close(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

CLOSE(2)                Linux Programmer's Manual               CLOSE(2)

NAME         top

       close - close a file descriptor

SYNOPSIS         top

       #include <unistd.h>

       int close(int fd);

DESCRIPTION         top

       close() closes a file descriptor, so that it no longer refers to
       any file and may be reused.  Any record locks (see fcntl(2)) held
       on the file it was associated with, and owned by the process, are
       removed (regardless of the file descriptor that was used to
       obtain the lock).

       If fd is the last file descriptor referring to the underlying
       open file description (see open(2)), the resources associated
       with the open file description are freed; if the file descriptor
       was the last reference to a file which has been removed using
       unlink(2), the file is deleted.

RETURN VALUE         top

       close() returns zero on success.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       EBADF  fd isn't a valid open file descriptor.

       EINTR  The close() call was interrupted by a signal; see
              signal(7).

       EIO    An I/O error occurred.

       ENOSPC, EDQUOT
              On NFS, these errors are not normally reported against the
              first write which exceeds the available storage space, but
              instead against a subsequent write(2), fsync(2), or
              close().

       See NOTES for a discussion of why close() should not be retried
       after an error.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES         top

       A successful close does not guarantee that the data has been
       successfully saved to disk, as the kernel uses the buffer cache
       to defer writes.  Typically, filesystems do not flush buffers
       when a file is closed.  If you need to be sure that the data is
       physically stored on the underlying disk, use fsync(2).  (It will
       depend on the disk hardware at this point.)

       The close-on-exec file descriptor flag can be used to ensure that
       a file descriptor is automatically closed upon a successful
       execve(2); see fcntl(2) for details.

   Multithreaded processes and close()
       It is probably unwise to close file descriptors while they may be
       in use by system calls in other threads in the same process.
       Since a file descriptor may be reused, there are some obscure
       race conditions that may cause unintended side effects.

       Furthermore, consider the following scenario where two threads
       are performing operations on the same file descriptor:

       1. One thread is blocked in an I/O system call on the file
          descriptor.  For example, it is trying to write(2) to a pipe
          that is already full, or trying to read(2) from a stream
          socket which currently has no available data.

       2. Another thread closes the file descriptor.

       The behavior in this situation varies across systems.  On some
       systems, when the file descriptor is closed, the blocking system
       call returns immediately with an error.

       On Linux (and possibly some other systems), the behavior is
       different: the blocking I/O system call holds a reference to the
       underlying open file description, and this reference keeps the
       description open until the I/O system call completes.  (See
       open(2) for a discussion of open file descriptions.)  Thus, the
       blocking system call in the first thread may successfully
       complete after the close() in the second thread.

   Dealing with error returns from close()
       A careful programmer will check the return value of close(),
       since it is quite possible that errors on a previous write(2)
       operation are reported only on the final close() that releases
       the open file description.  Failing to check the return value
       when closing a file may lead to silent loss of data.  This can
       especially be observed with NFS and with disk quota.

       Note, however, that a failure return should be used only for
       diagnostic purposes (i.e., a warning to the application that
       there may still be I/O pending or there may have been failed I/O)
       or remedial purposes (e.g., writing the file once more or
       creating a backup).

       Retrying the close() after a failure return is the wrong thing to
       do, since this may cause a reused file descriptor from another
       thread to be closed.  This can occur because the Linux kernel
       always releases the file descriptor early in the close operation,
       freeing it for reuse; the steps that may return an error, such as
       flushing data to the filesystem or device, occur only later in
       the close operation.

       Many other implementations similarly always close the file
       descriptor (except in the case of EBADF, meaning that the file
       descriptor was invalid) even if they subsequently report an error
       on return from close().  POSIX.1 is currently silent on this
       point, but there are plans to mandate this behavior in the next
       major release of the standard.

       A careful programmer who wants to know about I/O errors may
       precede close() with a call to fsync(2).

       The EINTR error is a somewhat special case.  Regarding the EINTR
       error, POSIX.1-2008 says:

              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.

       This permits the behavior that occurs on Linux and many other
       implementations, where, as with other errors that may be reported
       by close(), the file descriptor is guaranteed to be closed.
       However, it also permits another possibility: that the
       implementation returns an EINTR error and keeps the file
       descriptor open.  (According to its documentation, HP-UX's
       close() does this.)  The caller must then once more use close()
       to close the file descriptor, to avoid file descriptor leaks.
       This divergence in implementation behaviors provides a difficult
       hurdle for portable applications, since on many implementations,
       close() must not be called again after an EINTR error, and on at
       least one, close() must be called again.  There are plans to
       address this conundrum for the next major release of the POSIX.1
       standard.

SEE ALSO         top

       close_range(2), fcntl(2), fsync(2), open(2), shutdown(2),
       unlink(2), fclose(3)

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                       CLOSE(2)

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