I can get things to work using the following in os_defines.h: #undef _ANSI_C_SOURCE #undef _POSIX_SOURCE #undef _XOPEN_SOURCE #undef _H_STANDARDS #include <standards.h> #define __off_t off_t #define __off64_t off64_t #define __ssize_t ssize_t The macro definitions which need to be unset require tracking the AIX standards.h header which is what I was trying to avoid. David