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 to indicate the error.

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.

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

COLOPHON
       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)