C++ plugin thread crashes if main program doesn't have a thread - c++

Question

I have a main program that uses dlopen to load a library that is written in C++ and call a routine.
handle = dlopen(LIBNAME,RTLD_LAZY);
if(handle == NULL){
fprintf(stderr,"Could not open library %s: %s\n",LIBNAME,dlerror());
exit(1);
}
...
startptr = (int (*)(char*,char*,char*))dlsym(handle,"PluginStart");
if (startptr == NULL){
fprintf(stderr,"Could not find start\n");
exit(1);
}
...
int res = (*startptr)(name,sig,descr);
The start code in the plugin creates an instance of a class,
and spins of a thread to do the startup so that the start code can return
to the main program. The thread runs a loop so that the plugin is running independently from the main program.
Manager(){
std::thread mgrThread(&Manager::threadFunc,this);
mgrThread.detach();
};
void Manager::threadFunc(void)
{
//setup stuff that ends in a loop
}
This worked fine when the main program was written in C. I decided to add a map
to the main program so I changed to C++, and the creation of the thread in the plugin now segfaults.
Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) where
#0 0x0000000000000000 in ?? ()
#1 0x00007ffff7e740a9 in std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) () from /lib/x86_64-linux-gnu/libstdc++.so.6
#2 0x00007ffff7fac0c7 in std::thread::thread<void (Manager::*)(), Manager*, void> (this=0x7fffffffddc0, __f=
#0x7fffffffddd0: (void (Manager::*)(class Manager * const)) 0x7ffff7face36 <Manager::threadFunc()>)
at /usr/include/c++/9/thread:130
#3 0x00007ffff7fabf76 in Manager::Manager (this=0x555555580500) at Manager.h:13
#4 0x00007ffff7fabc21 in PluginStart (outName=0x555555561080 <name> "", outSig=0x555555561480 <sig> "",
outDesc=0x555555561880 <descr> "") at testPlugin.cpp:35
#5 0x000055555555b678 in main (argc=1, argv=0x7fffffffdf68) at main.cpp:122
Creating a thread in the main program that does a single printf and exits fixes the failure.
void tstFunc(void){
printf("xyzzy\n");
}
...
std::thread tstThread(tstFunc);
tstThread.detach();
...
So apparently the pthread library is can't dynamically load correctly from the plugin if the C++ library is loaded without pthread in the main program?
I was using -Wl,--dynamic-list=./mexports.txt in the link of the main program to make the callbacks the plugin uses visible to the plugin. I changed it to -Wl,--export-dynamic to see if a more generous symbol export worked, but it only works if I create and use a thread in the main program (for both link options).
Any help would be appreciated.
Ubuntu 20.04 LTS, g++ 9.4.0, no c++ std specified in the makefile.