"Renewc"
  The XSUB-writer's interface to the C "realloc" function, with cast.

  Memory obtained by this should ONLY be freed with "Safefree".

    void  Renewc(void* ptr, int nitems, type, cast)
[download]

So actually not how you commented it. The Renewc function modifies the pointer rather than returning anything.

Basically the OP's new version of the code works because sizeof(int) < 2*sizeof(char*)

The renewc is unneeded though, it could just be

message= (EdjeMessageStringSet*) safemalloc(
  sizeof(EdjeMessageStringSet) + (count-1)*sizeof(char*)
)
[download]

For this code, the Newx type casting & sizing behavior is simply the wrong API to try to use.