Tcl_Interp
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 struc-
ture. This pointer is then passed into other Tcl procedures to process
commands in the interpreter and perform other operations on the inter-
preter. Interpreter structures contain many many fields that are used
by Tcl, but only three that may be accessed by clients: result, freeP-
roc, and errorLine.
The result and freeProc fields are used to return results or error mes-
sages from commands. This information is returned by command proce-
dures 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 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 allo-
cated, 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->freeP-
roc 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 pur-
pose).
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_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 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)
