If interp->freeProc is non-zero, then Tcl will call freeProc
to free the space pointed to by interp->result before it
invokes the next command.
If a client procedure overwrites interp->result when
interp->freeProc is non-zero, then it is responsible for calling
freeProc to free the old interp->result (the Tcl_FreeResult
macro should be used for this purpose).
FreeProc should have arguments and result that match the
Tcl_FreeProc declaration above: it receives a single
argument which is a pointer to the result value to free.
In most applications TCL_DYNAMIC is the only non-zero value ever
used for freeProc.
However, an application may store a different procedure address
in freeProc in order to use an alternate memory allocator
or in order to do other cleanup when the result memory is freed.
As part of processing each command, Tcl_Eval initializes
and interp->freeProc just before calling the command procedure for
the command. The freeProc field will be initialized to zero,
and interp->result will point to an empty string. Commands that
do not return any value can simply leave the fields alone.
Furthermore, the empty string pointed to by result is actually
part of an array of TCL_RESULT_SIZE characters (approximately 200).
If a command wishes to return a short string, it can simply copy
it to the area pointed to by interp->result. Or, it can use
the sprintf procedure to generate a short result string at the location
pointed to by interp->result.
It is a general convention in Tcl-based applications that the result
of an interpreter is normally in the initialized state described
in the previous paragraph.
Procedures that manipulate an interpreters result (e.g. by
returning an error) will generally assume that the result
has been initialized when the procedure is called.
If such a procedure is to be called after the result has been
changed, then Tcl_ResetResult should be called first to
reset the result to its initialized state. The direct use of
interp->result is strongly deprecated (see Tcl_SetResult).
field is valid only after Tcl_Eval returns
a TCL_ERROR return code. In this situation the errorLine
field identifies the line number of the command being executed when
the error occurred. The line numbers are relative to the command
being executed: 1 means the first line of the command passed to
Tcl_Eval, 2 means the second line, and so on.
The errorLine field is typically used in conjunction with
Tcl_AddErrorInfo to report information about where an error
ErrorLine should not normally be modified except by Tcl_Eval.