Tcl_Interp(3) Tcl Library Procedures Tcl_Interp(3)

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NAME

Tcl_Interp - client-visible fields of interpreter structures

SYNOPSIS

#include <tcl.h>

typedef struct {
char *result;
Tcl_FreeProc *freeProc;
int errorLine;
} Tcl_Interp;

typedef void Tcl_FreeProc(
char *blockPtr);
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DESCRIPTION

The Tcl_CreateInterp procedure returns a pointer to a Tcl_Interp
structure. Callers of Tcl_CreateInterp should use this pointer as an
opaque token, suitable for nothing other than passing back to other
routines in the Tcl interface. Accessing fields directly through the
pointer as described below is no longer supported. The supported
public routines Tcl_SetResult, Tcl_GetResult, Tcl_SetErrorLine,
Tcl_GetErrorLine must be used instead.

For legacy programs and extensions no longer being maintained,
compiles against the Tcl 8.6 header files are only possible with the
compiler directives
#define USE_INTERP_RESULT
and/or
#define USE_INTERP_ERRORLINE
depending on which fields of the Tcl_Interp struct are accessed.
These directives may be embedded in code or supplied via compiler
options.

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 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 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 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 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
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 initialized state described in
the previous paragraph. Procedures 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_SetResult).

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 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 occurred. ErrorLine should not normally be modified except
by Tcl_Eval.

KEYWORDS

free, initialized, interpreter, malloc, result

Tcl 7.5 Tcl_Interp(3)