close

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



NAME
       close - close a file descriptor

SYNOPSIS
       #include <unistd.h>

       int close(int fd);

DESCRIPTION
       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
       close() returns zero on success.  On error, -1 is returned, and errno
       is set appropriately.

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

NOTES
       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.

       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.

   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.1-2013 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
       fcntl(2), fsync(2), open(2), shutdown(2), unlink(2), fclose(3)

COLOPHON
       This page is part of release 5.03 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                             2019-10-10                          CLOSE(2)