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open(2)
NAME
open - open for reading or writing
C SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open (const char *path, int oflag, ... /* mode_t mode */);
DESCRIPTION
path points to a path name naming a file. open opens a file descriptor
for the named file and sets the file status flags according to the value
of oflag. oflag values are constructed by OR-ing Flags from the
following list (only one of the first three flags below may be used):
O_RDONLY
Open for reading only.
O_WRONLY
Open for writing only.
O_RDWR Open for reading and writing.
O_NDELAY or O_NONBLOCK
These flags may affect subsequent reads and writes [see read(2)
and write(2)]. If both O_NDELAY and O_NONBLOCK are set,
O_NONBLOCK will take precedence.
When opening a FIFO with O_RDONLY or O_WRONLY set:
If O_NDELAY or O_NONBLOCK is set: An open for reading-
only will return without delay; an open for writing-only
will return an error if no process currently has the file
open for reading.
If O_NDELAY and O_NONBLOCK are clear: An open for
reading-only will block until a process opens the file for
writing; an open for writing-only will block until a
process opens the file for reading.
When opening a file associated with a communication line:
If O_NDELAY or O_NONBLOCK is set: The open will return
without waiting for the device to be ready or available;
subsequent behavior of the device is device specific.
If O_NDELAY and O_NONBLOCK are clear: The open will block
until the device is ready or available.
O_APPEND
If set, the file pointer will be set to the end of the file prior
to each write.
O_SYNC When opening a regular file, this flag affects subsequent writes.
If set, each write(2) will wait for both the file data and file
status to be physically updated.
O_DSYNCWhen opening a regular file, this flag affects subsequent writes.
If set, each write(2) will wait for the file data to be physically
updated.
O_RSYNCWhen opening a regular file, this flag affects subsequent reads.
If set, and either O_SYNC or O_DSYNC is set then each read(2) will
wait for the file to be in the same state as O_SYNC or O_DSYNC
would require for writes.
O_NOCTTY
If set and the file is a terminal, the terminal will not be
allocated as the calling process's controlling terminal.
O_CREATIf the file exists, this flag has no effect, except as noted under
O_EXCL below. Otherwise, the file is created and the owner ID of
the file is set to the effective user IDs of the process, the
group ID of the file is set to the effective group IDs of the
process or to the group ID of the directory in which the file is
being created. This is determined as follows:
If the underlying filesystem was mounted with the BSD file
creation semantics flag [see fstab(4)] or the S_ISGID bit is
set [see chmod(2)] on the parent directory, then the group ID
of the new file is set to the group ID of the parent
directory, otherwise it is set to the effective group ID of
the calling process.
If the group ID of the new file does not match the effective group
ID or one of the supplementary groups IDs, the S_ISGID bit is
cleared. The access permission bits of the file mode are set to
the value of mode, modified as follows [see creat(2)]:
All bits set in the file mode creation mask of the process
are cleared [see umask(2)].
The ``save text image after execution bit'' of the mode is
cleared [see chmod(2)].
O_TRUNCIf the file exists and is a regular file, and the file is
successfully opened O_RDWR or O_WRONLY, its length is truncated to
0 and the mode and owner are unchanged. It will have no effect on
FIFO special files or terminal device files. It's effect on other
files types is implementation-dependent. The result of using
O_TRUNC with O_RDONLY is undefined.
O_EXCL If O_EXCL and O_CREAT are set, open will fail if the file exists.
The check for the existence of the file and the creation of the
file if it does not exist is atomic with respect to other
processes executing open naming the same filename in the same
directory with O_EXCL and O_CREAT set.
O_LCFLUSH
If set, when all copies of a file descriptor are closed, all
modified buffers associated with the file will be written back to
the physical medium. O_LCFLUSH is a Silicon Graphics extension.
O_LCINVAL
If set, when all copies of a file descriptor are closed, all
modified buffers associated with the file will be written back to
the physical medium, then invalidated, immediately freeing the
buffers for other use. The process doing the last close must be
able to aquire write access at the time of last close for the
cache to be invalidated. O_LCINVAL is a Silicon Graphics
extension.
O_DIRECT
If set, reads and writes on the resulting file descriptor will be
performed directly to or from the user program buffer, provided
appropriate size and alignment restrictions are met and pages are
not locked in memory by any process. Refer to the F_SETFL and
F_DIOINFO commands in the fcntl(2) manual entry for information
about how to determine the alignment constraints. Refer to the
mlock(3C), read(2) and write(2) manual entries for information on
direct I/O involving pages that are locked in memory. O_DIRECT is
a Silicon Graphics extension and is only supported on local EFS
and XFS file systems, and remote BDS file systems. In Irix 6.5.24
and beyond, O_DIRECT is also supported on remote NFS Version 3
file systems.
When opening a STREAMS file, oflag may be constructed from O_NDELAY or
O_NONBLOCK OR-ed with either O_RDONLY, O_WRONLY , or O_RDWR. Other flag
values are not applicable to STREAMS devices and have no effect on them.
The values of O_NDELAY and O_NONBLOCK affect the operation of STREAMS
drivers and certain system calls [see read(2), getmsg(2), putmsg(2), and
write(2)]. For drivers, the implementation of O_NDELAY and O_NONBLOCK is
device specific. Each STREAMS device driver may treat these options
differently.
When open is invoked to open a named stream, and the connld module [see
connld(7)] has been pushed on the pipe, open blocks until the server
process has issued an I_RECVFD ioctl [see streamio(7)] to receive the
file descriptor.
If path is a symbolic link and O_CREAT and O_EXCL are set, the link is
not followed.
The file pointer used to mark the current position within the file is set
to the beginning of the file.
The new file descriptor is the lowest numbered file descriptor available
and is set to remain open across exec system calls [see fcntl(2)].
Certain flag values can be set following open as described in fcntl(2).
If O_CREAT is set and the file did not previously exist, upon successful
completion open marks for update the st_atime, st_ctime and st_mtime
fields of the file and the st_ctime and st_mtime fields of the parent
directory.
If O_TRUNC is set and the file did previously exist, upon successful
completion open marks for update the st_ctime and st_mtime fields of the
file.
There is a system enforced limit on the number of open file descriptors
per process {OPEN_MAX}, whose value is returned by the getdtablesize(2)
function.
The named file is opened unless one or more of the following are true:
EACCES The file does not exist and write permission is denied by the
parent directory of the file to be created.
EACCES O_CREAT or O_TRUNC is specified and write permission is denied.
EACCES A component of the path prefix denies search permission.
EACCES The file is a character or block device file and the file system
in which it resides has been mounted with the nodev option [see
fstab(4)].
EACCES oflag permission is denied for an existing file.
EAGAIN The file exists, O_CREAT or O_TRUNC are specified, mandatory
file/record locking is set, and there are outstanding record locks
on the file [see chmod(2)].
EBUSY path points to a device special file and the device is in the
closing state.
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 either because the user's quota of disk blocks on the
file system containing the directory has been exhausted or 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 are set, and the named file exists.
EFAULT path points outside the allocated address space of the process.
EINTR A signal was caught during the open system call.
EINVAL An attempt was made to open a file not in an EFS, XFS, BDS or NFS
Version 3 file system with O_DIRECT set.
EIO A hangup or error occurred during the open of the STREAMS-based
device.
EISDIR The named file is a directory and oflag is write or read/write.
ELOOP Too many symbolic links were encountered in translating path.
EMFILE The process has too many open files [see getrlimit(2)].
ENAMETOOLONG
The length of the path argument exceeds {PATH_MAX}, or the length
of a path component exceeds {NAME_MAX} while {_POSIX_NO_TRUNC} is
in effect.
ENFILE The system file table is full.
ENODEV path points to a device special file and the device is not in the
activated state.
ENOENT O_CREAT is not set and the named file does not exist.
ENOENT O_CREAT is set and a component of the path prefix does not exist
or is the null pathname.
ENOMEM The system is unable to allocate a send descriptor.
ENOSPC O_CREAT and O_EXCL are set, and the file system is out of inodes
or 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 set and the directory that would contain the file
cannot be extended.
ENOSR Unable to allocate a stream.
ENOTDIRA component of the path prefix 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.
ENXIO O_NDELAY or O_NONBLOCK is set, the named file is a FIFO, O_WRONLY
is set, and no process has the file open for reading.
ENXIO A STREAMS module or driver open routine failed.
EOPNOTSUPP
An attempt was made to open a socket (not currently supported).
EPERM path points to a device special file, the device is in the setup
state, and the calling process does not have the P_DEV privilege.
ETIMEDOUT
The object of the open is located on a remote system which is not
available [see intro(2)].
EROFS The named file resides on a read-only file system and either
O_WRONLY, O_RDWR, O_CREAT, or O_TRUNC is set in oflag (if the file
does not exist).
SEE ALSO
chmod(2), close(2), creat(2), dup(2), exec(2), fcntl(2),
getdtablesize(2), getmsg(2), getrlimit(2), intro(2), lseek(2), putmsg(2),
read(2), stat(2), stat(5), umask(2), write(2)
DIAGNOSTICS
Upon successful completion, the file descriptor is returned. Otherwise,
a value of -1 is returned and errno is set to indicate the error.
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