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
flock(2)).
When a process exits, all associated 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 calls
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() 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 only if the
execve() succeeds. For this reason, the system call
is provided, which arranges that a descriptor “
d” will be closed after a successful
execve(); the system call
restores the default, which is to not close descriptor “
d”.