here is my simple test:
where is my mistake??

#include "ruby.h"
#include "stdio.h"
static VALUE
tests(){
   char *s1="a ";
    char *s2=" b";
  char *buf;
    sprintf(buf,"%s after %s",s1,s2);
    printf(buf);
  return Qnil;
}
void Init_hello(){
 rb_define_global_function("tests",tests,0);
}

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