GCC Linking "undefined reference" with third part Shared Object (.so) file

Banging my head against a wall with making a quick proof of concept app which links to a third party .so file.

I've produces a simple C file which calls a simple function in the .so file, it includes the header file also supplied by the vendor. Linking produces the following errors:

/usr/local/lib/xxx.so: undefined reference to `sprintf_s'
/usr/local/lib/xxx.so: undefined reference to `sendWCmdThread'
/usr/local/lib/xxx.so: undefined reference to `pthread_create'
/usr/local/lib/xxx.so: undefined reference to `pthread_cancel'

I can handle the pthread issues (-pthread option) but I'm at a loss for the other two errors. Expecially as Google provides zero results for 'sendWCmdThread'

Anyone help?