illumos gate

There's an extra description of O_SYNC in the middle of the description for
O_CREAT in open(2):

[ ... ] Thus, all bits set in the process's file mode creation mask
(see umask(2)) are correspondingly cleared in the file's per- mission
mask. The "save text image after execution bit" of the mode is cleared
(see chmod(2)). O_SYNC Write I/O operations on the file descriptor
complete as defined by synchronized I/O file integrity
completion (see fcntl.h(3HEAD) definition of O_SYNC.) When bits
other than the file permission bits are set, the effect is
unspecified. [ ... ]

(emphasis mine)