When make unloads and then loads a shared object make supplies an  uninitialized floc to the plugin setup routine.

$ cat makefile
load hello.so
all:; :
hello.so: force; :
force:;
.PHONY: force
$ cat hello.c
#include <assert.h>
#include <gnumake.h>
int plugin_is_GPL_compatible;


int hello_gmk_setup (const gmk_floc *floc)
{
  assert (floc->filenm);
  return 1;
}
$ ~/src/gmake/make/m64/make
hello plugin loaded from makefile:1
:
make: hello.c:9: hello_gmk_setup: Assertion `floc->filenm'
failed.
Aborted