GNU Info

(libc.info)Operating Modes

---

Next: Getting File Status Flags Prev: Open-time Flags Up: File Status Flags

---

Enter node , (file) or (file)node

I/O Operating Modes
-------------------

   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; Note: 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.  Note:
     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 returning.  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.  Note: 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.