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.  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 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_GetReturnOp- |
       tions instead.

       The result and freeProc fields are used to return results or error messages 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 is not needed anymore.  The easiest way for com-
       mand 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->freeP-
       roc 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 depre-
       cated (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)