SWIG convert C-Pointer stringvalue to tcl string - c

Question

because of my limited knowledge in C and SWIG i couldn't manage to adopt any public example for converting c-pointer chars to tcl strings ....
I always get stuck at the problem that my tcl variable just doesn't get dereferenced
like this :
tcl_str = _30e84c05ef550000_p_stringout2
string_pointer.c
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#include "string_pointer.h"
stringout2 Itla_Get_Model_Version (int laser, char * mv_string)
{
stringout2 * pointer2;
char *mod_ver ="PPCL600";
pointer2 = malloc( sizeof(stringout2) );
pointer2-> modelvers= *mod_ver;
printf ( "Itla_Get_Model_Version : read %s \n", mod_ver );
return *pointer2 ;
}
string_pointer.h
#include <sys/types.h>
#include <sys/resource.h>
typedef struct {
char * modelvers;
} stringout2;
stringout2 Itla_Get_Model_Version (int laser, char * mv_string) ;
string_pointer.swig
/* File : string_pointer.swig */
%module string_pointer
%{
#include "string_pointer.h"
%}
%include "typemaps.i"
%include "cpointer.i"
%include "cstring.i"
%typemap(argout) char* (char tmp) %{
$1 = &tmp;
%}
stringout2 Itla_Get_Model_Version (int laser, char *OUTPUT) ;
%include "string_pointer.h"
test.tcl
load ./string_pointer.so
proc test { laser } {
scan [Itla_Get_Model_Version $laser ] %s a
puts "$a "
return $a
}
set name [test 1 ]
puts "Itla_Get_Model_Version= $name"
when executing the tcl-script you get :
Itla_Get_Model_Version : read PPCL600
_f0a759f8d9550000_p_stringout2
Itla_Get_Model_Version= _f0a759f8d9550000_p_stringout2
so i finally need to dereference the Pointer to its value ...
But i don't know how to succeed.....
The C-function is given and can't be modified !
Anybody out there, knowing how to do it ?

Answer 1

If your strings are basically ASCII or UTF-8, all you need to do is to tell SWIG that your function has allocated the string it is returning. For details see, the SWIG docs on C strings.
yourcode.c
char *Itla_Get_Model_Version (int laser, char * mv_string) {
// I assume this is a proxy for something more complicated...
const char *mod_ver ="PPCL600";
size_t len = strlen(mod_ver) + 1;
char *output = malloc(len);
memcpy(output, mod_ver, len);
printf ( "Itla_Get_Model_Version : read %s \n", mod_ver );
return output;
}
yourcode.h
char *Itla_Get_Model_Version(int laser, char * mv_string);
yourcode.swig
/* Tell SWIG that this function returns something to be freed */
%newobject Itla_Get_Model_Version
/* And now we can use the standard C header */
%include "yourcode.h"
If the above simple solution doesn't work…
Things get a lot more complicated if you are using a different encoding for your strings or if you wrap them inside a structure (as you did in your question). That's when you need a typemap, particularly ones of the Tcl variety. Correctly writing a typemap depends on understanding the semantics of the values that you are producing and/or consuming and the semantics of the language that you're using. Assuming you want the wrapping, here's a very simple output typemap that might work:
%typemap(out) stringout2* {
Tcl_SetObjResult(interp, Tcl_NewStringObj($1->modelvers, -1));
free($1);
}
Your function also needs to be modified to return a stringout2* by doing return pointer2;, and not a stringout2 since otherwise you will be leaking memory on every call. You can return a stringout2, but if you are doing that then you should not allocate it with malloc, but rather keep it as a structure directly in a local variable.
In that case, the typemap you'd use is:
%typemap(out) stringout2 {
Tcl_SetObjResult(interp, Tcl_NewStringObj($1.modelvers, -1));
}
(Note the different type, different access to the field, and lack of free.)
And your structure should be declared as containing a const char * if it really is that.
If you have strings in a different encoding (and it isn't ISO 8859-1, for which you can cheat and use a binary string using Tcl_NewByteArrayObj; that's also what you want for slabbing a chunk of binary data over) then you'll need to write a typemap using Tcl_ExternalToUtfDString, and the amount of boilerplate code goes up. Tcl insists that its internal strings are in (almost) UTF-8, and ASCII is OK too as that's a strict subset; everything else must be converted.
Ask another question if that's what you need. You probably are either dealing with ASCII or binary data, so I'll leave (quite a bit more complex!) encoding conversion alone until requested.