path: root/man-pages-posix-2017/man3p/close.3p

'\" et
.TH CLOSE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
close
\(em close a file descriptor
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
int close(int \fIfildes\fP);
.fi
.SH DESCRIPTION
The
\fIclose\fR()
function shall deallocate the file descriptor indicated by
.IR fildes .
To deallocate means to make the file descriptor available for return by
subsequent calls to
\fIopen\fR()
or other functions that allocate file descriptors. All outstanding
record locks owned by the process on the file associated with the file
descriptor shall be removed (that is, unlocked).
.P
If
\fIclose\fR()
is interrupted by a signal that is to be caught, it shall return
\-1 with
.IR errno
set to
.BR [EINTR] 
and the state of
.IR fildes
is unspecified. If an I/O error occurred while reading from or writing
to the file system during
\fIclose\fR(),
it may return \-1 with
.IR errno
set to
.BR [EIO] ;
if this error is returned, the state of
.IR fildes
is unspecified.
.P
When all file descriptors associated with a pipe or FIFO special file
are closed, any data remaining in the pipe or FIFO shall be discarded.
.P
When all file descriptors associated with an open file description have
been closed, the open file description shall be freed.
.P
If the link count of the file is 0, when all file descriptors
associated with the file are closed, the space occupied by the file
shall be freed and the file shall no longer be accessible.
.P
If a STREAMS-based
.IR fildes
is closed and the calling process was previously registered to receive
a SIGPOLL signal
for events associated with that STREAM, the calling process shall be
unregistered for events associated with the STREAM. The last
\fIclose\fR()
for a STREAM shall cause the STREAM associated with
.IR fildes
to be dismantled. If O_NONBLOCK is not set and there have been no
signals posted for the STREAM,
and if there is data on the module's write queue,
\fIclose\fR()
shall wait for an unspecified time (for each module and driver) for
any output to drain before dismantling the STREAM. The time delay
can be changed via an I_SETCLTIME
\fIioctl\fR()
request. If the O_NONBLOCK flag is set, or if there are any pending
signals,
\fIclose\fR()
shall not wait for output to drain, and shall dismantle the STREAM
immediately.
.P
If the implementation supports STREAMS-based pipes, and
.IR fildes
is associated with one end of a pipe, the last
\fIclose\fR()
shall cause a hangup to occur on the other end of the pipe. In
addition, if the other end of the pipe has been named by
\fIfattach\fR(),
then the last
\fIclose\fR()
shall force the named end to be detached by
\fIfdetach\fR().
If the named end has no open file descriptors associated with it and
gets detached, the STREAM associated with that end shall also be
dismantled.
.P
If
.IR fildes
refers to the master side of a pseudo-terminal, and this is the last
close, a SIGHUP signal shall be sent to the
controlling process, if any, for which the slave side of the
pseudo-terminal is the controlling terminal. It is unspecified whether
closing the master side of the pseudo-terminal flushes all queued input
and output.
.P
If
.IR fildes
refers to the slave side of a STREAMS-based pseudo-terminal, a
zero-length message may be sent to the master.
.P
When there is an outstanding cancelable asynchronous I/O operation
against
.IR fildes
when
\fIclose\fR()
is called, that I/O operation may be canceled. An I/O operation that
is not canceled completes as if the
\fIclose\fR()
operation had not yet occurred. All operations that are not canceled
shall complete as if the
\fIclose\fR()
blocked until the operations completed. The
\fIclose\fR()
operation itself need not block awaiting such I/O completion. Whether
any I/O operation is canceled, and which I/O operation may be
canceled upon
\fIclose\fR(),
is implementation-defined.
.P
If a memory mapped file
or a shared memory object
remains referenced at the last close (that is, a process has
it mapped), then the entire contents of the memory object shall
persist until the memory object becomes unreferenced.
If this is the last close of a memory mapped file
or a shared memory object
and the close results in the memory object becoming unreferenced,
and the memory object has been unlinked, then the memory object
shall be removed.
.P
If
.IR fildes
refers to a socket,
\fIclose\fR()
shall cause the socket to be destroyed. If the socket is in
connection-mode, and the SO_LINGER option is set for the socket with
non-zero linger time, and the socket has untransmitted data, then
\fIclose\fR()
shall block for up to the current linger interval until all data is
transmitted.
.SH "RETURN VALUE"
Upon successful completion, 0 shall be returned; otherwise, \-1
shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIclose\fR()
function shall fail if:
.TP
.BR EBADF
The
.IR fildes
argument is not a open file descriptor.
.TP
.BR EINTR
The
\fIclose\fR()
function was interrupted by a signal.
.P
The
\fIclose\fR()
function may fail if:
.TP
.BR EIO
An I/O error occurred while reading from or writing to the file system.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Reassigning a File Descriptor"
.P
The following example closes the file descriptor associated with
standard output for the current process, re-assigns standard output to
a new file descriptor, and closes the original file descriptor to clean
up. This example assumes that the file descriptor 0 (which is the
descriptor for standard input) is not closed.
.sp
.RS 4
.nf

#include <unistd.h>
\&...
int pfd;
\&...
close(1);
dup(pfd);
close(pfd);
\&...
.fi
.P
.RE
.P
Incidentally, this is exactly what could be achieved using:
.sp
.RS 4
.nf

dup2(pfd, 1);
close(pfd);
.fi
.P
.RE
.SS "Closing a File Descriptor"
.P
In the following example,
\fIclose\fR()
is used to close
a file descriptor after an unsuccessful attempt is made to associate that
file descriptor with a stream.
.sp
.RS 4
.nf

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
.P
#define LOCKFILE "/etc/ptmp"
\&...
int pfd;
FILE *fpfd;
\&...
if ((fpfd = fdopen (pfd, "w")) == NULL) {
    close(pfd);
    unlink(LOCKFILE);
    exit(1);
}
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
An application that had used the
.IR stdio
routine
\fIfopen\fR()
to open a file should use the corresponding
\fIfclose\fR()
routine rather than
\fIclose\fR().
Once a file is closed, the file descriptor no longer exists, since the
integer corresponding to it no longer refers to a file.
.P
Implementations may use file descriptors that must be inherited into
child processes for the child process to remain conforming, such as for
message catalog or tracing purposes. Therefore, an application that calls
\fIclose\fR()
on an arbitrary integer risks non-conforming behavior, and
\fIclose\fR()
can only portably be used on file descriptor values that the application
has obtained through explicit actions, as well as the three file
descriptors corresponding to the standard file streams. In multi-threaded
parent applications, the practice of calling
\fIclose\fR()
in a loop after
\fIfork\fR()
and before an
.IR exec
call in order to avoid a race condition of leaking an unintended file
descriptor into a child process, is therefore unsafe, and the race should
instead be combatted by opening all file descriptors with the FD_CLOEXEC
bit set unless the file descriptor is intended to be inherited across
.IR exec .
.P
Usage of
\fIclose\fR()
on file descriptors STDIN_FILENO, STDOUT_FILENO, or STDERR_FILENO
should immediately be followed by an operation to reopen these file
descriptors. Unexpected behavior will result if any of these file
descriptors is left in a closed state (for example, an
.BR [EBADF] 
error from
\fIperror\fR())
or if an unrelated
\fIopen\fR()
or similar call later in the application accidentally allocates
a file to one of these well-known file descriptors. Furthermore, a
\fIclose\fR()
followed by a reopen operation (e.g.,
\fIopen\fR(),
\fIdup\fR(),
etc.) is not atomic;
\fIdup2\fR()
should be used to change standard file descriptors.
.SH RATIONALE
The use of interruptible device close routines should be discouraged to
avoid problems with the implicit closes of file descriptors by
.IR exec
and
\fIexit\fR().
This volume of POSIX.1\(hy2017 only intends to permit such behavior by specifying the
.BR [EINTR] 
error condition.
.P
Note that the requirement for
\fIclose\fR()
on a socket to block for up to the current linger interval is not
conditional on the O_NONBLOCK setting.
.P
The standard developers rejected a proposal to add
\fIclosefrom\fR()
to the standard. Because the standard permits implementations to
use inherited file descriptors as a means of providing a conforming
environment for the child process, it is not possible to standardize an
interface that closes arbitrary file descriptors above a certain value
while still guaranteeing a conforming environment.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.6" ", " "STREAMS",
.IR "\fIdup\fR\^(\|)",
.IR "\fIexec\fR\^",
.IR "\fIexit\fR\^(\|)",
.IR "\fIfattach\fR\^(\|)",
.IR "\fIfclose\fR\^(\|)",
.IR "\fIfdetach\fR\^(\|)",
.IR "\fIfopen\fR\^(\|)",
.IR "\fIfork\fR\^(\|)",
.IR "\fIioctl\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIperror\fR\^(\|)",
.IR "\fIunlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .