tcl_interp(3)

NAME

Tcl_Interp - client-visible fields of interpreter struc
tures

SYNOPSIS

#include <tcl.h>
typedef struct {
     char *result;
     Tcl_FreeProc *freeProc;
     int errorLine;
} Tcl_Interp;
typedef void Tcl_FreeProc(char *blockPtr);

DESCRIPTION

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 inter
preter and perform other operations on the interpreter.
Interpreter structures contain many many fields that are
used by Tcl, but only three that may be accessed by
clients: result, freeProc, and errorLine.

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 isn't 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 manipu
late the fields directly.

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 stati
cally 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 Tcl_Alloc. 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 over
writes 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 interp->result 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 initial
ized state described in the previous paragraph. Proce
dures that manipulate an interpreter's 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_SetRe sult).

The errorLine 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 exe
cuted when the error occurred. The line numbers are rela
tive 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 occurred. ErrorLine should not nor mally be modified except by Tcl_Eval.

KEYWORDS

free, initialized, interpreter, malloc, result

Tcl 7.5 Tcl_Interp(3)