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(libc.info)Open-time Flags

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Open-time Flags
---------------

   The open-time flags specify options affecting how `open' will behave.
These options are not preserved once the file is open.  The exception to
this is `O_NONBLOCK', which is also an I/O operating mode and so it
_is_ saved.  Note: Opening and Closing Files, for how to call `open'.

   There are two sorts of options specified by open-time flags.

   * "File name translation flags" affect how `open' looks up the file
     name to locate the file, and whether the file can be created.

   * "Open-time action flags" specify extra operations that `open' will
     perform on the file once it is open.

   Here are the file name translation flags.

 - Macro: int O_CREAT
     If set, the file will be created if it doesn't already exist.

 - Macro: int O_EXCL
     If both `O_CREAT' and `O_EXCL' are set, then `open' fails if the
     specified file already exists.  This is guaranteed to never
     clobber an existing file.

 - Macro: int O_NONBLOCK
     This prevents `open' from blocking for a "long time" to open the
     file.  This is only meaningful for some kinds of files, usually
     devices such as serial ports; when it is not meaningful, it is
     harmless and ignored.  Often opening a port to a modem blocks
     until the modem reports carrier detection; if `O_NONBLOCK' is
     specified, `open' will return immediately without a carrier.

     Note that the `O_NONBLOCK' flag is overloaded as both an I/O
     operating mode and a file name translation flag.  This means that
     specifying `O_NONBLOCK' in `open' also sets nonblocking I/O mode;
     Note: Operating Modes.  To open the file without blocking but do
     normal I/O that blocks, you must call `open' with `O_NONBLOCK' set
     and then call `fcntl' to turn the bit off.

 - Macro: int O_NOCTTY
     If the named file is a terminal device, don't make it the
     controlling terminal for the process.  Note: Job Control, for
     information about what it means to be the controlling terminal.

     In the GNU system and 4.4 BSD, opening a file never makes it the
     controlling terminal and `O_NOCTTY' is zero.  However, other
     systems may use a nonzero value for `O_NOCTTY' and set the
     controlling terminal when you open a file that is a terminal
     device; so to be portable, use `O_NOCTTY' when it is important to
     avoid this.

   The following three file name translation flags exist only in the
GNU system.

 - Macro: int O_IGNORE_CTTY
     Do not recognize the named file as the controlling terminal, even
     if it refers to the process's existing controlling terminal
     device.  Operations on the new file descriptor will never induce
     job control signals.  Note: Job Control.

 - Macro: int O_NOLINK
     If the named file is a symbolic link, open the link itself instead
     of the file it refers to.  (`fstat' on the new file descriptor will
     return the information returned by `lstat' on the link's name.)

 - Macro: int O_NOTRANS
     If the named file is specially translated, do not invoke the
     translator.  Open the bare file the translator itself sees.

   The open-time action flags tell `open' to do additional operations
which are not really related to opening the file.  The reason to do them
as part of `open' instead of in separate calls is that `open' can do
them atomically.

 - Macro: int O_TRUNC
     Truncate the file to zero length.  This option is only useful for
     regular files, not special files such as directories or FIFOs.
     POSIX.1 requires that you open the file for writing to use
     `O_TRUNC'.  In BSD and GNU you must have permission to write the
     file to truncate it, but you need not open for write access.

     This is the only open-time action flag specified by POSIX.1.
     There is no good reason for truncation to be done by `open',
     instead of by calling `ftruncate' afterwards.  The `O_TRUNC' flag
     existed in Unix before `ftruncate' was invented, and is retained
     for backward compatibility.

   The remaining operating modes are BSD extensions.  They exist only
on some systems.  On other systems, these macros are not defined.

 - Macro: int O_SHLOCK
     Acquire a shared lock on the file, as with `flock'.  Note: File
     Locks.

     If `O_CREAT' is specified, the locking is done atomically when
     creating the file.  You are guaranteed that no other process will
     get the lock on the new file first.

 - Macro: int O_EXLOCK
     Acquire an exclusive lock on the file, as with `flock'.  Note:
     File Locks.  This is atomic like `O_SHLOCK'.