The operating modes affect how input and output operations using a file
descriptor work. These flags are set by open and can be fetched
and changed with fcntl.
— Macro: int O_APPEND
The bit that enables append mode for the file. If set, then all
write operations write the data at the end of the file, extending
it, regardless of the current file position. This is the only reliable
way to append to a file. In append mode, you are guaranteed that the
data you write will always go to the current end of the file, regardless
of other processes writing to the file. Conversely, if you simply set
the file position to the end of file and write, then another process can
extend the file after you set the file position but before you write,
resulting in your data appearing someplace before the real end of file.
— Macro: int O_NONBLOCK
The bit that enables nonblocking mode for the file. If this bit is set,
read requests on the file can return immediately with a failure
status if there is no input immediately available, instead of blocking.
Likewise, write requests can also return immediately with a
failure status if the output can't be written immediately.
Note that the O_NONBLOCK flag is overloaded as both an I/O
operating mode and a file name translation flag; see Open-time Flags.
— Macro: int O_NDELAY
This is an obsolete name for O_NONBLOCK, provided for
compatibility with BSD. It is not defined by the POSIX.1 standard.
The remaining operating modes are BSD and GNU extensions. They exist only
on some systems. On other systems, these macros are not defined.
— Macro: int O_ASYNC
The bit that enables asynchronous input mode. If set, then SIGIO
signals will be generated when input is available. See Interrupt Input.
Asynchronous input mode is a BSD feature.
— Macro: int O_FSYNC
The bit that enables synchronous writing for the file. If set, each
write call will make sure the data is reliably stored on disk before
Synchronous writing is a BSD feature.
— Macro: int O_SYNC
This is another name for O_FSYNC. They have the same value.
— Macro: int O_NOATIME
If this bit is set, read will not update the access time of the
file. See File Times. This is used by programs that do backups, so
that backing a file up does not count as reading it.
Only the owner of the file or the superuser may use this bit.
This is a GNU extension.
Published under the terms of the GNU General Public License