This function is missing on some platforms:
AIX 4.3.2, IRIX 6.5, OSF/1 5.1, Solaris 9, mingw.
@item
+This function is declared but not defined on some platforms:
+IRIX 6.5.30.
+@item
This function returns 0 for all possible arguments on some platforms:
Linux libc5.
@end itemize