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.\" @(#)open.2 8.2 (Berkeley) 11/16/93
.\"
.Dd November 10, 2010
.Dt OPEN 2
.Os BSD 4
.Sh NAME
.Nm open , openat
.Nd open or create a file for reading or writing
.Sh SYNOPSIS
.\" OH??? .Fd #include <sys/stat.h>
.Fd #include <fcntl.h>
.Ft int
.Fo open
.Fa "const char *path"
.Fa "int oflag"
.Fa "..."
.Fc
.Ft int
.Fn openat "int fd" "const char *path" "int oflag" "..."
.Sh DESCRIPTION
The file name specified by
.Fa path
is opened
for reading and/or writing,
as specified by the argument
.Fa oflag ;
the file descriptor is returned to the calling process.
.Pp
The
.Fa oflag
argument may indicate that the file is to be
created if it does not exist (by specifying the
.Dv O_CREAT
flag). In this case,
.Fn open
and
.Fn openat
require an additional argument
.Fa "mode_t mode" ;
the file is created with mode
.Fa mode
as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.Pp
The
.Fn openat
function is equivalent to the
.Fn open
function except in the case where the
.Fa path
specifies a relative path.
In this case the file to be opened is determined relative to the directory
associated with the file descriptor
.Fa fd
instead of the current working directory.
The
.Fa oflag
argument and the optional fourth argument correspond exactly to
the arguments for
.Fn open .
If
.Fn openat
is passed the special value
.Dv AT_FDCWD
in the
.Fa fd
argument, the current working directory is used
and the behavior is identical to a call to
.Fn open .
.Pp
The flags specified
for the
.Fa oflag
argument are formed by
.Em or Ns 'ing
the following values:
.Pp
.Bd -literal -offset indent -compact
O_RDONLY open for reading only
O_WRONLY open for writing only
O_RDWR open for reading and writing
O_NONBLOCK do not block on open or for data to become available
O_APPEND append on each write
O_CREAT create file if it does not exist
O_TRUNC truncate size to 0
O_EXCL error if O_CREAT and the file exists
O_SHLOCK atomically obtain a shared lock
O_EXLOCK atomically obtain an exclusive lock
O_NOFOLLOW do not follow symlinks
O_SYMLINK allow open of symlinks
O_EVTONLY descriptor requested for event notifications only
O_CLOEXEC mark as close-on-exec
.Ed
.Pp
Opening a file with
.Dv O_APPEND
set causes each write on the file to be appended to the end. If
.Dv O_TRUNC
is specified and the
file exists, the file is truncated to zero length.
If
.Dv O_EXCL
is set with
.Dv O_CREAT
and the file already
exists,
.Fn open
returns an error.
This may be used to implement a simple exclusive-access locking mechanism.
If
.Dv O_EXCL
is set with
.Dv O_CREAT
and the last component of the pathname is a symbolic link,
.Fn open
will fail even if the symbolic link points to a non-existent name.
.Pp
If the
.Dv O_NONBLOCK
flag is specified, do not wait for the device or file
to be ready or available. If the
.Fn open
call would result
in the process being blocked for some reason
(e.g., waiting for carrier on a dialup line),
.Fn open
returns immediately.
This flag also has the effect of making all subsequent I/O
on the open file non-blocking.
.Pp
When opening a file, a lock with
.Xr flock 2
semantics can be obtained by setting
.Dv O_SHLOCK
for a shared lock, or
.Dv O_EXLOCK
for an exclusive lock.
If creating a file with
.Dv O_CREAT ,
the request for the lock will never fail
(provided that the underlying filesystem supports locking).
.Pp
If
.Dv O_NOFOLLOW
is used in the mask and the target file passed to
.Fn open
is a symbolic link then the
.Fn open
will fail.
.Pp
If
.Dv O_SYMLINK
is used in the mask and the target file passed to
.Fn open
is a symbolic link then the
.Fn open
will be for the symbolic link itself, not what it links to.
.Pp
The
.Dv O_EVTONLY
flag is only intended for monitoring a file for changes (e.g. kqueue). Note: when
this flag is used, the opened file will not prevent an unmount
of the volume that contains the file.
.Pp
The
.Dv O_CLOEXEC
flag causes the file descriptor to be marked as close-on-exec,
setting the
.Dv FD_CLOEXEC
flag. The state of the file descriptor flags can be inspected
using the F_GETFD fcntl. See
.Xr fcntl 2 .
.Pp
If successful,
.Fn open
returns a non-negative integer, termed a file descriptor.
It returns -1 on failure.
The file pointer (used to mark the current position within the file)
is set to the beginning of the file.
.Pp
When a new file is created,
it is given the group of the directory which contains it.
.Pp
The new descriptor is set to remain open across
.Xr execve
system calls; see
.Xr close 2
and
.Xr fcntl 2 .
.Pp
The system imposes a limit on the number of file descriptors
that can be held open simultaneously by one process.
.Xr Getdtablesize 2
returns the current system limit.
.Sh RETURN VALUES
If successful,
.Fn open
returns a non-negative integer, termed a file descriptor.
It returns -1 on failure, and sets
.Va errno
to indicate the error.
.Sh ERRORS
The named file is opened unless:
.Bl -tag -width Er
.\" ===========
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.\" ===========
.It Bq Er EACCES
The required permissions (for reading and/or writing)
are denied for the given flags.
.\" ===========
.It Bq Er EACCES
.Dv O_CREAT
is specified,
the file does not exist,
and the directory in which it is to be created
does not permit writing.
.\" ===========
.It Bq Er EACCES
.Dv O_TRUNC
is specified and write permission is denied.
.\" ===========
.It Bq Er EAGAIN
.Fa path
specifies the slave side of a locked pseudo-terminal device.
.\" ===========
.It Bq Er EDQUOT
.Dv O_CREAT
is specified,
the file does not exist,
and the directory in which the entry for the new file
is being placed cannot be extended because the
user's quota of disk blocks on the file system
containing the directory has been exhausted.
.\" ===========
.It Bq Er EDQUOT
.Dv O_CREAT
is specified,
the file does not exist,
and the user's quota of inodes on the file system
on which the file is being created has been exhausted.
.\" ===========
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
are specified and the file exists.
.\" ===========
.It Bq Er EFAULT
.Fa Path
points outside the process's allocated address space.
.\" ===========
.It Bq Er EINTR
The
.Fn open
operation is interrupted by a signal.
.\" ===========
.It Bq Er EINVAL
The value of
.Fa oflag
is not valid.
.\" ===========
.It Bq Er EIO
An I/O error occurs while making the directory entry or
allocating the inode for
.Dv O_CREAT .
.\" ===========
.It Bq Er EISDIR
The named file is a directory, and the arguments specify
that it is to be opened for writing.
.\" ===========
.It Bq Er ELOOP
Too many symbolic links are encountered in translating the pathname.
This is taken to be indicative of a looping symbolic link.
.\" ===========
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.\" ===========
.It Bq Er ENAMETOOLONG
A component of a pathname exceeds
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.\" ===========
.It Bq Er ENFILE
The system file table is full.
.\" ===========
.It Bq Er ELOOP
.Dv O_NOFOLLOW
was specified and the target is a symbolic link.
.\" ===========
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named file does not exist.
.\" ===========
.It Bq Er ENOENT
A component of the path name that must exist does not exist.
.\" ===========
.It Bq Er ENOSPC
.Dv O_CREAT
is specified,
the file does not exist,
and the directory in which the entry for the new file is being placed
cannot be extended because there is no space left on the file
system containing the directory.
.\" ===========
.It Bq Er ENOSPC
.Dv O_CREAT
is specified,
the file does not exist,
and there are no free inodes on the file system on which the
file is being created.
.\" ===========
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.\" ===========
.It Bq Er ENXIO
The named file is a character-special or block-special file
and the device associated with this special file does not exist.
.\" ===========
.It Bq Er ENXIO
O_NONBLOCK and O_WRONLY are set, the file is a FIFO,
and no process has it open for reading.
.\" ===========
.It Bq Er EOPNOTSUPP
.Dv O_SHLOCK
or
.Dv O_EXLOCK
is specified, but the underlying filesystem does not support locking.
.\" ===========
.It Bq Er EOPNOTSUPP
An attempt is made to open a socket (not currently implemented).
.\" ===========
.It Bq Er EOVERFLOW
The named file is a regular file
and its size does not fit in an object of type off_t.
.\" ===========
.It Bq Er EROFS
The named file resides on a read-only file system,
and the file is to be modified.
.\" ===========
.It Bq Er ETXTBSY
The file is a pure procedure (shared text) file that is being
executed and the
.Fn open
call requests write access.
.It Bq Eq 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 Eq 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.
.El
.Sh COMPATIBILITY
.Fn open
on a terminal device (i.e., /dev/console)
will now make that device a controlling terminal for the process.
Use the O_NOCTTY flag to open a terminal device
without changing your controlling terminal.
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr close 2 ,
.Xr dup 2 ,
.Xr getdtablesize 2 ,
.Xr lseek 2 ,
.Xr read 2 ,
.Xr umask 2 ,
.Xr write 2
.Sh HISTORY
An
.Fn open
function call appeared in
.At v6 .
The
.Fn openat
function was introduced in OS X 10.10