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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);
_________________________________________________________________
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 interpreter and per-
form 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 manip-
ulate the fields directly.
Whenever a command procedure returns, it must ensure that the result field of its inter-
preter 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->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 proce-
dure 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 alter-
nate 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->freeP-
roc 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 (approxi-
mately 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)
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