Hi, We are porting come code to Linux using gcc... gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-49) Some of our code includes <sys/msg.h> for the definition of 'msgbuf' but it doesn't compile unless you use D_GNU_SOURCE. I thought that this was SVID functionality? Should we be using <linux/msg.h>? When should you use <linux/...h> as opposed to <sys/...h>? Cheers, Barry Send instant messages to your online friends http://uk.messenger.yahoo.com