[PATCH RFC] fcntl.2: update manpage with verbiage about file-private locks

[Jeff Layton]

On Tue, 15 Apr 2014 22:08:50 +0200
"Michael Kerrisk (man-pages)" <mtk.manpages at gmail.com> wrote:

> Hello Jeff,
> 
> On Thu, Jan 9, 2014 at 3:23 PM, Jeff Layton <jlayton at redhat.com> wrote:
> > Please don't merge this yet, as the kernel patches are still a work in
> > progress...
> 
> Now that this has hit mainline, is this man page patch still current?
> 
> Cheers,
> 
> Michael
> 

No, it needs a bit of a revision. I'm sorting through the glibc patches
now, and will plan to send a respin of this once that's complete.

Thanks!


> > Signed-off-by: Jeff Layton <jlayton at redhat.com>
> > ---
> >  man2/fcntl.2 | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 94 insertions(+), 3 deletions(-)
> >
> > diff --git a/man2/fcntl.2 b/man2/fcntl.2
> > index 72dcd7b..74c67b6 100644
> > --- a/man2/fcntl.2
> > +++ b/man2/fcntl.2
> > @@ -208,7 +208,7 @@ struct flock {
> >      off_t l_start;   /* Starting offset for lock */
> >      off_t l_len;     /* Number of bytes to lock */
> >      pid_t l_pid;     /* PID of process blocking our lock
> > -                        (F_GETLK only) */
> > +                        (F_GETLK and F_GETLKP only) */
> >      ...
> >  };
> >  .fi
> > @@ -344,9 +344,13 @@ returns details about one of these locks in the
> >  .IR l_type ", " l_whence ", " l_start ", and " l_len
> >  fields of
> >  .I lock
> > -and sets
> > +.
> > +If the conflicting lock is a traditional POSIX lock, then the
> >  .I l_pid
> > -to be the PID of the process holding that lock.
> > +will be set to the PID of the process holding that lock. If the
> > +conflicting lock is a file-private lock, then the
> > +.I l_pid
> > +will be set to -1.
> >  .P
> >  In order to place a read lock,
> >  .I fd
> > @@ -386,6 +390,93 @@ should be avoided; use
> >  and
> >  .BR write (2)
> >  instead.
> > +.SS File-private locking
> > +(Currently non-POSIX, but being proposed)
> > +.PP
> > +.BR F_GETLKP ", " F_SETLKP " and  " F_SETLKPW
> > +are used to acquire, release and test file-private record locks. These
> > +are byte-range locks that work identically to the traditional advisory
> > +record locks described above, but are associated with the open file on
> > +which they were acquired rather than the process, much like locks
> > +acquired with
> > +.BR flock (2)
> > +.
> > +.PP
> > +Unlike traditional advisory record locks, these locks are inherited
> > +across
> > +.BR fork (2) ", " dup (2) " and " dup2 (2)
> > +and are only released on the last close of the open file instead of being
> > +released on any close of the file.
> > +.PP
> > +File-private locks always conflict with traditional record locks, even
> > +when they are acquired by the same process on the same file descriptor.
> > +They only conflict with each other when they are acquired on different
> > +open file descriptors.
> > +.TP
> > +.BR F_SETLKP " (\fIstruct flock *\fP)"
> > +Acquire a lock (when
> > +.I l_type
> > +is
> > +.B F_RDLCK
> > +or
> > +.BR F_WRLCK )
> > +or release a lock (when
> > +.I l_type
> > +is
> > +.BR F_UNLCK )
> > +on the bytes specified by the
> > +.IR l_whence ", " l_start ", and " l_len
> > +fields of
> > +.IR lock .
> > +If a conflicting lock is held by another process,
> > +this call returns \-1 and sets
> > +.I errno
> > +to
> > +.B EACCES
> > +or
> > +.BR EAGAIN .
> > +.TP
> > +.BR F_SETLKPW " (\fIstruct flock *\fP)"
> > +As for
> > +.BR F_SETLKP ,
> > +but if a conflicting lock is held on the file, then wait for that
> > +lock to be released.
> > +If a signal is caught while waiting, then the call is interrupted
> > +and (after the signal handler has returned)
> > +returns immediately (with return value \-1 and
> > +.I errno
> > +set to
> > +.BR EINTR ;
> > +see
> > +.BR signal (7)).
> > +.TP
> > +.BR F_GETLKP " (\fIstruct flock *\fP)"
> > +On input to this call,
> > +.I lock
> > +describes a lock we would like to place on the file.
> > +If the lock could be placed,
> > +.BR fcntl ()
> > +does not actually place it, but returns
> > +.B F_UNLCK
> > +in the
> > +.I l_type
> > +field of
> > +.I lock
> > +and leaves the other fields of the structure unchanged.
> > +If one or more incompatible locks would prevent
> > +this lock being placed, then
> > +.BR fcntl ()
> > +returns details about one of these locks in the
> > +.IR l_type ", " l_whence ", " l_start ", and " l_len
> > +fields of
> > +.I lock
> > +.
> > +If the conflicting lock is a traditional POSIX lock, then the
> > +.I l_pid
> > +will be set to the PID of the process holding that lock. If the
> > +conflicting lock is a file-private lock, then the
> > +.I l_pid
> > +will be set to -1.
> >  .SS Mandatory locking
> >  (Non-POSIX.)
> >  The above record locks may be either advisory or mandatory,
> > --
> > 1.8.4.2
> >
> > --
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> 
> 


-- 
Jeff Layton <jlayton at redhat.com>