Use another language to call a function: Difference between revisions

=={{header|Tcl}}==

The way you would tackle this problem depends on whether you are working with ‘In’ or ‘Out’ parameters. (It is normal model ‘inout’ parameters as Tcl variables; omitted for brevity.)

===‘In’ Parameters===

To connect a function to Tcl that passes an arbitrary C string as input, you'd use a short C thunk, like this:

<lang c>int Query (char * Data, size_t * Length) {

Tcl_Obj *arguments[2];

int code;

arguments[0] = Tcl_NewStringObj("Query", -1); /* -1 for "use up to zero byte" */

arguments[1] = Tcl_NewStringObj(Data, Length);

Tcl_IncrRefCount(arguments[0]);

Tcl_IncrRefCount(arguments[1]);

if (Tcl_EvalObjv(interp, 2, arguments, 0) != TCL_OK) {

/* Was an error or other exception; report here... */

Tcl_DecrRefCount(arguments[0]);

Tcl_DecrRefCount(arguments[1]);

return 0;

}

Tcl_DecrRefCount(arguments[0]);

Tcl_DecrRefCount(arguments[1]);

if (Tcl_GetObjResult(NULL, Tcl_GetObjResult(interp), &code) != TCL_OK) {

/* Not an integer result */

return 0;

}

return code;

}</lang>

Which would lead to a <code>Query</code> implementation like this:

<lang tcl>proc Query data {

puts "Query was $data"

return 1;

}</lang>

===‘Out’ Parameters===

However, in the specific case of writing to a user-specified buffer (an “out” parameter) the thunk code would instead manage copying the result from the interpreter back to the buffer:

<lang c>int Query (char * Data, size_t * Length) {

const char *str;

int len;

if (Tcl_Eval(interp, "Query") != TCL_OK) {

return 0;

}

str = Tcl_GetStringFromObj(Tcl_GetObjResult(interp), &len);

if (len+1 > Length) {

return 0;

}

memcpy(Data, str, len+1);

return 1;

}</lang>

And the implementation of <code>Query</code> would be just:

<lang tcl>proc Query {} {

return "Here am I"

}</lang>

(Since this is working with a literal, this would actually be efficient and just result in references being passed.)

===Connecting up the pieces===

You would also need a short piece of code in <code>main()</code> to initialize the Tcl library and create an interpreter instance, and you would need to build and link against [[libtcl]].

<lang c>#include <tcl.h>

Tcl_Interp *interp;

int main(int argc, char **argv) {

Tcl_FindExecutable(argv[0]); /* Initializes library */

interp = Tcl_CreateInterp(); /* Make an interpreter */

/* Rest of contents of main() from task header... */

}</lang>