From the Solaris getpwnam(3C) manpage:

       Solaris 2.4 and earlier releases provided definitions of the getpw-
       nam_r() and getpwuid_r() functions as specified in POSIX.1c Draft 6.
       The final POSIX.1c standard changed the interface for these functions.
       Support for the Draft 6 interface is provided for compatibility only
       and might not be supported in future releases. New applications and
       libraries should use the standard-conforming interface.

The getgrnam(3C) manpage contains a similar section.

The easiest way to use the standard-conforming interfaces is to #define _POSIX_PTHREAD_SEMANTICS before #including <pwd.h> or <gid.h>