TCL extensions that depends on each other - c

Question

My question is a little bit similar to this but it is about TCL extensions.
I am using C on Linux (gcc) and I have a package with three modules A, B, and C. Module A contains functions and also define (not only declare) global variables. I compile and link module A into a dynamic library (libA.so).
Now, I want that B and C are TCL extensions. Both are using functions and global variables from A, while C is also using functions from B. I have made B and C shared library (B.so and C.so) but without using "-Wl -soname". I made B.so depends on A.so, while C.so is without user dependencies. Although this is strange, bot extensions loaded and worked properly. Here is, what I have (A=libbiddy.so, B=bddscout.so, C=bddscoutIFIP.so):
meolic#meolic:/usr/lib/bddscout$ ldd *.so
bddscout.so:
linux-gate.so.1 => (0x00177000)
libbiddy.so.1 => /usr/lib/libbiddy.so.1 (0x00eca000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x00342000)
/lib/ld-linux.so.2 (0x0061f000)
bddscoutIFIP.so:
linux-gate.so.1 => (0x00fc2000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x00110000)
/lib/ld-linux.so.2 (0x00c75000)
meolic#meolic:/usr/lib/bddscout$ wish
% puts $tcl_patchLevel
8.5.8
% load ./bddscout.so
% load ./bddscoutIFIP.so
% info loaded
{./bddscoutIFIP.so Bddscoutifip} {./bddscout.so Bddscout} {{} Tk}
The problem is, that exactly the same package is not working everywhere. On a new computer extension C.so does not load.
meolic#altair:/usr/lib/bddscout$ ldd *.so
bddscout.so:
linux-gate.so.1 => (0xb76ef000)
libbiddy.so.1 => /usr/lib/libbiddy.so.1 (0xb76c9000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb754d000)
/lib/ld-linux.so.2 (0xb76f0000)
bddscoutIFIP.so:
linux-gate.so.1 => (0xb7780000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb75e8000)
/lib/ld-linux.so.2 (0xb7781000)
meolic#altair:/usr/lib/bddscout$ wish
% puts $tcl_patchLevel
8.5.10
% load ./bddscout.so
% load ./bddscoutIFIP.so
couldn't load file "./bddscoutIFIP.so": ./bddscoutIFIP.so: undefined symbol: biddy_termFalse
The reported undefined symbol is one of global variables from A. Question1: is my approach correct as it works on some systems? Question2: why it does not work on a new system?

Answer 1

Tcl's load command uses dlopen() under the covers (on Linux; it's different on other platforms of course) and it uses it with the RTLD_LOCAL flag; symbols in the library are not exported to the rest of the application. Because of this, unbound symbols in one dynamically-loaded library will not resolve against another one; this boosts isolation, but forces you to do more work to make things all function correctly where you want such a dependency to actually exist.
Your options are:
If libscoutIFIP.so depends on libbiddy.so's symbols, tell this to the linker when building the library and the dynamic linker engine will sort it all out so that the dependency doesn't get loaded multiple times. That is, if a library depends on a symbol in another library, it should explicitly list that library as a dependency.
Arrange for libbiddy.so to export its symbols as a stub table (i.e., structure of pointers to functions/variables) through Tcl's package API (Tcl_PkgProvide()). Then when libscoutIFIP.so does Tcl_PkgRequireEx() on the biddy package, it will get a pointer to that stub table and can use the references within it instead of doing direct linking. This is how Tcl's stub mechanism works, and its awesome and portable and lets you do fairly complex API version management (if necessary). It's a bit more work to set up though. The Tcler's Wiki goes into quite a lot more depth on this topic.
If option 1 works for you, go with that; for Linux-specific code that should be just fine as the system dynamic linker isn't desperately dense (unlike the situation on Windows).
[EDIT]: Note that older versions of Tcl (up to 8.5.9) used RTLD_GLOBAL instead. It seems that this change should have been labelled ***POTENTIAL INCOMPATIBILITY*** in the release notes and trailed more widely. Apologies on behalf of the Tcl developers.