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open.2: Merge text from openat(2)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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man2/open.2
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man2/open.2
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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
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.\" and Copyright (C) 2008 Greg Banks
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.\" and Copyright (C) 2008, 2013, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" and Copyright (C) 2006, 2008, 2013, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" O_TTYINIT. Eventually these may need to be documented. --mtk
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.\" FIXME Linux 2.6.33 has O_DSYNC, and a hidden __O_SYNC.
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.\"
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.TH OPEN 2 2014-02-20 "Linux" "Linux Programmer's Manual"
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.TH OPEN 2 2014-02-21 "Linux" "Linux Programmer's Manual"
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.SH NAME
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open, creat \- open and possibly create a file
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open, openat, creat \- open and possibly create a file
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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@ -62,7 +62,30 @@ open, creat \- open and possibly create a file
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.BI "int open(const char *" pathname ", int " flags ", mode_t " mode );
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.BI "int creat(const char *" pathname ", mode_t " mode );
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.sp
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.BI "int openat(int " dirfd ", const char *" pathname ", int " flags );
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.BI "int openat(int " dirfd ", const char *" pathname ", int " flags \
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", mode_t " mode );
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.fi
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.BR openat ():
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.PD 0
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.ad l
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.RS 4
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.TP 4
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Since glibc 2.10:
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_XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L
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.TP
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Before glibc 2.10:
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_ATFILE_SOURCE
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.RE
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.ad
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.PD
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.SH DESCRIPTION
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Given a
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.I pathname
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@ -652,7 +675,7 @@ flag is ignored.
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Otherwise the effect of
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.B O_TRUNC
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is unspecified.
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.PP
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.SS creat()
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.BR creat ()
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is equivalent to
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.BR open ()
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@ -660,8 +683,43 @@ with
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.I flags
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equal to
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.BR O_CREAT|O_WRONLY|O_TRUNC .
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.SS openat()
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The
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.BR openat ()
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system call operates in exactly the same way as
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.BR open (2),
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except for the differences described here.
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If the pathname given in
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.I pathname
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is relative, then it is interpreted relative to the directory
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referred to by the file descriptor
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.I dirfd
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(rather than relative to the current working directory of
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the calling process, as is done by
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.BR open (2)
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for a relative pathname).
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If
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.I pathname
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is relative and
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.I dirfd
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is the special value
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.BR AT_FDCWD ,
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then
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.I pathname
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is interpreted relative to the current working
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directory of the calling process (like
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.BR open (2)).
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If
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.I pathname
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is absolute, then
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.I dirfd
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is ignored.
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.SH RETURN VALUE
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.BR open ()
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.BR open (),
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.BR openat (),
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and
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.BR creat ()
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return the new file descriptor, or \-1 if an error occurred
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@ -669,6 +727,11 @@ return the new file descriptor, or \-1 if an error occurred
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.I errno
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is set appropriately).
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.SH ERRORS
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.BR open (),
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.BR openat (),
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and
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.BR creat ()
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can fail with the following errors:
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.TP
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.B EACCES
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The requested access to the file is not allowed, or search permission
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flag was specified, and an incompatible lease was held on the file
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(see
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.BR fcntl (2)).
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.PP
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The following additional errors can occur for
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.BR openat ():
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.TP
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.B EBADF
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.I dirfd
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is not a valid file descriptor.
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.TP
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.B ENOTDIR
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.I pathname
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is relative and
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.I dirfd
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is a file descriptor referring to a file other than a directory.
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.SH VERSIONS
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.BR openat ()
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was added to Linux in kernel 2.6.16;
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library support was added to glibc in version 2.4.
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.SH CONFORMING TO
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.BR open (),
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.BR creat ()
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SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
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.BR openat ():
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POSIX.1-2008.
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A similar system call exists on Solaris.
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The
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.BR O_DIRECT ,
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.BR O_NOATIME ,
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Otherwise, if the file is modified because of the
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.B O_TRUNC
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flag, its st_ctime and st_mtime fields are set to the current time.
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.\"
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.\"
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.SS Rationale for openat() and the other *at() calls
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.BR openat ()
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and other similar system calls and library functions suffixed "at"
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(i.e.,
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.BR faccessat (2),
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.BR fchmodat (2),
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.BR fchownat (2),
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.BR fstatat (2),
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.BR futimesat (2),
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.BR linkat (2),
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.BR mkdirat (2),
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.BR mknodat (2),
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.BR readlinkat (2),
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.BR renameat (2),
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.BR symlinkat (2),
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.BR unlinkat (2),
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.BR utimensat (2)
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and
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.BR mkfifoat (3))
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are supported
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for two reasons.
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Here, the explanation is in terms of the
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.BR openat ()
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call, but the rationale is the analogous for the other interfaces.
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First,
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.BR openat ()
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allows an application to avoid race conditions that could
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occur when using
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.BR open (2)
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to open files in directories other than the current working directory.
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These race conditions result from the fact that some component
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of the directory prefix given to
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.BR open (2)
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could be changed in parallel with the call to
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.BR open (2).
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Such races can be avoided by
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opening a file descriptor for the target directory,
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and then specifying that file descriptor as the
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.I dirfd
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argument of
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.BR openat ().
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Second,
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.BR openat ()
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allows the implementation of a per-thread "current working
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directory", via file descriptor(s) maintained by the application.
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(This functionality can also be obtained by tricks based
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on the use of
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.IR /proc/self/fd/ dirfd,
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but less efficiently.)
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.\"
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.\"
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.SS O_DIRECT
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.LP
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The
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@ -1173,7 +1314,6 @@ functionality.
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.BR mknod (2),
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.BR mmap (2),
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.BR mount (2),
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.BR openat (2),
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.BR read (2),
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.BR socket (2),
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.BR stat (2),
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