1Tcl_Interp(3) Tcl Library Procedures Tcl_Interp(3)
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8 Tcl_Interp - client-visible fields of interpreter structures
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11 #include <tcl.h>
12 typedef struct {
14 char *result;
15 Tcl_FreeProc *freeProc;
16 int errorLine;
17 } Tcl_Interp;
18 typedef void Tcl_FreeProc(char *blockPtr);
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24 The Tcl_CreateInterp procedure returns a pointer to a Tcl_Interp struc‐
25 ture. This pointer is then passed into other Tcl procedures to process
26 commands in the interpreter and perform other operations on the inter‐
27 preter. Interpreter structures contain many many fields that are used
28 by Tcl, but only three that may be accessed by clients: result, freeP‐
29 roc, and errorLine.
30 The result and freeProc fields are used to return results or error mes‐
32 sages from commands. This information is returned by command proce‐
33 dures back to Tcl_Eval, and by Tcl_Eval back to its callers. The
34 result field points to the string that represents the result or error
35 message, and the freeProc field tells how to dispose of the storage for
36 the string when it isn't needed anymore. The easiest way for command
37 procedures to manipulate these fields is to call procedures like
38 Tcl_SetResult or Tcl_AppendResult; they will hide all the details of
39 managing the fields. The description below is for those procedures
40 that manipulate the fields directly.
41 Whenever a command procedure returns, it must ensure that the result
43 field of its interpreter points to the string being returned by the
44 command. The result field must always point to a valid string. If a
45 command wishes to return no result then interp->result should point to
46 an empty string. Normally, results are assumed to be statically allo‐
47 cated, which means that the contents will not change before the next
48 time Tcl_Eval is called or some other command procedure is invoked. In │
49 this case, the freeProc field must be zero. Alternatively, a command │
50 procedure may dynamically allocate its return value (e.g. using │
51 Tcl_Alloc) and store a pointer to it in interp->result. In this case, │
52 the command procedure must also set interp->freeProc to the address of │
53 a procedure that can free the value, or TCL_DYNAMIC if the storage was │
54 allocated directly by Tcl or by a call to Tcl_Alloc. If interp->freeP‐
55 roc is non-zero, then Tcl will call freeProc to free the space pointed
56 to by interp->result before it invokes the next command. If a client
57 procedure overwrites interp->result when interp->freeProc is non-zero,
58 then it is responsible for calling freeProc to free the old
59 interp->result (the Tcl_FreeResult macro should be used for this pur‐
60 pose).
61 FreeProc should have arguments and result that match the Tcl_FreeProc
63 declaration above: it receives a single argument which is a pointer to
64 the result value to free. In most applications TCL_DYNAMIC is the only │
65 non-zero value ever used for freeProc. However, an application may
66 store a different procedure address in freeProc in order to use an
67 alternate memory allocator or in order to do other cleanup when the
68 result memory is freed.
69 As part of processing each command, Tcl_Eval initializes interp->result
71 and interp->freeProc just before calling the command procedure for the
72 command. The freeProc field will be initialized to zero, and
73 interp->result will point to an empty string. Commands that do not
74 return any value can simply leave the fields alone. Furthermore, the
75 empty string pointed to by result is actually part of an array of
76 TCL_RESULT_SIZE characters (approximately 200). If a command wishes to
77 return a short string, it can simply copy it to the area pointed to by
78 interp->result. Or, it can use the sprintf procedure to generate a
79 short result string at the location pointed to by interp->result.
80 It is a general convention in Tcl-based applications that the result of
82 an interpreter is normally in the initialized state described in the
83 previous paragraph. Procedures that manipulate an interpreter's result
84 (e.g. by returning an error) will generally assume that the result has
85 been initialized when the procedure is called. If such a procedure is
86 to be called after the result has been changed, then Tcl_ResetResult
87 should be called first to reset the result to its initialized state.
88 The direct use of interp->result is strongly deprecated (see Tcl_SetRe‐
89 sult).
90 The errorLine field is valid only after Tcl_Eval returns a TCL_ERROR
92 return code. In this situation the errorLine field identifies the line
93 number of the command being executed when the error occurred. The line
94 numbers are relative to the command being executed: 1 means the first
95 line of the command passed to Tcl_Eval, 2 means the second line, and so
96 on. The errorLine field is typically used in conjunction with
97 Tcl_AddErrorInfo to report information about where an error occurred.
98 ErrorLine should not normally be modified except by Tcl_Eval.
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102 free, initialized, interpreter, malloc, result
103Tcl 7.5 Tcl_Interp(3)