NAME
open
, openat
— open or create a file for
reading or writing
LIBRARY
library “libc”
SYNOPSIS
#include
<fcntl.h>
int
open
(const
char *path, int
flags, ...);
int
openat
(int
fd, const char
*path, int flags,
...);
DESCRIPTION
The file name specified by path is opened for reading and/or writing as specified by the argument flags and the file descriptor returned to the calling process.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
- Do not block on open or for data to become available.
O_APPEND
- Append to the file on each write.
O_CREAT
- Create the file if it does not exist. The third argument of type mode_t is used to compute the mode bits of the file as described in chmod(2) and modified by the process' umask value (see umask(2)).
O_TRUNC
- Truncate size to 0.
O_EXCL
- Error if
O_CREAT
and the file already exists. O_SHLOCK
- Atomically obtain a shared lock.
O_EXLOCK
- Atomically obtain an exclusive lock.
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.
O_CLOEXEC
- Set the close(2) on exec(3) flag.
O_NOSIGPIPE
- Return
EPIPE
instead of raisingSIGPIPE
. 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.
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.
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
O_SYNC
, each read will wait for the file status to be committed to stable storage.Combining
O_RSYNC
withO_DSYNC
only, or specifying it without any other synchronized I/O integrity completion flag set, has no further effect. 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.
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 NetBSD, since the system defaults to the abovementioned behaviour. The flag is present only for standards conformance.
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.
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
- Fail if the file is not a directory.
O_REGULAR
- Fail if the path does not refer to a regular file.
O_ASYNC
- Enable the
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.
RETURN VALUES
If successful, 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.
ERRORS
The named file is opened unless:
- [
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
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; orO_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
andO_EXCL
were specified and the file exists.- [
EFAULT
] - path points outside the process's allocated address space.
- [
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
] - The
open
() operation was interrupted by a signal. - [
EIO
] - An I/O error occurred while making the directory entry or allocating the
inode for
O_CREAT
. - [
EISDIR
] - The named file is a directory, and the arguments specify it is to be opened for writing.
- [
ELOOP
] - Too many symbolic links were encountered in translating the pathname.
- [
EMFILE
] - The process has already reached its limit for open file descriptors.
- [
ENAMETOOLONG
] - A component of a pathname exceeded {
NAME_MAX
} characters, or an entire path name exceeded {PATH_MAX
} characters. - [
ENFILE
] - The system file table is full.
- [
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; orO_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
] - A component of the path prefix is not a directory; or
O_DIRECTORY
is specified and the last path component is not a directory. - [
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,
O_NONBLOCK
andO_WRONLY
is set and no process has the file open for reading. - [
EOPNOTSUPP
] O_SHLOCK
orO_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
] - The file's flags (see chflags(2)) don't allow the file to be opened.
- [
EROFS
] - The named file resides on a read-only file system, and the file is to be modified.
- [
ETXTBSY
] - The file is a pure procedure (shared text) file that is being executed and
the
open
() call requests write access.
In addition, openat
() will fail if:
- [
EBADF
] - path does not specify an absolute path and
fd is neither
AT_FDCWD
nor a valid file descriptor open for reading or searching. - [
ENOTDIR
] - path is not an absolute path and fd is a file descriptor associated with a non-directory file.
SEE ALSO
chmod(2), close(2), dup(2), faccessat(2), fchmodat(2), fchownat(2), fstatat(2), linkat(2), lseek(2), mkdirat(2), mkfifoat(2), mknodat(2), read(2), readlinkat(2), symlinkat(2), umask(2), unlinkat(2), utimensat(2), write(2), getdtablesize(3)
STANDARDS
The 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.
HISTORY
An open
() function call appeared in
Version 1 AT&T UNIX.