porting a unix programme to linux??what matters

My question is: difference between unix and linux in terms of compiler, file system, ANSI
compliance, standard liabrary issues, threading issues, operating system
calls memory issues..ie whether these things wud cause problems while
porting a unix programme to linux.?
I think answer is depend upon the flavor type.and for most of them ,answer is yes.can you help me to find out by sharing ur experience. :rolleyes:

there aren't a lot of differences between linux and a posix-compliant unix. sometimes , you 'll have simply to recompile your unix program to port it under linux.