Some replies seem to hold openat() for superfluous. In fact, IMHO the 
traditional syscall open() should be deprecated.

In FreeBSD at least, the libc  open() does in fact use the real thing 
__sys_openat(). in /usr/src/lib/libc/sys/open.c
via the interposing table.

Some days ago, I experimented with a user space implementation of the of the 
file lookup in namei() to get the benefits of a  proc specific namespace on my 
FreeBSDsystem, making Plan9 style bind and mount  available. This involved 
changing every  slash in a path name into  openat() calls for the splltted path 
components. pkeus.de/~wb/ns.c

Btw, there is another syscall in FreeBSD, which overlaps open() and openat() , 
namely fhopen(). This seems to isolate the namespace lookup operation and the 
actual file access.


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