OPEN(2) | System Calls Manual | OPEN(2) |
open
, openat
—
#include <fcntl.h>
int
open
(const
char *path, int
flags, ...);
int
openat
(int
fd, const char
*path, int flags,
...);
O_CREAT
flag), in
which case the file is created with a mode specified by an additional argument
of type mode_t as described in
chmod(2) and modified by the process' umask
value (see umask(2)).
The flags specified are a bitwise OR of the following values. Exactly one of the first three values (file access modes) must be specified:
Any combination of the following flags may additionally be used:
O_NONBLOCK
O_APPEND
O_CREAT
O_TRUNC
O_EXCL
O_CREAT
is set and file exists.O_SYNC
O_SHLOCK
O_EXLOCK
O_NOFOLLOW
O_CLOEXEC
FD_CLOEXEC
(the close-on-exec flag) on the new
file descriptor.O_DIRECTORY
Opening a file with O_APPEND
set causes
each write on the file to be appended to the end. If
O_TRUNC
and a writing mode are specified and the
file exists, the file is truncated to zero length. If
O_EXCL
is set with O_CREAT
and the file already exists, open
() returns an
error. This may be used to implement a simple exclusive access locking
mechanism. If either of O_EXCL
or
O_NOFOLLOW
are set and the last component of the
pathname is a symbolic link, open
() will fail even
if the symbolic link points to a non-existent name. If the
O_NONBLOCK
flag is specified, do not wait for the
device or file to be ready or available. If the
open
() call would result in the process being
blocked for some reason (e.g., waiting for carrier on a dialup line),
open
() returns immediately. This flag also has the
effect of making all subsequent I/O on the open file non-blocking. If the
O_SYNC
flag is set, all I/O operations on the file
will be done synchronously.
A FIFO should either be opened with
O_RDONLY
or with O_WRONLY
.
The behavior for opening a FIFO with O_RDWR
is
undefined.
When opening a file, a lock with
flock(2) semantics can be obtained by
setting O_SHLOCK
for a shared lock, or
O_EXLOCK
for an exclusive lock. If creating a file
with O_CREAT
, the request for the lock will never
fail (provided that the underlying filesystem supports locking).
If open
() is successful, the file pointer
used to mark the current position within the file is set to the beginning of
the file.
When a new file is created it is given the group of the directory which contains it.
The new descriptor is set to remain open across execve(2) system calls; see close(2) and fcntl(2).
The system imposes a limit on the number of file descriptors open simultaneously by one process. getdtablesize(3) returns the current system limit.
The openat
() function is equivalent to
open
() except that where path
specifies a relative path, the file to be opened is determined relative to
the directory associated with file descriptor fd
instead of the current working directory.
If openat
() is passed the special value
AT_FDCWD
(defined in
<fcntl.h>
) in the
fd parameter, the current working directory is used
and the behavior is identical to a call to
open
().
open
() returns a non-negative integer,
termed a file descriptor. Otherwise, a value of -1 is returned and
errno is set to indicate the error.
open
() and openat
()
functions will fail if:
ENOTDIR
]ENOTDIR
]O_DIRECTORY
is specified and path does not name a
directory.ENAMETOOLONG
]NAME_MAX
characters, or an entire pathname (including the terminating NUL) exceeded
PATH_MAX
bytes.ENOENT
]O_CREAT
is not set and the named file does not exist.ENOENT
]EACCES
]EACCES
]EACCES
]O_CREAT
is specified, the file does not exist, and the directory in which it is to
be created does not permit writing.ELOOP
]O_NOFOLLOW
flag was specified and the target
is a symbolic link.EISDIR
]EINVAL
]EROFS
]EMFILE
]ENFILE
]ENXIO
]ENXIO
]O_NONBLOCK
and
O_WRONLY
flags are set, and no process has the
file open for reading.EINTR
]open
() operation was interrupted by a
signal.EOPNOTSUPP
]O_SHLOCK
or O_EXLOCK
is specified but the underlying
filesystem does not support locking.EWOULDBLOCK
]O_NONBLOCK
and one of O_SHLOCK
or
O_EXLOCK
is specified and the file is already
locked.ENOSPC
]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.ENOSPC
]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.EDQUOT
]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.EDQUOT
]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.EIO
]O_CREAT
.ETXTBSY
]open
() call requests write access.EFAULT
]EEXIST
]O_CREAT
and O_EXCL
were specified and the file
exists.EPERM
]O_APPEND
was not specified in
flags.EOPNOTSUPP
]EBUSY
]Additionally, the openat
() function will
fail if:
EBADF
]AT_FDCWD
nor a valid file descriptor.ENOTDIR
]EACCES
]open
() and openat
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”).
POSIX
specifies three different flavors
for synchronous I/O: O_SYNC
,
O_DSYNC
, and O_RSYNC
. In
OpenBSD, these are all equivalent.
The O_SHLOCK
and
O_EXLOCK
flags are non-standard extensions and
should not be used if portability is of concern.
open
() system call first appeared in
Version 1 AT&T UNIX. The
flags argument has been supported since
4.2BSD. Before that, a dedicated
creat
() system call had to be used to create new
files; it appeared in Version 1 AT&T UNIX,
was deprecated in 4.3BSD-Reno, and removed in
OpenBSD 5.0.
The openat
() system call has been
available since OpenBSD 5.0.
O_TRUNC
flag requires that one of
O_RDWR
or O_WRONLY
also be
specified, else EINVAL
is returned.January 19, 2015 | OpenBSD-current |