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

       close - close a file descriptor

       #include <unistd.h>

       int close(int fd);

       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 (re-
       gardless 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.

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

       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.

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

       A  successful  close does not guarantee that the data has been success-
       fully 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 hard-
       ware 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 per-
       forming 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 re-
       ported only on the final close() that releases the open  file  descrip-
       tion.   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  in-
       valid)  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 stan-

       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 implemen-
       tations,  where,  as with other errors that may be reported by close(),
       the file descriptor is guaranteed to be closed.  However, it also  per-
       mits  another possibility: that the implementation returns an EINTR er-
       ror and keeps the file descriptor open.  (According to  its  documenta-
       tion,  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.

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

       This  page  is  part of release 5.07 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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