On 02/05/2012 02:20 PM, Richard Shaw wrote: No, nowadays, presuming unistd.h to be present is pretty safe, because it has been part of POSIX for ages and is likely present on all POSIX'sh environments. c.f.: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html Somewhat oversimplified, in practice, this means, #include <unistd.h> is safe except on Windows. Whether a particular version of <unistd.h> will provide the missing decls, is a different story. Some (ancient) OSes still could be shipping ancient or broken versions of unistd.h which do not supply the decls current POSIX would expect unistd.h to supply. However, this would not be worse than the current situation - Compilers could continue to complain about missing symbols/decls. Ralf