close(2) — Linux manual page


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 appropriately.

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.

              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

       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

       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

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-06-09                         CLOSE(2)

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