What does -D_XOPEN_SOURCE do/mean?

Question

I recently encountered some code that gcc would not compile without this arg. I checked the gcc man page, but did not find this specific option. I did find XOPEN_SOURC

Answer 1

For some unknown reason, Mac OS/X (XCode) requires 600 to define strdup(), even though it's in the 1995 specification. Mozilla and others have run into this...