Tcl_Interp(3) Tcl Library Procedures Tcl_Interp(3) 







Tcl_Interp - client-visible fields of interpreter structures








#include <tcl.h>

typedef struct {
        char *result;
        Tcl_FreeProc *freeProc;
        int errorLine;
} Tcl_Interp;
typedef void Tcl_FreeProc(char *blockPtr);











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_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 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->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 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.










free, initialized, interpreter, malloc, result







  

    7.5
    Tcl