Hi @cavaliercoder,
gcc[1] reports this warning,
libzbxsystemd.c: In function ‘SYSTEMD_UNIT_DISCOVERY’:
libzbxsystemd.c:214:35: warning: comparison between pointer and zero character constant [-Wpointer-compare]
  214 |         if(NULL != filter || '\0' != filter)
      |                                   ^~
libzbxsystemd.c:214:38: note: did you mean to dereference the pointer?
  214 |         if(NULL != filter || '\0' != filter)
      |                                      ^Why do you need to check that in parallel to the correspondent NULL?
[1] gcc (GCC) 9.3.1 20200317 (Red Hat 9.3.1-1)