The
close()
system call deletes a descriptor from the per-process object
reference table.
If this is the last reference to the underlying object, the
object will be deactivated.
For example, on the last close of a file
the current
seek
pointer associated with the file is lost;
on the last close of a
socket(2)
associated naming information and queued data are discarded;
on the last close of a file holding an advisory lock
the lock is released (see further
flock(2)).
However, the semantics of System V and
IEEE Std 1003.1-88 ("
POSIX.1")
dictate that all
fcntl(2)
advisory record locks associated with a file for a given process
are removed when
any
file descriptor for that file is closed by that process.
When a process exits,
all associated file descriptors are freed, but since there is
a limit on active descriptors per processes, the
close()
system call
is useful when a large quantity of file descriptors are being handled.
When a process forks (see
fork(2)),
all descriptors for the new child process reference the same
objects as they did in the parent before the fork.
If a new process is then to be run using
execve(2),
the process would normally inherit these descriptors.
Most
of the descriptors can be rearranged with
dup2(2)
or deleted with
close()
before the
execve(2)
is attempted, but if some of these descriptors will still
be needed if the execve fails, it is necessary to arrange for them
to be closed if the execve succeeds.
For this reason, the call
"fcntl(d, F_SETFD, FD_CLOEXEC)"
is provided,
which arranges that a descriptor will be closed after a successful
execve; the call
"fcntl(d, F_SETFD, 0)"
restores the default,
which is to not close the descriptor.
