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,
...);
openat
() works the same way as
open
() except if path is
relative. In that case, it is looked up from a directory whose file
descriptor was passed as fd. Search permission is
required on this directory. fd can be set to
AT_FDCWD
in order to specify the current
directory.
The flags are specified by or'ing the values listed below. Applications must specify exactly one of the first three values (file access methods):
Any combination of the following may be used:
O_NONBLOCK
O_APPEND
O_CREAT
O_TRUNC
O_EXCL
O_CREAT
and the file already exists.O_SHLOCK
O_EXLOCK
O_NOFOLLOW
O_CLOEXEC
O_NOSIGPIPE
EPIPE
instead of raising
SIGPIPE
.O_DSYNC
O_SYNC
O_RSYNC
O_SYNC
, each read will wait for the file status to
be committed to stable storage.
Combining O_RSYNC
with
O_DSYNC
only, or specifying it without any other
synchronized I/O integrity completion flag set, has no further
effect.
O_ALT_IO
O_NOCTTY
O_DIRECT
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 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.
O_DIRECTORY
O_REGULAR
O_ASYNC
SIGIO
signal to be sent to the process
group when I/O is possible, e.g., upon availability of data to be
read.Opening a file with O_APPEND
set causes
each write on the file to be appended to the end. If
O_TRUNC
is 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
O_EXCL
is 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.
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 file system 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. Calling getdtablesize(3) returns the current system limit.
open
() and
openat
() returns a non-negative integer, termed a file
descriptor. Otherwise, a value of -1 is returned and
errno is set to indicate the error.
EACCES
]O_CREAT
is specified, the file does not
exist, and the directory in which it is to be created does not permit
writing.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; or 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.EEXIST
]O_CREAT
and O_EXCL
were specified and the file
exists.EFAULT
]EFTYPE
]O_NOFOLLOW
was specified, but the last path component is a symlink.
Note: IEEE Std 1003.1-2008
(“POSIX.1”) specifies returning
[ELOOP
]
for this case.EINTR
]open
() operation was interrupted by a
signal.EIO
]O_CREAT
.EISDIR
]ELOOP
]EMFILE
]ENAMETOOLONG
]NAME_MAX
}
characters, or an entire path name exceeded
{PATH_MAX
} characters.ENFILE
]ENOENT
]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.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; or
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.ENOTDIR
]O_DIRECTORY
is specified and the last path
component is not a directory.ENXIO
]O_NONBLOCK
and
O_WRONLY
is set and no process has the file open
for reading.EOPNOTSUPP
]O_SHLOCK
or 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).EPERM
]EROFS
]ETXTBSY
]open
() call requests write access.In addition, openat
() will fail if:
open
() function conforms to IEEE
Std 1003.1-1990 (“POSIX.1”).
openat
() conforms to IEEE Std
1003.1-2008 (“POSIX.1”).
The flags values
O_DSYNC
, O_SYNC
and
O_RSYNC
are extensions defined in
IEEE Std 1003.1b-1993
(“POSIX.1b”).
The O_SHLOCK
and
O_EXLOCK
flags are non-standard extensions and
should not be used if portability is of concern.
open
() function call appeared in
Version 1 AT&T UNIX.
September 2, 2019 | NetBSD 9.2 |