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.\"     @(#)open.2	8.2 (Berkeley) 11/16/93
.\"
.Dd March 5, 2023
.Dt OPEN 2
.Os
.Sh NAME
.Nm open ,
.Nm openat
.Nd open or create a file for reading, writing or executing
.Sh LIBRARY
.Lb libc
.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 either execution or 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 this case
.Fn open
and
.Fn openat
require an additional argument
.Fa "mode_t mode" ,
and 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
is relative.
In that case, it is looked up from a directory whose file
descriptor was passed as
.Fa fd .
Search permission is required on this directory.
.\"    (These alternatives await a decision about the semantics of O_SEARCH)
.\" Search permission is required on this directory
.\" except if
.\" .Fa fd
.\" was opened with the
.\" .Dv O_SEARCH
.\" flag.
.\"    - or -
.\" This file descriptor must have been opened with the
.\" .Dv O_SEARCH
.\" flag.
.Fa fd
can be set to
.Dv AT_FDCWD
in order to specify the current directory.
.Pp
The
.Fa flags
are specified by
.Em or Ns 'ing
the values listed below.
Applications must specify exactly one of these four values
(file access methods):
.Bl -tag -offset indent -width O_DIRECTORY
.It Dv O_RDONLY
Open for reading only.
.It Dv O_WRONLY
Open for writing only.
.It Dv O_EXEC
Open for execute only.
.It Dv O_RDWR
Open for reading and writing.
.El
.Pp
Any combination of the following may be used as well:
.Bl -tag -offset indent -width O_DIRECTORY
.It Dv O_NONBLOCK
Do not block on open or for data to become available.
.It Dv O_APPEND
Append to the file on each write.
.It Dv O_CREAT
Create the file if it does not exist.
The third argument of type
.Ft mode_t
is used to compute the mode bits of the file as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.It Dv O_TRUNC
Truncate size to 0.
.It Dv O_EXCL
Error if
.Dv O_CREAT
and the file already exists.
.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.
This option is provided for compatibility with other operating
systems, but its security value is questionable.
.It Dv O_CLOEXEC
Set the
.Xr close 2
on
.Xr exec 3
flag.
.It Dv O_NOSIGPIPE
Return
.Er EPIPE
instead of raising
.Dv SIGPIPE .
.It Dv O_DSYNC
If set, write operations will be performed according to synchronized
I/O data integrity completion:
each write will wait for the file data to be committed to stable
storage.
.It Dv O_SYNC
If set, write operations will be performed according to synchronized
I/O file integrity completion:
each write will wait for both the file data and file status to be
committed to stable storage.
.It Dv O_RSYNC
If set, read operations will complete at the same level of
integrity which is in effect for write operations:
if specified together with
.Dv O_SYNC ,
each read will wait for the file status to be committed to stable
storage.
.Pp
Combining
.Dv O_RSYNC
with
.Dv O_DSYNC
only, or specifying it without any other synchronized I/O integrity
completion flag set, has no further effect.
.It Dv O_ALT_IO
Alternate I/O semantics will be used for read and write operations
on the file descriptor.
Alternate semantics are defined by the underlying layers and will not
have any alternate effect in most cases.
.It Dv O_NOCTTY
If the file is a terminal device, the opened device is not
made the controlling terminal for the session.
This flag has no effect on
.Nx ,
since the system defaults to the abovementioned behaviour.
The flag is present only for standards conformance.
.It Dv O_DIRECT
If set on a regular file, data I/O operations will not buffer the data
being transferred in the kernel's cache, but rather transfer the data
directly between user memory and the underlying device driver if possible.
This flag is advisory; the request may be performed in the normal
buffered fashion if certain conditions are not met, e.g. if the request
is not sufficiently aligned or if the file is mapped.
.Pp
To meet the alignment requirements for direct I/O, the file offset,
the length of the I/O and the address of the buffer in memory must all
be multiples of
.Dv DEV_BSIZE
(512 bytes).
If the I/O request is made
using an interface that supports scatter/gather via struct iovec, each
element of the request must meet the above alignment constraints.
.It Dv O_DIRECTORY
Fail if the file is not a directory.
.It Dv O_REGULAR
Fail if the path does not refer to a regular file.
.It Dv O_ASYNC
Enable the
.Dv SIGIO
signal to be sent to the process group
when I/O is possible, e.g.,
upon availability of data to be read.
.Pp
Note: This is broken in
.Fn open ;
it must be set explicitly with the
.Dv F_SETFL
command to
.Xr fcntl 2 .
.\"    (This block awaits a decision about the semantics of O_SEARCH)
.\" .It Dv O_SEARCH
.\" If opening a directory, search permission checks will not be performed on
.\" subsequent usage of the file descriptor for looking up relative paths by
.\" .Xr faccessat 2 ,
.\" .Xr fchmodat 2 ,
.\" .Xr fchownat 2 ,
.\" .Xr fstatat 2 ,
.\" .Xr linkat 2 ,
.\" .Xr mkdirat 2 ,
.\" .Xr mkfifoat 2 ,
.\" .Xr mknodat 2 ,
.\" .Xr openat 2 ,
.\" .Xr readlinkat 2 ,
.\" .Xr symlinkat 2 ,
.\" .Xr unlinkat 2 ,
.\" and
.\" .Xr utimensat 2 .
.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
is specified and the
file exists, the file is truncated to zero length.
.Pp
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 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 file system 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.
Calling
.Xr getdtablesize 3
returns the current system limit.
.Sh RETURN VALUES
If successful,
.Fn open
and
.Fn openat
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 named file is opened unless:
.Bl -tag -width Er
.It Bq Er EACCES
Search permission is denied for a component of the path prefix,
the required permissions (for reading and/or writing)
are denied for the given flags, or
.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 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; or
.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
were specified and the file exists.
.It Bq Er EFAULT
.Fa path
points outside the process's allocated address space.
.It Bq Er EFTYPE
.Dv O_NOFOLLOW
was specified, but the last path component is a symlink.
.Em Note :
.St -p1003.1-2008
specifies returning
.Bq Er ELOOP
for this case.
.It Bq Er EFTYPE
.Dv O_REGULAR
is specified and the last path component is not a regular file.
.It Bq Er EINTR
The
.Fn open
operation was interrupted by a signal.
.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 EISDIR
The named file is a directory, and the arguments specify
it is to be opened for writing.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Brq Dv NAME_MAX
characters, or an entire path name exceeded
.Brq Dv PATH_MAX
characters.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named file does not exist, or
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; or
.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; or
.Dv O_DIRECTORY
is specified and the last path component 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, or the named file is a FIFO,
.Dv O_NONBLOCK
and
.Dv O_WRONLY
is set and no process has the file open for reading.
.It Bq Er EOPNOTSUPP
.Dv O_SHLOCK
or
.Dv O_EXLOCK
is specified but the underlying file system does not support locking; or
an attempt was made to open a socket (not currently implemented).
.It Bq Er EPERM
The file's flags (see
.Xr chflags 2 )
don't allow the file to be opened.
.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.
.El
.Pp
In addition,
.Fn openat
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa path
does not specify an absolute path and
.Fa fd
is neither
.Dv AT_FDCWD
nor a valid file descriptor open for reading or searching.
.It Bq Er EINVAL
An attempt was made to open a descriptor with an illegal combination
of
.Dv O_RDONLY ,
.Dv O_WRONLY ,
.Dv O_RDWR ,
and
.Dv O_EXEC .
.It Bq Er ENOTDIR
.Fa path
is not an absolute path and
.Fa fd
is a file descriptor associated with a non-directory file.
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr close 2 ,
.Xr dup 2 ,
.Xr faccessat 2 ,
.Xr fchmodat 2 ,
.Xr fchownat 2 ,
.Xr fstatat 2 ,
.Xr linkat 2 ,
.Xr lseek 2 ,
.Xr mkdirat 2 ,
.Xr mkfifoat 2 ,
.Xr mknodat 2 ,
.Xr read 2 ,
.Xr readlinkat 2 ,
.Xr symlinkat 2 ,
.Xr umask 2 ,
.Xr unlinkat 2 ,
.Xr utimensat 2 ,
.Xr write 2 ,
.Xr getdtablesize 3
.Sh STANDARDS
The
.Fn open
function conforms to
.St -p1003.1-90 .
.Fn openat
conforms to
.St -p1003.1-2008 .
.Pp
The
.Fa flags
values
.Dv O_DSYNC ,
.Dv O_SYNC
and
.Dv O_RSYNC
are extensions defined in
.St -p1003.1b-93 .
.\"    (This block awaits a decision about the semantics of O_SEARCH)
.\" .Dv O_SEARCH
.\" is defined in
.\" .St -p1003.1-2008 .
.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
function call appeared in
.At v1 .
.Sh BUGS
.Dv O_ASYNC
doesn't actually work as advertised with
.Nm ;
you must set
.Dv O_ASYNC
explicitly with the
.Dv F_SETFL
command to
.Xr fcntl 2 .