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From Unable to build OpenPLC emulator. C warning: no return statement in function on Stack Overflow:

I encountered a problem trying to run build.sh for OpenPCL master. I get three warnings and the build fails:

openplc.cpp: in function 'void* modbusThread
openplc.cpp:52:1: warning: no return statement in function returning non-void [-Wreturn-type]
persistent_storage.cpp: in function 'int readPersistentStorage()':
persistent_storage.cpp:107:1 warning: control reaches end of non-void function [-Wreturn-type]
server.cpp: in function 'void* handleConnections(void*)':
server.cpp:138:1 warning: no return statement in function returning non-void [-Wreturn-type]

The OP did not state the failures.

Looking at some of the sources, like openplc.cpp:52:

void *modbusThread(void *arg)
{
	startServer(502);
}

It may be a good idea to clear the warnings so inexperienced folks don't have to attempt it. It looks like startServer does not return a value:

$ grep -IR startServer
core/server.cpp:void startServer(int port)
core/openplc.cpp:	startServer(502);
core/ladder.h:void startServer(int port);

So maybe change modbusThread to something like:

void *modbusThread(void *arg)
{
	startServer(502);
	return nullptr;
}