Getting "cannot find symbol .... while executing load ..." error when trying to run Hello World as a C extension (dll) example - gcc

Question

I have used the C code from the following verbatim: https://wiki.tcl-lang.org/page/Hello+World+as+a+C+extension
/*
* hello.c -- A minimal Tcl C extension.
*/
#include <tcl.h>
static int
Hello_Cmd(ClientData cdata, Tcl_Interp *interp, int objc, Tcl_Obj *const objv[])
{
Tcl_SetObjResult(interp, Tcl_NewStringObj("Hello, World!", -1));
return TCL_OK;
}
/*
* Hello_Init -- Called when Tcl loads your extension.
*/
int DLLEXPORT
Hello_Init(Tcl_Interp *interp)
{
if (Tcl_InitStubs(interp, TCL_VERSION, 0) == NULL) {
return TCL_ERROR;
}
/* changed this to check for an error - GPS */
if (Tcl_PkgProvide(interp, "Hello", "1.0") == TCL_ERROR) {
return TCL_ERROR;
}
Tcl_CreateObjCommand(interp, "hello", Hello_Cmd, NULL, NULL);
return TCL_OK;
}
My command for compiling is nearly verbatim except for the last character, indicating Tcl version 8.6 rather than 8.4, and it compiles without error:
gcc -shared -o hello.dll -DUSE_TCL_STUBS -I$TCLINC -L$TCLLIB -ltclstub86
Then I created the following Tcl program:
load hello.dll Hello
puts "got here"
But when running it with tclsh get the following error:
cannot find symbol "Hello_Init"
while executing
"load ./hello.dll Hello"
(file "hello.tcl" line 1)
So I am essentially following a couple of suggestions from Donal Fellows answer here: cannot find symbol "Embeddedrcall_Init" The OP there however commented that, like me, the suggestion(s) hadn't resolved their issue. One thing that I didn't try from that answer was "You should have an exported (extern "C") function symbol in your library" -- could that be the difference maker? Shouldn't it have been in the example all along then?
At the suggestion of somebody on comp.lang.tcl I found "DLL Export Viewer" but when I run it against the DLL it reports 0 functions found :( What am I doing wrong?
Could it be an issue with MinGW/gcc on Windows, and I need to bite the bullet and do this with Visual Studio? That's overkill I'd like to avoid if possible.

Answer 1

The core of the problem is that your function Hello_Init is not ending up in the global symbol table exported by the resulting DLL. (Some linkers would put such things in as _Hello_Init instead of Hello_Init; Tcl adapts to them transparently.) The symbol must be there for Tcl's load command to work: without it, there's simply no consistent way to tell your extension code what the Tcl_Interp context handle is (which allows it to make commands, variables, etc.)
(If you'd been working with C++, one of the possible problem is a missing extern "C" whose actual meaning is to turn off name mangling. That's probably not the problem here.)
Since you are on Windows — going by the symbols in your DLL, such as EnterCriticalSection and GetLastError — the problem is probably linked to exactly how you are linking. I'm guessing that Tcl is defining your function to have __declspec(dllexport) (assuming you've not defined STATIC_BUILD, which absolutely should not be used when building a DLL) and yet that's not getting respected. Assuming you're using a modern-enough version of GCC… which you probably are.

Answer 2

I'm also going through the process of how to build tcl extensions in C and had exactly the same problem when working though this same example using tcl 8.6.
i.e. I was compiling using MinGW GCC (64-bit), and used the following:
gcc -shared -o hello.dll -DUSE_TCL_STUBS "-IC:\\ActiveTcl\\include" "-LC:\\ActiveTcl\\lib" -ltclstub86
And like the OP I got no compile error, but when loading the dll at a tclsh prompt tcl complained :
'cannot find symbol "Hello_Init"'
I can't say that I understand, but I was able to find a solution that works thanks to some trial and error, and some information on the tcl wiki here
https://wiki.tcl-lang.org/page/Building+Tcl+DLL%27s+for+Windows
In my case I had to adjust the compiler statement to the following
gcc -shared -o hello.dll hello.c "-IC:\\ActiveTcl\\include" "-LC:\\ActiveTcl\\bin" -ltcl86t
Obviously those file paths are specific to my system, but basically
I had to add an explicit reference to the .c file
I had to include the tcl86t dll library from the tcl bin directory
I had to remove the -DUSE_TCL_STUBS flag ( meaning that the references -LC:\\ActiveTcl\\lib and -ltclstub86 could also be removed)
(attempting to use the -DUSE_TCL_STUBS flag caused the compiler to complain with C:\ActiveTcl\lib/tclstub86.lib: error adding symbols: File format not recognized )
This successfully compiled a dll that I could load, and then call the hello function to print my 'Hello World' message.
Something else I stumbled over, and which wasn't immediately obvious:
reading https://www.tcl.tk/man/tcl8.6/TclCmd/load.htm, tcl epxects to find an 'init' function based on a certain naming convention.
if the C extension does not define a package name then the name of that init function will be derived from the dll filename.
This caused a few problems for me (when compiling via Eclipse IDE), as the dll name was being automatically determined from the eclipse projet name.
For example, if I recompile the same example, but call the .dll something else, eg.
gcc -shared -o helloWorldExenstion.dll hello.c "-IC:\\ActiveTcl\\include" "-LC:\\ActiveTcl\\bin" -ltcl86t
Then at tclsh prompt:
% load helloWorldExtension
cannot find symbol "Helloworldextension_Init"