On Mon, Apr 21, 2003 at 03:03:47PM +0900, Jorgen Lundman wrote:
  | 
  | Just out of curiosity,
  | 
  | If I write a piece of code, run as root, or setuid root, that does:
  | 
  |    unlink("testfile");
  |    seteuid(10000);
  |    F=fopen("testfile", "w");
  | 
  | It creates the file as the user I want. Now I also want to do the same with 
  | groups, so we add:
  | 
  |    unlink("testfile");
  |    setegid(10001);
  |    seteuid(10000);
  |    F=fopen("testfile", "w");
  | 
  | It creates the file as 10000, but the group is "wheel". Or to be precise, 
  | it is the same group as "." of the directory.

That's standard "BSD" file creation semantics, no matter what group
you're a part of.

(System V file creation semantics use the "primary" group of the user
as the group of the file, BSD uses the group of the parent directory).