HelenOS sources

O_EXCL            128 uspace/lib/posix/src/fcntl.c 		if (posix_flags & O_EXCL)
O_EXCL             60 uspace/lib/posix/test/stdlib.c 	file = open(p, O_CREAT | O_EXCL | O_RDWR, 0600);
O_EXCL             68 uspace/lib/posix/test/unistd.c 	file = open(name, O_CREAT | O_EXCL | O_RDWR, 0600);
HelenOS homepage, sources at GitHub