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.\"     @(#)open.2	8.2 (Berkeley) 11/16/93
.\"
.Dd $Mdocdate: March 31 2022 $
.Dt OPEN 2
.Os
.Sh NAME
.Nm open ,
.Nm openat
.Nd open or create a file for reading or writing
.Sh SYNOPSIS
.In fcntl.h
.Ft int
.Fn open "const char *path" "int flags" ...
.Ft int
.Fn openat "int fd" "const char *path" "int flags" ...
.Sh DESCRIPTION
The file name specified by
.Fa path
is opened
for reading and/or writing as specified by the
argument
.Fa flags
and the file descriptor returned to the calling process.
The
.Fa flags
argument may indicate the file is to be
created if it does not exist (by specifying the
.Dv O_CREAT
flag), in which case the file is created with a mode
specified by an additional argument of type
.Vt mode_t
as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.Pp
The
.Fa flags
specified are a bitwise OR of the following values.
Exactly one of the first three values (file access modes) must be specified:
.Pp
.Bl -tag -width O_DIRECTORY -offset indent -compact
.It Dv O_RDONLY
Open for reading only.
.It Dv O_WRONLY
Open for writing only.
.It Dv O_RDWR
Open for reading and writing.
.El
.Pp
Any combination of the following flags may additionally be used:
.Pp
.Bl -tag -width O_DIRECTORY -offset indent -compact
.It Dv O_NONBLOCK
Do not block on open or for data to become available.
.It Dv O_APPEND
Append on each write.
.It Dv O_CREAT
Create file if it does not exist.
An additional argument of type
.Vt mode_t
must be supplied to the call.
.It Dv O_TRUNC
Truncate size to 0.
.It Dv O_EXCL
Error if
.Dv O_CREAT
is set and file exists.
.It Dv O_SYNC
Perform synchronous I/O operations.
.It Dv O_SHLOCK
Atomically obtain a shared lock.
.It Dv O_EXLOCK
Atomically obtain an exclusive lock.
.It Dv O_NOFOLLOW
If last path element is a symlink, don't follow it.
.It Dv O_CLOEXEC
Set
.Dv FD_CLOEXEC
(the close-on-exec flag)
on the new file descriptor.
.It Dv O_DIRECTORY
Error if
.Fa path
does not name a directory.
.El
.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
and a writing mode are 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 either of
.Dv O_EXCL
or
.Dv O_NOFOLLOW
are set 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.
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.
If the
.Dv O_SYNC
flag is set, all I/O operations on the file will be done synchronously.
.Pp
A FIFO should either be opened with
.Dv O_RDONLY
or with
.Dv O_WRONLY .
The behavior for opening a FIFO with
.Dv O_RDWR
is undefined.
.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
.Fn open
is successful, 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 2
system calls; see
.Xr close 2
and
.Xr fcntl 2 .
.Pp
The system imposes a limit on the number of file descriptors
open simultaneously by one process.
.Xr getdtablesize 3
returns the current system limit.
.Pp
The
.Fn openat
function is equivalent to
.Fn open
except that where
.Fa path
specifies a relative path,
the file to be opened is determined relative to
the directory associated with file descriptor
.Fa fd
instead of the current working directory.
.Pp
If
.Fn openat
is passed the special value
.Dv AT_FDCWD
(defined in
.In fcntl.h )
in the
.Fa fd
parameter, the current working directory is used
and the behavior is identical to a call to
.Fn open .
.Sh RETURN VALUES
If successful,
.Fn open
returns a non-negative integer, termed a file descriptor.
Otherwise, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn open
and
.Fn openat
functions will fail if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENOTDIR
.Dv O_DIRECTORY
is specified and
.Fa path
does not name a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv NAME_MAX
characters, or an entire pathname (including the terminating NUL)
exceeded
.Dv PATH_MAX
bytes.
.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 pathname that must exist does not exist.
.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
.Fa 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 ELOOP
Too many symbolic links were encountered in translating the pathname,
or the
.Dv O_NOFOLLOW
flag was specified and the target is a symbolic link.
.It Bq Er EISDIR
The named file is a directory, and the arguments specify
it is to be opened for writing.
.It Bq Er EINVAL
The
.Fa flags
specified for opening the file are not valid.
.It Bq Er EROFS
The named file resides on a read-only file system,
and the file is to be modified.
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENFILE
The system file table is full.
.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
The named file is a FIFO, the
.Dv O_NONBLOCK
and
.Dv O_WRONLY
flags are set, and no process has the file open for reading.
.It Bq Er EINTR
The
.Fn open
operation was interrupted by a signal.
.It Bq Er EOPNOTSUPP
.Dv O_SHLOCK
or
.Dv O_EXLOCK
is specified but the underlying filesystem does not support locking.
.It Bq Er EWOULDBLOCK
.Dv O_NONBLOCK
and one of
.Dv O_SHLOCK
or
.Dv O_EXLOCK
is specified and the file is already locked.
.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 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 EIO
An I/O error occurred while making the directory entry or
allocating the inode for
.Dv O_CREAT .
.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 Er EFAULT
.Fa path
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 EPERM
The file named by
.Fa path
is flagged append-only but
.Dv O_APPEND
was not specified in
.Fa flags .
.It Bq Er EOPNOTSUPP
An attempt was made to open a socket (not currently implemented).
.It Bq Er EBUSY
An attempt was made to open a terminal device that requires exclusive
access and the specified device has already be opened.
.El
.Pp
Additionally, the
.Fn openat
function will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa path
argument specifies a relative path and the
.Fa fd
argument is neither
.Dv AT_FDCWD
nor a valid file descriptor.
.It Bq Er ENOTDIR
The
.Fa path
argument specifies a relative path and the
.Fa fd
argument is a valid file descriptor but it does not reference a directory.
.It Bq Er EACCES
The
.Fa path
argument specifies a relative path but search permission is denied
for the directory which the
.Fa fd
file descriptor references.
.El
.Sh SEE ALSO
.Xr chflags 2 ,
.Xr chmod 2 ,
.Xr close 2 ,
.Xr dup 2 ,
.Xr flock 2 ,
.Xr lseek 2 ,
.Xr read 2 ,
.Xr umask 2 ,
.Xr write 2 ,
.Xr getdtablesize 3
.Sh STANDARDS
The
.Fn open
and
.Fn openat
functions conform to
.St -p1003.1-2008 .
.Pp
.Dv POSIX
specifies three different flavors for synchronous I/O:
.Dv O_SYNC ,
.Dv O_DSYNC ,
and
.Dv O_RSYNC .
In
.Ox ,
these are all equivalent.
.Pp
The
.Dv O_SHLOCK
and
.Dv O_EXLOCK
flags are non-standard extensions and should not be used if portability
is of concern.
.Sh HISTORY
An
.Fn open
system call first appeared in
.At v1 .
The
.Fa flags
argument has been supported since
.Bx 4.2 .
Before that, a dedicated
.Fn creat
system call had to be used to create new files;
it appeared in
.At v1 ,
was deprecated in
.Bx 4.3 Reno ,
and removed in
.Ox 5.0 .
.Pp
The
.Fn openat
system call has been available since
.Ox 5.0 .
.Sh CAVEATS
The
.Dv O_TRUNC
flag requires that one of
.Dv O_RDWR
or
.Dv O_WRONLY
also be specified, else
.Er EINVAL
is returned.