Solved: I started from a fresh project and everything worked. I had a Function Block in the project at one point and deleted it. I think Codesys doesn't fully clean up on deletes.
I'm installed the CODESYS Control for Linux SL package and am trying to use the Extension API to call a simple C function. When I get to the step to run "make all" on the Linux machine, I get the error: build interface header: out/CmpFirstTestItf.h from CmpFirstTestItf.m4 ERROR: This functionname does not contain _cext (this is mandatory, see help for details): DEF_API(void',CDECL',external_struct__fb_init',(external_struct_fb_init_struct *p)',1,0x1F77E075,0x00000001) Why does "Generate Runtime System...