Manpages - close.3p

Table of Contents

Home DTOS Other Projects Knowledge Base Linux Manpages Community Contribute

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.

NAME

close — close a file descriptor

SYNOPSIS

  #include <unistd.h>
  int close(int fildes);

DESCRIPTION

The close/() function shall deallocate the file descriptor indicated by /fildes. To deallocate means to make the file descriptor available for return by subsequent calls to /open/() 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).

If close/() is interrupted by a signal that is to be caught, it shall return -1 with /errno set to [EINTR] and the state of fildes is unspecified. If an I/O error occurred while reading from or writing to the file system during close/(), it may return -1 with /errno set to [EIO]; if this error is returned, the state of fildes is unspecified.

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.

When all file descriptors associated with an open file description have been closed, the open file description shall be freed.

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.

If a STREAMS-based 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 close/() for a STREAM shall cause the STREAM associated with /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, /close/() 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 /ioctl/() request. If the O_NONBLOCK flag is set, or if there are any pending signals, /close/() shall not wait for output to drain, and shall dismantle the STREAM immediately.

If the implementation supports STREAMS-based pipes, and fildes is associated with one end of a pipe, the last /close/() 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 /fattach/(), then the last /close/() shall force the named end to be detached by /fdetach/(). 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.

If 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.

If fildes refers to the slave side of a STREAMS-based pseudo-terminal, a zero-length message may be sent to the master.

When there is an outstanding cancelable asynchronous I/O operation against fildes when /close/() is called, that I/O operation may be canceled. An I/O operation that is not canceled completes as if the /close/() operation had not yet occurred. All operations that are not canceled shall complete as if the /close/() blocked until the operations completed. The /close/() 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 /close/(), is implementation-defined.

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.

If fildes refers to a socket, /close/() 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 /close/() shall block for up to the current linger interval until all data is transmitted.

RETURN VALUE

Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS

The /close/() function shall fail if:

EBADF
The fildes argument is not a open file descriptor.
EINTR
The /close/() function was interrupted by a signal.

The /close/() function may fail if:

EIO
An I/O error occurred while reading from or writing to the file system.

The following sections are informative.

EXAMPLES

Reassigning a File Descriptor

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. 


    #include <unistd.h>
    ...
    int pfd;
    ...
    close(1);
    dup(pfd);
    close(pfd);
    ...

Incidentally, this is exactly what could be achieved using: 


    dup2(pfd, 1);
    close(pfd);

Closing a File Descriptor

In the following example, /close/() is used to close a file descriptor after an unsuccessful attempt is made to associate that file descriptor with a stream. 


    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    #define LOCKFILE "/etc/ptmp"
    ...
    int pfd;
    FILE *fpfd;
    ...
    if ((fpfd = fdopen (pfd, "w")) == NULL) {
        close(pfd);
        unlink(LOCKFILE);
        exit(1);
    }
    ...

APPLICATION USAGE

An application that had used the stdio routine /fopen/() to open a file should use the corresponding /fclose/() routine rather than /close/(). Once a file is closed, the file descriptor no longer exists, since the integer corresponding to it no longer refers to a file.

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 /close/() on an arbitrary integer risks non-conforming behavior, and /close/() 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 close/() in a loop after /fork/() and before an /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 exec.

Usage of /close/() 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 [EBADF] error from /perror/()) or if an unrelated /open/() or similar call later in the application accidentally allocates a file to one of these well-known file descriptors. Furthermore, a /close/() followed by a reopen operation (e.g., /open/(), /dup/(), etc.) is not atomic; /dup2/() should be used to change standard file descriptors.

RATIONALE

The use of interruptible device close routines should be discouraged to avoid problems with the implicit closes of file descriptors by exec and /exit/(). This volume of POSIX.1‐2017 only intends to permit such behavior by specifying the [EINTR] error condition.

Note that the requirement for /close/() on a socket to block for up to the current linger interval is not conditional on the O_NONBLOCK setting.

The standard developers rejected a proposal to add /closefrom/() 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.

FUTURE DIRECTIONS

None.

SEE ALSO

Section 2.6, STREAMS, /dup ( )/, /exec , //exit ( )/, /fattach ( )/, /fclose ( )/, /fdetach ( )/, /fopen ( )/, /fork ( )/, /ioctl ( )/, /open ( )/, /perror ( )/, /unlink ( )/

The Base Definitions volume of POSIX.1‐2017, <unistd.h>

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 .

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 .

Author: dt

Created: 2022-02-20 Sun 09:11