Tcl_Interp(3Tcl)Tcl_Interp(3Tcl)NAMETcl_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 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
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 malloc)
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 (usually free). 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).
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Tcl_Interp(3Tcl)Tcl_Interp(3Tcl)
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 free 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 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
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