Combine C and TCL using Swig - c

Question

I have been following a tutorial to combine C with TCL using Swig. The tutorial seemed to be properly working but at the end I ran into an error that I cannot solve. The situation is as follows:
The tutorial I was following is:
http://www.swig.org/tutorial.html.
I have a file named test.c:
char *HelloWorld()
{
return "hello world";
}
and another named test.i:
%module test
%{
/* Put header files here or function declarations like below */
extern char *HelloWorld();
%}
extern char *HelloWorld();
I then used the following command line arguments to ready the correct files:
gcc -c test.c -o test.o
swig -tcl test.i
gcc -c test_wrap.c -o test_wrap.o
gcc -dynamiclib -framework Tcl test.o test_wrap.o -o test.so
And finally I tried to load it using:
tclsh
% load test.so test
This is the point where I received the following error:
dlsym(0x100600090, Test_Unload): symbol not founddlsym(0x100600090, Test_SafeUnload): symbol not found
As far as I know I did not stray from the tutorial. Can anyone tell me how it is that I got this error and more importantly how to get rid of it?
Thanks in advance!

Answer 1

Are those error messages stopping the load from working? They shouldn't; they're reporting that the low-level API for supporting unloading of the extension isn't present, but that's OK (lots of extensions can't be unloaded; it's tricky to write code that supports it).
You don't mention exactly which version of Tcl you are using — but it must be at least 8.5 for those symbols to be even searched for in the first place — so it is a little hard to guess what the exact underlying issue is. (The message should simply not be reported.) I advise filing a bug report on this; make sure you include all exact versions in your report.

Answer 2

It's a long time since I used SWIG, so I'm not sure whether it gives you sufficient control over the code it generates for you to be able to apply this fix. Glossing over that detail, I can reproduce (and fix) the issue with the following:
In 'ext.c':
#include <tcl.h>
int DLLEXPORT Ext_Init(Tcl_Interp *interp) {
if (Tcl_InitStubs(interp, TCL_VERSION, 0) == NULL) {
return TCL_ERROR;
}
if (Tcl_PkgProvide(interp, "Ext", "0.0") == TCL_ERROR) {
return TCL_ERROR;
}
return TCL_OK;
}
Build, run tclsh, load extension:
$ gcc -dynamiclib -framework Tcl ext.c -o ext.so
$ tclsh8.5
% load ./ext.so
dlsym(0x400000, Ext_SafeInit): symbol not found
dlsym(0x400000, Ext_Unload): symbol not found
dlsym(0x400000, Ext_SafeUnload): symbol not found
Something internal to the library loading code is putting that error message into the interpreters result. To stop the message ever surfacing, set or reset the result so that the _Init() function ends with one or other of:
// Set the result to a message of your choosing
Tcl_SetObjResult(interp, Tcl_NewStringObj("ok", -1));
// Or clear out the result altogether
Tcl_ResetResult(interp);
return TCL_OK;
}
The init block feature of swig might insert code in the right place to achieve the same thing:
%init %{
Tcl_ResetResult(interp);
%}