f05698585d
On FreeBSD, this is the default behaviour. According to the spec, we may give this flag a value of zero, but I'd rather not do this. If we define it to a non-zero value, we can always change default behaviour without changing the ABI. This is very unlikely to happen, though.
451 lines
12 KiB
Groff
451 lines
12 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)open.2 8.2 (Berkeley) 11/16/93
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.\" $FreeBSD$
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.\"
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.Dd February 28, 2009
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.Dt OPEN 2
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.Os
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.Sh NAME
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.Nm open , openat
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.Nd open or create a file for reading, writing or executing
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In fcntl.h
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.Ft int
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.Fn open "const char *path" "int flags" "..."
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.Ft int
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.Fn openat "int fd" "const char *path" "int flags" "..."
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.Sh DESCRIPTION
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The file name specified by
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.Fa path
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is opened
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for either execution or reading and/or writing as specified by the
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argument
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.Fa flags
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and the file descriptor returned to the calling process.
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The
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.Fa flags
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argument may indicate the file is to be
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created if it does not exist (by specifying the
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.Dv O_CREAT
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flag).
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In this case
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.Fn open
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and
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.Fn openat
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require an additional argument
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.Fa "mode_t mode" ,
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and the file is created with mode
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.Fa mode
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as described in
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.Xr chmod 2
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and modified by the process' umask value (see
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.Xr umask 2 ) .
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.Pp
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The
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.Fn openat
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function is equivalent to the
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.Fn open
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function except in the case where the
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.Fa path
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specifies a relative path.
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In this case the file to be opened is determined relative to the directory
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associated with the file descriptor
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.Fa fd
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instead of the current working directory.
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The
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.Fa flag
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parameter and the optional fourth parameter correspond exactly to
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the parameters of
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.Fn open .
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If
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.Fn openat
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is passed the special value
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.Dv AT_FDCWD
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in the
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.Fa fd
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parameter, the current working directory is used
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and the behavior is identical to a call to
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.Fn open .
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.Pp
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The flags specified are formed by
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.Em or Ns 'ing
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the following values
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.Pp
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.Bd -literal -offset indent -compact
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O_RDONLY open for reading only
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O_WRONLY open for writing only
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O_RDWR open for reading and writing
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O_EXEC open for execute only
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O_NONBLOCK do not block on open
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O_APPEND append on each write
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O_CREAT create file if it does not exist
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O_TRUNC truncate size to 0
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O_EXCL error if create and file exists
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O_SHLOCK atomically obtain a shared lock
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O_EXLOCK atomically obtain an exclusive lock
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O_DIRECT eliminate or reduce cache effects
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O_FSYNC synchronous writes
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O_SYNC synchronous writes
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O_NOFOLLOW do not follow symlinks
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O_NOCTTY don't assign controlling terminal
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O_TTY_INIT restore default terminal attributes
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.Ed
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.Pp
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Opening a file with
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.Dv O_APPEND
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set causes each write on the file
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to be appended to the end.
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If
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.Dv O_TRUNC
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is specified and the
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file exists, the file is truncated to zero length.
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If
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.Dv O_EXCL
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is set with
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.Dv O_CREAT
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and the file already
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exists,
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.Fn open
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returns an error.
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This may be used to
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implement a simple exclusive access locking mechanism.
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If
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.Dv O_EXCL
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is set and the last component of the pathname is
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a symbolic link,
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.Fn open
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will fail even if the symbolic
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link points to a non-existent name.
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If the
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.Dv O_NONBLOCK
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flag is specified and the
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.Fn open
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system call would result
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in the process being blocked for some reason (e.g., waiting for
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carrier on a dialup line),
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.Fn open
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returns immediately.
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The descriptor remains in non-blocking mode for subsequent operations.
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.Pp
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If
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.Dv O_FSYNC
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is used in the mask, all writes will
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immediately be written to disk,
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the kernel will not cache written data
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and all writes on the descriptor will not return until
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the data to be written completes.
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.Pp
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.Dv O_SYNC
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is a synonym for
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.Dv O_FSYNC
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required by
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.Tn POSIX .
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.Pp
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If
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.Dv O_NOFOLLOW
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is used in the mask and the target file passed to
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.Fn open
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is a symbolic link then the
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.Fn open
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will fail.
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.Pp
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When opening a file, a lock with
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.Xr flock 2
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semantics can be obtained by setting
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.Dv O_SHLOCK
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for a shared lock, or
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.Dv O_EXLOCK
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for an exclusive lock.
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If creating a file with
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.Dv O_CREAT ,
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the request for the lock will never fail
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(provided that the underlying file system supports locking).
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.Pp
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.Dv O_DIRECT
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may be used to minimize or eliminate the cache effects of reading and writing.
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The system will attempt to avoid caching the data you read or write.
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If it cannot avoid caching the data,
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it will minimize the impact the data has on the cache.
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Use of this flag can drastically reduce performance if not used with care.
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.Pp
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.Dv O_NOCTTY
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may be used to ensure the OS does not assign this file as the
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controlling terminal when it opens a tty device.
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This is the default on
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.Fx ,
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but is present for
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.Tn POSIX
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compatibility.
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The
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.Fn open
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system call will not assign controlling terminals on
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.Fx .
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.Pp
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.Dv O_TTY_INIT
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may be used to ensure the OS restores the terminal attributes when
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initially opening a TTY.
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This is the default on
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.Fx ,
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but is present for
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.Tn POSIX
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compatibility.
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The initial call to
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.Fn open
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on a TTY will always restore default terminal attributes on
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.Fx .
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.Pp
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If successful,
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.Fn open
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returns a non-negative integer, termed a file descriptor.
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It returns \-1 on failure.
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The file pointer used to mark the current position within the
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file is set to the beginning of the file.
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.Pp
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When a new file is created it is given the group of the directory
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which contains it.
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.Pp
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The new descriptor is set to remain open across
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.Xr execve 2
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system calls; see
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.Xr close 2
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and
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.Xr fcntl 2 .
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.Pp
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The system imposes a limit on the number of file descriptors
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open simultaneously by one process.
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The
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.Xr getdtablesize 2
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system call returns the current system limit.
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.Sh RETURN VALUES
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If successful,
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.Fn open
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and
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.Fn openat
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return a non-negative integer, termed a file descriptor.
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They return \-1 on failure, and set
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.Va errno
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to indicate the error.
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.Sh ERRORS
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The named file is opened unless:
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.Bl -tag -width Er
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.It Bq Er ENOTDIR
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A component of the path prefix is not a directory.
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.It Bq Er ENAMETOOLONG
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A component of a pathname exceeded 255 characters,
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or an entire path name exceeded 1023 characters.
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.It Bq Er ENOENT
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.Dv O_CREAT
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is not set and the named file does not exist.
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.It Bq Er ENOENT
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A component of the path name that must exist does not exist.
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.It Bq Er EACCES
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Search permission is denied for a component of the path prefix.
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.It Bq Er EACCES
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The required permissions (for reading and/or writing)
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are denied for the given flags.
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.It Bq Er EACCES
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.Dv O_TRUNC
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is specified and write permission is denied.
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.It Bq Er EACCES
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.Dv O_CREAT
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is specified,
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the file does not exist,
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and the directory in which it is to be created
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does not permit writing.
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.It Bq Er EPERM
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.Dv O_CREAT
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is specified, the file does not exist, and the directory in which it is to be
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created has its immutable flag set, see the
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.Xr chflags 2
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manual page for more information.
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.It Bq Er EPERM
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.Dv The named file has its immutable flag set and the file is to be modified.
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.It Bq Er EPERM
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.Dv The named file has its append-only flag set, the file is to be modified, and
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.Dv O_TRUNC
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is specified or
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.Dv O_APPEND
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is not specified.
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.It Bq Er ELOOP
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Too many symbolic links were encountered in translating the pathname.
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.It Bq Er EISDIR
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The named file is a directory, and the arguments specify
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it is to be modified.
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.It Bq Er EROFS
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The named file resides on a read-only file system,
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and the file is to be modified.
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.It Bq Er EROFS
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.Dv O_CREAT
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is specified and the named file would reside on a read-only file system.
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.It Bq Er EMFILE
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The process has already reached its limit for open file descriptors.
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.It Bq Er ENFILE
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The system file table is full.
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.It Bq Er EMLINK
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.Dv O_NOFOLLOW
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was specified and the target is a symbolic link.
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.It Bq Er ENXIO
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The named file is a character special or block
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special file, and the device associated with this special file
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does not exist.
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.It Bq Er ENXIO
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.Dv O_NONBLOCK
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is set, the named file is a fifo,
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.Dv O_WRONLY
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is set, and no process has the file open for reading.
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.It Bq Er EINTR
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The
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.Fn open
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operation was interrupted by a signal.
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.It Bq Er EOPNOTSUPP
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.Dv O_SHLOCK
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or
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.Dv O_EXLOCK
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is specified but the underlying file system does not support locking.
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.It Bq Er EOPNOTSUPP
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The named file is a special file mounted through a file system that
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does not support access to it (e.g.\& NFS).
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.It Bq Er EWOULDBLOCK
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.Dv O_NONBLOCK
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and one of
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.Dv O_SHLOCK
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or
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.Dv O_EXLOCK
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is specified and the file is locked.
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.It Bq Er ENOSPC
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.Dv O_CREAT
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is specified,
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the file does not exist,
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and the directory in which the entry for the new file is being placed
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cannot be extended because there is no space left on the file
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system containing the directory.
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.It Bq Er ENOSPC
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.Dv O_CREAT
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is specified,
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the file does not exist,
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and there are no free inodes on the file system on which the
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file is being created.
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.It Bq Er EDQUOT
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.Dv O_CREAT
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is specified,
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the file does not exist,
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and the directory in which the entry for the new file
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is being placed cannot be extended because the
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user's quota of disk blocks on the file system
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containing the directory has been exhausted.
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.It Bq Er EDQUOT
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.Dv O_CREAT
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is specified,
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the file does not exist,
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and the user's quota of inodes on the file system on
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which the file is being created has been exhausted.
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.It Bq Er EIO
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An I/O error occurred while making the directory entry or
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allocating the inode for
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.Dv O_CREAT .
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.It Bq Er ETXTBSY
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The file is a pure procedure (shared text) file that is being
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executed and the
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.Fn open
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system call requests write access.
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.It Bq Er EFAULT
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The
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.Fa path
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argument
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points outside the process's allocated address space.
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.It Bq Er EEXIST
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.Dv O_CREAT
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and
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.Dv O_EXCL
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were specified and the file exists.
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.It Bq Er EOPNOTSUPP
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An attempt was made to open a socket (not currently implemented).
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.It Bq Er EINVAL
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An attempt was made to open a descriptor with an illegal combination
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of
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.Dv O_RDONLY ,
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.Dv O_WRONLY ,
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.Dv O_RDWR
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and
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.Dv O_EXEC.
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.It Bq Eq EBADF
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The
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.Fa path
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argument does not specify an absolute path and the
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.Fa fd
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argument is
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neither
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.Dv AT_FDCWD
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nor a valid file descriptor open for searching.
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.It Bq Eq ENOTDIR
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The
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.Fa path
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argument is not an absolute path and
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.Fa fd
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is neither
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.Dv AT_FDCWD
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nor a file descriptor associated with a directory.
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.El
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.Sh SEE ALSO
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.Xr chmod 2 ,
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.Xr close 2 ,
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.Xr dup 2 ,
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.Xr fexecve 2 ,
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.Xr fhopen 2 ,
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.Xr getdtablesize 2 ,
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.Xr getfh 2 ,
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.Xr lgetfh 2 ,
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.Xr lseek 2 ,
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.Xr read 2 ,
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.Xr umask 2 ,
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.Xr write 2 ,
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.Xr fopen 3
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.Sh HISTORY
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The
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.Fn open
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function appeared in
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.At v6 .
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The
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.Fn openat
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function was introduced in
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.Fx 8.0 .
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.Sh BUGS
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The Open Group Extended API Set 2 specification requires that the test
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for whether
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.Fa fd
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is searchable is based on whether
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.Fa fd
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is open for searching, not whether the underlying directory currently
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permits searches.
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The present implementation of the
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.Fa openat
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checks the current permissions of directory instead.
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