OPEN(2) | System Calls Manual | OPEN(2) |
open
, openat
— open or create a file for reading, writing or
executing
Standard C Library (libc, -lc)
#include
<fcntl.h>
int
open
(const
char *path, int
flags, ...);
int
openat
(int
fd, const char
*path, int flags,
...);
The file name specified by path is opened
for either execution or reading and/or writing as specified by the argument
flags and the file descriptor returned to the calling
process. The flags argument may indicate the file is
to be created if it does not exist (by specifying the
O_CREAT
flag). In this case
open
()
and openat
() require an additional argument
mode_t mode, and the file is created with mode
mode as described in chmod(2) and
modified by the process' umask value (see umask(2)).
The
openat
()
function is equivalent to the open
() function except
in the case where the path specifies a relative path.
In this case the file to be opened is determined relative to the directory
associated with the file descriptor fd instead of the
current working directory. The flag parameter and the
optional fourth parameter correspond exactly to the parameters of
open
(). If openat
() is
passed the special value AT_FDCWD
in the
fd parameter, the current working directory is used
and the behavior is identical to a call to
open
().
In capsicum(4) capability mode,
open
() is
not permitted. The path argument to
openat
() must be strictly relative to a file
descriptor fd, as defined in
sys/kern/vfs_lookup.c. path
must not be an absolute path and must not contain ".." components.
Additionally, no symbolic link in path may contain
".." components either. fd must not be
AT_FDCWD
.
The flags specified are formed by or'ing the following values
O_RDONLY open for reading only O_WRONLY open for writing only O_RDWR open for reading and writing O_EXEC open for execute only O_NONBLOCK do not block on open 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 create and file exists O_SHLOCK atomically obtain a shared lock O_EXLOCK atomically obtain an exclusive lock O_DIRECT eliminate or reduce cache effects O_FSYNC synchronous writes O_SYNC synchronous writes O_NOFOLLOW do not follow symlinks O_NOCTTY ignored O_TTY_INIT ignored O_DIRECTORY error if file is not a directory O_CLOEXEC set FD_CLOEXEC upon open O_VERIFY verify the contents of the file
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 and the
open
() system call would result in the process being
blocked for some reason (e.g., waiting for carrier on a dialup line),
open
() returns immediately. The descriptor remains
in non-blocking mode for subsequent operations.
If O_FSYNC
is used in the mask, all writes
will immediately be written to disk, the kernel will not cache written data
and all writes on the descriptor will not return until the data to be
written completes.
O_SYNC
is a synonym for
O_FSYNC
required by POSIX.
If O_NOFOLLOW
is used in the
mask and the target file passed to
open
() is a
symbolic link then the open
() will fail.
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).
O_DIRECT
may be used to minimize or
eliminate the cache effects of reading and writing. The system will attempt
to avoid caching the data you read or write. If it cannot avoid caching the
data, it will minimize the impact the data has on the cache. Use of this
flag can drastically reduce performance if not used with care.
O_NOCTTY
may be used to ensure
the OS does not assign this file as the controlling terminal when it opens a
tty device. This is the default on FreeBSD, but is
present for POSIX compatibility. The
open
()
system call will not assign controlling terminals on
FreeBSD.
O_TTY_INIT
may be used to
ensure the OS restores the terminal attributes when initially opening a TTY.
This is the default on FreeBSD, but is present for
POSIX compatibility. The initial call to
open
() on a
TTY will always restore default terminal attributes on
FreeBSD.
O_DIRECTORY
may be used to ensure the
resulting file descriptor refers to a directory. This flag can be used to
prevent applications with elevated privileges from opening files which are
even unsafe to open with O_RDONLY
, such as device
nodes.
O_CLOEXEC
may be used to set
FD_CLOEXEC
flag for the newly returned file
descriptor.
O_VERIFY
may be used to indicate to the
kernel that the contents of the file should be verified before allowing the
open to proceed. The details of what “verified” means is
implementation specific. The run-time linker (rtld) uses this flag to ensure
shared objects have been verified before operating on them.
If successful,
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.
If a sleeping open of a device node from
devfs(5) is interrupted by a signal, the call always fails
with EINTR
, even if the
SA_RESTART
flag is set for the signal. A sleeping
open of a fifo (see mkfifo(2)) is restarted as normal.
When a new file is created it is given the group of the directory which contains it.
Unless O_CLOEXEC
flag was specified, the
new descriptor is set to remain open across execve(2)
system calls; see close(2), fcntl(2) and
O_CLOEXEC
description.
The system imposes a limit on the number of file descriptors open simultaneously by one process. The getdtablesize(2) system call returns the current system limit.
If successful, open
() and
openat
() return a non-negative integer, termed a
file descriptor. They return -1 on failure, and set
errno to indicate the error.
The named file is opened unless:
ENOTDIR
]ENAMETOOLONG
]ENOENT
]O_CREAT
is not set and the named file does not exist.ENOENT
]EACCES
]EACCES
]EACCES
]O_TRUNC
is specified and write permission is denied.EACCES
]O_CREAT
is specified, the file does not exist, and the directory in which it is to
be created does not permit writing.EPERM
]O_CREAT
is specified, the file does not exist, and the directory in which it is to
be created has its immutable flag set, see the
chflags(2) manual page for more information.EPERM
]EPERM
]O_TRUNC
is specified or
O_APPEND
is not specified.ELOOP
]EISDIR
]EROFS
]EROFS
]O_CREAT
is specified and the named file would reside on a read-only file
system.EMFILE
]ENFILE
]EMLINK
]O_NOFOLLOW
was specified and the target is a symbolic link.ENXIO
]ENXIO
]O_NONBLOCK
is set, the named file is a fifo, O_WRONLY
is 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 file
system does not support locking.EOPNOTSUPP
]EWOULDBLOCK
]O_NONBLOCK
and one of O_SHLOCK
or
O_EXLOCK
is specified and the file is 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
() system call requests write access.EFAULT
]EEXIST
]O_CREAT
and O_EXCL
were specified and the file
exists.EOPNOTSUPP
]EINVAL
]O_RDONLY
, O_WRONLY
,
O_RDWR
and O_EXEC
.EBADF
]AT_FDCWD
nor a valid file descriptor open for
searching.ENOTDIR
]AT_FDCWD
nor a
file descriptor associated with a directory.ENOTDIR
]O_DIRECTORY
is specified and the file is not a directory.ECAPMODE
]AT_FDCWD
is specified and the process is in capability mode.ECAPMODE
]open
() was called and the process is in capability
mode.ENOTCAPABLE
]chmod(2), close(2), dup(2), fexecve(2), fhopen(2), getdtablesize(2), getfh(2), lgetfh(2), lseek(2), read(2), umask(2), write(2), fopen(3), capsicum(4)
These functions are specified by IEEE Std
1003.1-2008 (“POSIX.1”).
FreeBSD sets errno to
EMLINK instead of
ELOOP
as
specified by POSIX when O_NOFOLLOW
is set in flags
and the final component of pathname is a symbolic link to distinguish it
from the case of too many symbolic link traversals in one of its non-final
components.
The open
() function appeared in
Version 1 AT&T UNIX. The
openat
() function was introduced in
FreeBSD 8.0.
The Open Group Extended API Set 2 specification requires that the test for whether fd is searchable is based on whether fd is open for searching, not whether the underlying directory currently permits searches. The present implementation of the openat checks the current permissions of directory instead.
December 1, 2017 | Debian |