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Message-ID: <CAKgNAkgykZJV59WJXW9c9gVwXhn=WNYKxo_W1GM7ocHEEFqEXg@mail.gmail.com> Date: Mon, 30 Apr 2012 09:39:11 +1200 From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> To: Mike Frysinger <vapier@...too.org> Cc: linux-man@...r.kernel.org, aneesh.kumar@...ux.vnet.ibm.com, viro@...iv.linux.org.uk, Richard Weinberger <richard@....at>, lkml <linux-kernel@...r.kernel.org>, Michael Kerrisk <mtk.manpages@...il.com> Subject: Re: [PATCH] open(2): document O_PATH Hi Mike, [Al, Aneesh, there is a question for you below] On Sun, Apr 29, 2012 at 3:25 PM, Mike Frysinger <vapier@...too.org> wrote: > Signed-off-by: Mike Frysinger <vapier@...too.org> > --- > man2/open.2 | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/man2/open.2 b/man2/open.2 > index a655fae..61689cf 100644 > --- a/man2/open.2 > +++ b/man2/open.2 > @@ -47,7 +47,6 @@ > .\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and > .\" O_TTYINIT. Eventually these may need to be documented. --mtk > .\" FIXME Linux 2.6.33 has O_DSYNC, and a hidden __O_SYNC. > -.\" FIXME: Linux 2.6.39 added O_PATH > .\" > .TH OPEN 2 2012-02-27 "Linux" "Linux Programmer's Manual" > .SH NAME > @@ -428,6 +427,21 @@ For a discussion of the effect of > in conjunction with mandatory file locks and with file leases, see > .BR fcntl (2). > .TP > +.B O_PATH > +The path is opened for accessing its file attributes only. Since the file > +itself is not opened, most operations (such as > +.BR read (2) > +or > +.BR write (2)) > +will return > +.BR EBADF . > +You may however use functions that operate on the file descriptor itself > +such as > +.BR close (2), > +functions that duplicate file descriptors, and as the dirfd argument with > +all the *at style of functions (e.g. > +.BR openat (2)). > +.TP > .B O_SYNC > The file is opened for synchronous I/O. > Any Thanks for the prod. Adding this has been on my list for a while. There's actually quite a lot more to say, and I've written the patch below. Could you check it over. Al, Aneesh, there is one question for you in a FIXME below. Could you take a look please? Thanks, Michael --- a/man2/open.2 +++ b/man2/open.2 @@ -428,6 +427,66 @@ For a discussion of the effect of in conjunction with mandatory file locks and with file leases, see .BR fcntl (2). .TP +.BR O_PATH " (since Linux 2.6.39)" +.\" commit 1abf0c718f15a56a0a435588d1b104c7a37dc9bd +Obtain a file descriptor that is used only for fetching file attributes. +The file itself is not opened, and most file operations (e.g., +.BR read (2), +.BR write (2)) +fail with the error +.BR EBADF . +The following operations +.I can +be performed on the resulting file descriptor: +.RS +.IP * 3 +Closing the file descriptor +.RB ( close (2)). +.\" FIXME Commit 1abf0c718f15a56a0a435588d1b104c7a37dc9bdcw +.\" message says that closing the file descriptor does not affect +.\" POSIX locks or dnotify. +.\" However, my testing shows that it DOES affect dnotify (and inotify). +.\" Does close() affect POSIX locks? +.IP * +Duplicating the file descriptor +.RB ( dup (2), +.BR fcntl (2) +.BR F_DUPFD , +etc.). +.IP * +Getting and setting file descriptor flags +.RB ( fcntl (2) +.BR F_GETFD +and +.BR F_SETFD ). +.IP * +Passing the file descriptor as the +.IR dirfd +argument of +.BR openat (2) +and the other "*at()" system calls. +.IP * +Passing the file descriptor to another process via a UNIX domain socket +(see +.BR SCM_RIGHTS +in +.BR unix (7)). +.RE +.IP +When +.B O_PATH +is specified in +.IR flags , +flag bits other than +.BR O_DIRECTORY +and +.BR O_NOFOLLOW +are ignored. +If the +.BR O_NOFOLLOW +flag is specified, +then the call returns a file descriptor referring to the symbolic link. +.TP .B O_SYNC The file is opened for synchronous I/O. Any @@ -612,8 +671,9 @@ SVr4, 4.3BSD, POSIX.1-2001. The .BR O_DIRECTORY , .BR O_NOATIME , +.BR O_NOFOLLOW , and -.B O_NOFOLLOW +.BR O_PATH flags are Linux-specific, and one may need to define .B _GNU_SOURCE (before including -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface"; http://man7.org/tlpi/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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