Why is open declared in fcntl.h, while close is declared in unistd.h?

Question

In POSIX, why is the open system call declared in fcntl.h, while the close system call is declared in unistd.h? I presume this is a historical artifact, but this pair strikes me as especially odd since you often need to close after open.

Answer 1

As previously pointed out, sockets and pipes also get passed to close. But probably the original reason is that open needs flags like O_RDONLY that are defined in fcntl.h, so you might as well put the prototype in that file.