The open-time flags specify options affecting how
open will behave.
These options are not preserved once the file is open. The exception to
O_NONBLOCK, which is also an I/O operating mode and so it
is saved. See Opening and Closing Files, for how to call
There are two sorts of options specified by open-time flags.
openlooks up the file name to locate the file, and whether the file can be created.
openwill perform on the file once it is open.
Here are the file name translation flags.
If set, the file will be created if it doesn’t already exist.
O_EXCL are set, then
if the specified file already exists. This is guaranteed to never
clobber an existing file.
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,
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
open also sets nonblocking I/O mode;
see I/O Operating Modes. To open the file without blocking but do normal
I/O that blocks, you must call
O_NONBLOCK set and
fcntl to turn the bit off.
If the named file is a terminal device, don’t make it the controlling terminal for the process. See Job Control, for information about what it means to be the controlling terminal.
On GNU/Hurd systems and 4.4 BSD, opening a file never makes it the
controlling terminal and
O_NOCTTY is zero. However, GNU/Linux systems
and some other systems 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 on GNU/Hurd systems.
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. See Job Control.
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.)
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
can do them atomically.
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
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
ftruncate afterwards. The
O_TRUNC flag existed in
ftruncate was invented, and is retained for backward
The remaining operating modes are BSD extensions. They exist only on some systems. On other systems, these macros are not defined.
Acquire a shared lock on the file, as with
See File Locks.
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.