OPEN(2) | System Calls Manual | OPEN(2) |
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:
Combining O_RSYNC with O_DSYNC only, or specifying it without any other synchronized I/O integrity completion flag set, has no further effect.
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.
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.
In addition, openat() will fail if:
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.
July 29, 2013 | NetBSD 7.2 |