Tcl_Interp(3) Tcl (7.5) 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. 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 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 |
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Tcl_Interp(3) Tcl (7.5) Tcl_Interp(3)
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 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.
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Tcl_Interp(3) Tcl (7.5) Tcl_Interp(3)
KEYWORDS
free, initialized, interpreter, malloc, result
Page 3 (printed 2/19/99)
