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Interp.3
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1995-07-25
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TTTTccccllll____IIIInnnntttteeeerrrrpppp((((3333)))) TTTTccccllll (((( )))) TTTTccccllll____IIIInnnntttteeeerrrrpppp((((3333))))
_________________________________________________________________
NNNNAAAAMMMMEEEE
Tcl_Interp - client-visible fields of interpreter structures
SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
####iiiinnnncccclllluuuuddddeeee <<<<ttttccccllll....hhhh>>>>
typedef struct {
char *_r_e_s_u_l_t;
Tcl_FreeProc *_f_r_e_e_P_r_o_c;
int _e_r_r_o_r_L_i_n_e;
} Tcl_Interp;
typedef void Tcl_FreeProc(char *_b_l_o_c_k_P_t_r);
_________________________________________________________________
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
The TTTTccccllll____CCCCrrrreeeeaaaatttteeeeIIIInnnntttteeeerrrrpppp 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:
_r_e_s_u_l_t, _f_r_e_e_P_r_o_c, and _e_r_r_o_r_L_i_n_e.
The _r_e_s_u_l_t and _f_r_e_e_P_r_o_c fields are used to return results or
error messages from commands. This information is returned
by command procedures back to TTTTccccllll____EEEEvvvvaaaallll, and by TTTTccccllll____EEEEvvvvaaaallll back
to its callers. The _r_e_s_u_l_t field points to the string that
represents the result or error message, and the _f_r_e_e_P_r_o_c
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 TTTTccccllll____SSSSeeeettttRRRReeeessssuuuulllltttt or TTTTccccllll____AAAAppppppppeeeennnnddddRRRReeeessssuuuulllltttt; 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 _r_e_s_u_l_t field of its interpreter points to the string
being returned by the command. The _r_e_s_u_l_t field must always
point to a valid string. If a command wishes to return no
result then _i_n_t_e_r_p->_r_e_s_u_l_t 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 TTTTccccllll____EEEEvvvvaaaallll is called or some other command procedure
is invoked. In this case, the _f_r_e_e_P_r_o_c field must be zero.
Alternatively, a command procedure may dynamically allocate
its return value (e.g. using mmmmaaaalllllllloooocccc) and store a pointer to
it in _i_n_t_e_r_p->_r_e_s_u_l_t. In this case, the command procedure
must also set _i_n_t_e_r_p->_f_r_e_e_P_r_o_c to the address of a procedure
Page 1 (printed 7/10/95)
TTTTccccllll____IIIInnnntttteeeerrrrpppp((((3333)))) TTTTccccllll (((( )))) TTTTccccllll____IIIInnnntttteeeerrrrpppp((((3333))))
that can free the value (usually ffffrrrreeeeeeee). If _i_n_t_e_r_p->_f_r_e_e_P_r_o_c
is non-zero, then Tcl will call _f_r_e_e_P_r_o_c to free the space
pointed to by _i_n_t_e_r_p->_r_e_s_u_l_t before it invokes the next
command. If a client procedure overwrites _i_n_t_e_r_p->_r_e_s_u_l_t
when _i_n_t_e_r_p->_f_r_e_e_P_r_o_c is non-zero, then it is responsible
for calling _f_r_e_e_P_r_o_c to free the old _i_n_t_e_r_p->_r_e_s_u_l_t (the
TTTTccccllll____FFFFrrrreeeeeeeeRRRReeeessssuuuulllltttt macro should be used for this purpose).
_F_r_e_e_P_r_o_c should have arguments and result that match the
TTTTccccllll____FFFFrrrreeeeeeeePPPPrrrroooocccc declaration above: it receives a single
argument which is a pointer to the result value to free. In
most applications ffffrrrreeeeeeee is the only non-zero value ever used
for _f_r_e_e_P_r_o_c. However, an application may store a different
procedure address in _f_r_e_e_P_r_o_c 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, TTTTccccllll____EEEEvvvvaaaallll initializes
_i_n_t_e_r_p->_r_e_s_u_l_t and _i_n_t_e_r_p->_f_r_e_e_P_r_o_c just before calling the
command procedure for the command. The _f_r_e_e_P_r_o_c field will
be initialized to zero, and _i_n_t_e_r_p->_r_e_s_u_l_t 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 _r_e_s_u_l_t is actually part of an array of
TTTTCCCCLLLL____RRRREEEESSSSUUUULLLLTTTT____SSSSIIIIZZZZEEEE characters (approximately 200). If a
command wishes to return a short string, it can simply copy
it to the area pointed to by _i_n_t_e_r_p->_r_e_s_u_l_t. Or, it can use
the sprintf procedure to generate a short result string at
the location pointed to by _i_n_t_e_r_p->_r_e_s_u_l_t.
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 TTTTccccllll____RRRReeeesssseeeettttRRRReeeessssuuuulllltttt should be called first to reset the
result to its initialized state.
The _e_r_r_o_r_L_i_n_e field is valid only after TTTTccccllll____EEEEvvvvaaaallll returns a
TTTTCCCCLLLL____EEEERRRRRRRROOOORRRR return code. In this situation the _e_r_r_o_r_L_i_n_e
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 TTTTccccllll____EEEEvvvvaaaallll, 2 means the second
line, and so on. The _e_r_r_o_r_L_i_n_e field is typically used in
conjunction with TTTTccccllll____AAAAddddddddEEEErrrrrrrroooorrrrIIIInnnnffffoooo to report information
about where an error occurred. _E_r_r_o_r_L_i_n_e should not
normally be modified except by TTTTccccllll____EEEEvvvvaaaallll.
Page 2 (printed 7/10/95)
TTTTccccllll____IIIInnnntttteeeerrrrpppp((((3333)))) TTTTccccllll (((( )))) TTTTccccllll____IIIInnnntttteeeerrrrpppp((((3333))))
KKKKEEEEYYYYWWWWOOOORRRRDDDDSSSS
free, initialized, interpreter, malloc, result
Page 3 (printed 7/10/95)