688 lines
17 KiB
Groff
688 lines
17 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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.\" 3. 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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.\"
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.Dd May 29, 2023
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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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For
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.Fn openat
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and relative
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.Fa path ,
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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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When
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.Fn openat
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is called with an absolute
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.Fa path ,
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it ignores the
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.Fa fd
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argument.
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.Pp
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In
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.Xr capsicum 4
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capability mode,
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.Fn open
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is not permitted.
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The
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.Fa path
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argument to
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.Fn openat
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must be strictly relative to a file descriptor
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.Fa fd .
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.Fa path
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must not be an absolute path and must not contain ".." components
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which cause the path resolution to escape the directory hierarchy
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starting at
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.Fa fd .
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Additionally, no symbolic link in
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.Fa path
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may target absolute path or contain escaping ".." components.
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.Fa fd
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must not be
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.Dv AT_FDCWD .
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.Pp
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If the
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.Dv vfs.lookup_cap_dotdot
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.Xr sysctl 3
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MIB is set to zero, ".." components in the paths,
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used in capability mode,
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are completely disabled.
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If the
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.Dv vfs.lookup_cap_dotdot_nonlocal
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MIB is set to zero, ".." is not allowed if found on non-local filesystem.
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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_SEARCH open for search only, an alias for O_EXEC
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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 (historical synonym for O_SYNC)
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O_SYNC synchronous writes
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O_DSYNC synchronous data writes
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O_NOFOLLOW do not follow symlinks
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O_NOCTTY ignored
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O_TTY_INIT ignored
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O_DIRECTORY error if file is not a directory
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O_CLOEXEC set FD_CLOEXEC upon open
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O_VERIFY verify the contents of the file
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O_RESOLVE_BENEATH path resolution must not cross the fd directory
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O_PATH record only the target path in the opened descriptor
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O_EMPTY_PATH openat, open file referenced by fd if path is empty
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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_SYNC
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is used in the mask, all writes will
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immediately and synchronously be written to disk.
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.Dv O_FSYNC
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is an historical synonym for
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.Dv O_SYNC .
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.Pp
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If
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.Dv O_DSYNC
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is used in the mask, all data and metadata required to read the data will be
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synchronously written to disk, but changes to metadata such as file access and
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modification timestamps may be written later.
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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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.Dv O_DIRECTORY
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may be used to ensure the resulting file descriptor refers to a
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directory.
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This flag can be used to prevent applications with elevated privileges
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from opening files which are even unsafe to open with
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.Dv O_RDONLY ,
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such as device nodes.
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.Pp
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.Dv O_CLOEXEC
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may be used to set
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.Dv FD_CLOEXEC
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flag for the newly returned file descriptor.
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.Pp
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.Dv O_VERIFY
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may be used to indicate to the kernel that the contents of the file should
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be verified before allowing the open to proceed.
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The details of what
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.Dq verified
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means is implementation specific.
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The run-time linker (rtld) uses this flag to ensure shared objects have
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been verified before operating on them.
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.Pp
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.Dv O_RESOLVE_BENEATH
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returns
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.Er ENOTCAPABLE
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if any intermediate component of the specified relative path does not
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reside in the directory hierarchy beneath the starting directory.
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Absolute paths or even the temporal escape from beneath of the starting
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directory is not allowed.
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.Pp
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When
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.Fa fd
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is opened with
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.Dv O_SEARCH ,
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execute permissions are checked at open time.
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The
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.Fa fd
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may not be used for any read operations like
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.Xr getdirentries 2 .
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The primary use for this descriptor will be as the lookup descriptor for the
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.Fn *at
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family of functions.
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.Pp
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.Dv O_PATH
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returns a file descriptor that can be used as a directory file descriptor for
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.Xr openat 2
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and other system calls taking a file descriptor argument, like
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.Xr fstatat 2
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and others.
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The other functionality of the returned file descriptor is limited to
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the descriptor-level operations.
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It can be used for
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.Bl -tag -width readlinkat(2) -offset indent -compact
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.It Xr fcntl 2
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but advisory locking is not allowed
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.It Xr dup 2
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.It Xr close 2
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.It Xr fstat 2
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.It Xr fexecve 2
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.It Dv SCM_RIGHTS
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can be passed over a
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.Xr unix 4
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socket using a
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.Dv SCM_RIGHTS
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message
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.It Xr kqueue 2
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using for
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.Dv EVFILT_VNODE
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.It Xr readlinkat 2
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.It Xr __acl_get_fd 2 , Xr __acl_aclcheck_fd 2
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.El
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But operations like
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.Xr read 2 ,
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.Xr ftruncate 2 ,
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and any other that operate on file and not on file descriptor (except
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.Xr fstat 2 ),
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are not allowed.
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.Pp
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A file descriptor created with the
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.Dv O_PATH
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flag can be opened into normal (operable) file descriptor by
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specifying it as the
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.Fa fd
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argument to
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.Fn openat
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with empty
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.Fa path
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and flag
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.Dv O_EMPTY_PATH .
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Such an open behaves as if the current path of the file referenced by
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.Fa fd
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is passed, except that the path walk permissions are not checked.
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See also the description of
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.Dv AT_EMPTY_PATH
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flag for
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.Xr fstatat 2
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and related syscalls.
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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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If a sleeping open of a device node from
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.Xr devfs 5
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is interrupted by a signal, the call always fails with
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.Er EINTR ,
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even if the
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.Dv SA_RESTART
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flag is set for the signal.
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A sleeping open of a fifo (see
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.Xr mkfifo 2 )
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is restarted as normal.
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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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Unless
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.Dv O_CLOEXEC
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flag was specified,
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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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.Xr fcntl 2
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and
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.Dv O_CLOEXEC
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description.
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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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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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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 EISDIR
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The named file is a directory, and the flags specified
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.Dv O_CREAT
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without
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.Dv O_DIRECTORY .
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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 EINTEGRITY
|
|
Corrupted data was detected while reading from the file system.
|
|
.It Bq Er ETXTBSY
|
|
The file is a pure procedure (shared text) file that is being
|
|
executed and the
|
|
.Fn open
|
|
system call requests write access.
|
|
.It Bq Er EFAULT
|
|
The
|
|
.Fa path
|
|
argument
|
|
points outside the process's allocated address space.
|
|
.It Bq Er EEXIST
|
|
.Dv O_CREAT
|
|
and
|
|
.Dv O_EXCL
|
|
were specified and the file exists.
|
|
.It Bq Er EOPNOTSUPP
|
|
An attempt was made to open a socket (not currently implemented).
|
|
.It Bq Er EINVAL
|
|
An attempt was made to open a descriptor with an illegal combination
|
|
of
|
|
.Dv O_RDONLY ,
|
|
.Dv O_WRONLY ,
|
|
or
|
|
.Dv O_RDWR ,
|
|
and
|
|
.Dv O_EXEC
|
|
or
|
|
.Dv O_SEARCH .
|
|
.It Bq Er EBADF
|
|
The
|
|
.Fa path
|
|
argument does not specify an absolute path and the
|
|
.Fa fd
|
|
argument is
|
|
neither
|
|
.Dv AT_FDCWD
|
|
nor a valid file descriptor open for searching.
|
|
.It Bq Er ENOTDIR
|
|
The
|
|
.Fa path
|
|
argument is not an absolute path and
|
|
.Fa fd
|
|
is neither
|
|
.Dv AT_FDCWD
|
|
nor a file descriptor associated with a directory.
|
|
.It Bq Er ENOTDIR
|
|
.Dv O_DIRECTORY
|
|
is specified and the file is not a directory.
|
|
.It Bq Er ECAPMODE
|
|
.Dv AT_FDCWD
|
|
is specified and the process is in capability mode.
|
|
.It Bq Er ECAPMODE
|
|
.Fn open
|
|
was called and the process is in capability mode.
|
|
.It Bq Er ENOTCAPABLE
|
|
.Fa path
|
|
is an absolute path and the process is in capability mode.
|
|
.It Bq Er ENOTCAPABLE
|
|
.Fa path
|
|
is an absolute path and
|
|
.Dv O_RESOLVE_BENEATH
|
|
is specified.
|
|
.It Bq Er ENOTCAPABLE
|
|
.Fa path
|
|
contains a ".." component leading to a directory outside
|
|
of the directory hierarchy specified by
|
|
.Fa fd
|
|
and the process is in capability mode.
|
|
.It Bq Er ENOTCAPABLE
|
|
.Fa path
|
|
contains a ".." component leading to a directory outside
|
|
of the directory hierarchy specified by
|
|
.Fa fd
|
|
and
|
|
.Dv O_RESOLVE_BENEATH
|
|
is specified.
|
|
.It Bq Er ENOTCAPABLE
|
|
.Fa path
|
|
contains a ".." component, the
|
|
.Dv vfs.lookup_cap_dotdot
|
|
.Xr sysctl 3
|
|
is set, and the process is in capability mode.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr chmod 2 ,
|
|
.Xr close 2 ,
|
|
.Xr dup 2 ,
|
|
.Xr fexecve 2 ,
|
|
.Xr fhopen 2 ,
|
|
.Xr getdtablesize 2 ,
|
|
.Xr getfh 2 ,
|
|
.Xr lgetfh 2 ,
|
|
.Xr lseek 2 ,
|
|
.Xr read 2 ,
|
|
.Xr umask 2 ,
|
|
.Xr write 2 ,
|
|
.Xr fopen 3 ,
|
|
.Xr capsicum 4
|
|
.Sh STANDARDS
|
|
These functions are specified by
|
|
.St -p1003.1-2008 .
|
|
.Fx
|
|
sets
|
|
.Va errno
|
|
to
|
|
.Er EMLINK instead of
|
|
.Er ELOOP
|
|
as specified by
|
|
.Tn POSIX
|
|
when
|
|
.Dv O_NOFOLLOW
|
|
is set in flags and the final component of pathname is a symbolic link
|
|
to distinguish it from the case of too many symbolic link traversals
|
|
in one of its non-final components.
|
|
.Sh HISTORY
|
|
The
|
|
.Fn open
|
|
function appeared in
|
|
.At v1 .
|
|
The
|
|
.Fn openat
|
|
function was introduced in
|
|
.Fx 8.0 .
|
|
.Dv O_DSYNC
|
|
appeared in 13.0.
|
|
.Sh BUGS
|
|
The Open Group Extended API Set 2 specification requires that the test
|
|
for whether
|
|
.Fa fd
|
|
is searchable is based on whether
|
|
.Fa fd
|
|
is open for searching, not whether the underlying directory currently
|
|
permits searches.
|
|
The present implementation of the
|
|
.Fa openat
|
|
checks the current permissions of directory instead.
|
|
.Pp
|
|
The
|
|
.Fa mode
|
|
argument is variadic and may result in different calling conventions
|
|
than might otherwise be expected.
|