--- a/lib/hash-pjw.h
+++ b/lib/hash-pjw.h
@@ -18,4 +18,8 @@
 
 #include <stddef.h>
 
-size_t hash_pjw (void const *x, size_t tablesize);
+/* Compute a hash code for a NUL-terminated string starting at X,
+   and return the hash code modulo TABLESIZE.
+   The result is platform dependent: it depends on the size of the 'size_t'
+   type and on the signedness of the 'char' type.  */
+extern size_t hash_pjw (void const *x, size_t tablesize);