✏ EDIT: fcntl-o.m4

# fcntl-o.m4 serial 4
dnl Copyright (C) 2006, 2009-2016 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.

dnl Written by Paul Eggert.

# Test whether the flags O_NOATIME and O_NOFOLLOW actually work.
# Define HAVE_WORKING_O_NOATIME to 1 if O_NOATIME works, or to 0 otherwise.
# Define HAVE_WORKING_O_NOFOLLOW to 1 if O_NOFOLLOW works, or to 0 otherwise.
AC_DEFUN([gl_FCNTL_O_FLAGS],
[
  dnl Persuade glibc <fcntl.h> to define O_NOATIME and O_NOFOLLOW.
  dnl AC_USE_SYSTEM_EXTENSIONS was introduced in autoconf 2.60 and obsoletes
  dnl AC_GNU_SOURCE.
  m4_ifdef([AC_USE_SYSTEM_EXTENSIONS],
    [AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])],
    [AC_REQUIRE([AC_GNU_SOURCE])])

AC_CHECK_HEADERS_ONCE([unistd.h])
  AC_CHECK_FUNCS_ONCE([symlink])
  AC_CACHE_CHECK([for working fcntl.h], [gl_cv_header_working_fcntl_h],
    [AC_RUN_IFELSE(
       [AC_LANG_PROGRAM(
          [[#include <sys/types.h>
           #include <sys/stat.h>
           #if HAVE_UNISTD_H
           # include <unistd.h>
           #else /* on Windows with MSVC */
           # include <io.h>
           # include <stdlib.h>
           # defined sleep(n) _sleep ((n) * 1000)
           #endif
           #include <fcntl.h>
           #ifndef O_NOATIME
            #define O_NOATIME 0
           #endif
           #ifndef O_NOFOLLOW
            #define O_NOFOLLOW 0
           #endif
           static int const constants[] =
            {
              O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC, O_APPEND,
              O_NONBLOCK, O_SYNC, O_ACCMODE, O_RDONLY, O_RDWR, O_WRONLY
            };
          ]],
          [[
            int result = !constants;
            #if HAVE_SYMLINK
            {
              static char const sym[] = "conftest.sym";
              if (symlink ("/dev/null", sym) != 0)
                result |= 2;
              else
                {
                  int fd = open (sym, O_WRONLY | O_NOFOLLOW | O_CREAT, 0);
                  if (fd >= 0)
                    {
                      close (fd);
                      result |= 4;
                    }
                }
              if (unlink (sym) != 0 || symlink (".", sym) != 0)
                result |= 2;
              else
                {
                  int fd = open (sym, O_RDONLY | O_NOFOLLOW);
                  if (fd >= 0)
                    {
                      close (fd);
                      result |= 4;
                    }
                }
              unlink (sym);
            }
            #endif
            {
              static char const file[] = "confdefs.h";
              int fd = open (file, O_RDONLY | O_NOATIME);
              if (fd < 0)
                result |= 8;
              else
                {
                  struct stat st0;
                  if (fstat (fd, &st0) != 0)
                    result |= 16;
                  else
                    {
                      char c;
                      sleep (1);
                      if (read (fd, &c, 1) != 1)
                        result |= 24;
                      else
                        {
                          if (close (fd) != 0)
                            result |= 32;
                          else
                            {
                              struct stat st1;
                              if (stat (file, &st1) != 0)
                                result |= 40;
                              else
                                if (st0.st_atime != st1.st_atime)
                                  result |= 64;
                            }
                        }
                    }
                }
            }
            return result;]])],
       [gl_cv_header_working_fcntl_h=yes],
       [case $? in #(
        4) gl_cv_header_working_fcntl_h='no (bad O_NOFOLLOW)';; #(
        64) gl_cv_header_working_fcntl_h='no (bad O_NOATIME)';; #(
        68) gl_cv_header_working_fcntl_h='no (bad O_NOATIME, O_NOFOLLOW)';; #(
         *) gl_cv_header_working_fcntl_h='no';;
        esac],
       [gl_cv_header_working_fcntl_h=cross-compiling])])

case $gl_cv_header_working_fcntl_h in #(
  *O_NOATIME* | no | cross-compiling) ac_val=0;; #(
  *) ac_val=1;;
  esac
  AC_DEFINE_UNQUOTED([HAVE_WORKING_O_NOATIME], [$ac_val],
    [Define to 1 if O_NOATIME works.])

case $gl_cv_header_working_fcntl_h in #(
  *O_NOFOLLOW* | no | cross-compiling) ac_val=0;; #(
  *) ac_val=1;;
  esac
  AC_DEFINE_UNQUOTED([HAVE_WORKING_O_NOFOLLOW], [$ac_val],
    [Define to 1 if O_NOFOLLOW works.])
])

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