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. The
flags are specified by
or'ing the values listed below. Applications must specify exactly one of the first three values (file access methods):
O_RDONLY
Open for reading only.
O_WRONLY
Open for writing only.
O_RDWR
Open for reading and writing.
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, in which case the file is created with mode
mode 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_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 with
O_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.
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 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. Calling
getdtablesize(3) returns the current system limit.