For legacy programs and extensions no longer being maintained, compiles against the Tcl 8.6 header files are only possible with the compiler directives
#define USE_INTERP_ERRORLINEdepending on which fields of the Tcl_Interp struct are accessed. These directives may be embedded in code or supplied via compiler options.
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.