A quick update to this topic. As I alluded to in my original post, the real issue is that the _syscall0 macro is not defined. Actually, none of the _syscallx (where x goes from 0 to 6) macros are defined in /usr/include/linux/unistd.h as they are on a CentOS 4.4 system. I grep'ed in all of /usr/include and couldn't find any references to these macros. Have they been deprecated or is this an oversight? At least one man page (gettid) still references _syscall0... Alfred