close
—
delete a descriptor
Standard C Library (libc, -lc)
#include
<unistd.h>
int
close
(
int
fd);
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
1003.1-1988 (“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.
The
close
() function returns the
value 0 if successful; otherwise the value -1 is returned and
the global variable
errno is set to indicate
the error.
The
close
() system call will fail if:
- [
EBADF
]
- The fd argument is not an active
descriptor.
- [
EINTR
]
- An interrupt was received.
- [
ENOSPC
]
- The underlying object did not fit, cached data was lost.
- [
ECONNRESET
]
- The underlying object was a stream socket that was shut down by the peer
before all pending data was delivered.
In case of any error except
EBADF
, the
supplied file descriptor is deallocated and therefore is no longer valid.
accept(2),
closefrom(2),
execve(2),
fcntl(2),
flock(2),
open(2),
pipe(2),
socket(2),
socketpair(2)
The
close
() system call is expected to
conform to
IEEE Std 1003.1-1990
(“POSIX.1”).
The
close
() function appeared in
Version 1 AT&T UNIX.