| OPEN(2) | System Calls Manual | OPEN(2) | 
open, openat —
#include <fcntl.h>
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:
O_NONBLOCKO_APPENDO_CREATO_TRUNCO_EXCLO_CREAT and the file already exists.O_SHLOCKO_EXLOCKO_NOFOLLOWO_CLOEXECO_NOSIGPIPEEPIPE instead of raising
      SIGPIPE.O_DSYNCO_SYNCO_RSYNCO_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_IOO_NOCTTYO_DIRECTTo 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_DIRECTORYO_REGULARO_ASYNCSIGIO 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.
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.
EACCES]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; or 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.EEXIST]O_CREAT
      and O_EXCL were specified and the file
    exists.EFAULT]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]open() operation was interrupted by a
    signal.EIO]O_CREAT.EISDIR]ELOOP]EMFILE]ENAMETOOLONG]NAME_MAX}
      characters, or an entire path name exceeded
      {PATH_MAX} characters.ENFILE]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; or
      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.ENOTDIR]O_DIRECTORY is specified and the last path
      component is not a directory.ENXIO]O_NONBLOCK and
      O_WRONLY is set and no process has the file open
      for reading.EOPNOTSUPP]O_SHLOCK
      or O_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]EROFS]ETXTBSY]open() call requests write access.In addition, openat() will fail if:
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.
open() function call appeared in
  Version 1 AT&T UNIX.
| September 2, 2019 | NetBSD 9.3 |