1 files changed, 100 insertions, 0 deletions

diff --git a/gl/m4/fflush.m4 b/gl/m4/fflush.m4
new file mode 100644
index 00000000..43266ef3
--- /dev/null
+++ b/gl/m4/fflush.m4
@@ -0,0 +1,100 @@
+# fflush.m4 serial 18
+
+# Copyright (C) 2007-2021 Free Software Foundation, Inc.
+# This file is free software; the Free Software Foundation
+# gives unlimited permission to copy and/or distribute it,
+# with or without modifications, as long as this notice is preserved.
+
+dnl From Eric Blake
+
+dnl Find out how to obey POSIX semantics of fflush(stdin) discarding
+dnl unread input on seekable streams, rather than C99 undefined semantics.
+
+AC_DEFUN([gl_FUNC_FFLUSH],
+[
+  AC_REQUIRE([gl_STDIO_H_DEFAULTS])
+  gl_FUNC_FFLUSH_STDIN
+  case "$gl_cv_func_fflush_stdin" in
+    *yes) ;;
+    *) REPLACE_FFLUSH=1 ;;
+  esac
+])
+
+dnl Determine whether fflush works on input streams.
+dnl Sets gl_cv_func_fflush_stdin.
+
+AC_DEFUN([gl_FUNC_FFLUSH_STDIN],
+[
+  AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
+  AC_CHECK_HEADERS_ONCE([unistd.h])
+  AC_CACHE_CHECK([whether fflush works on input streams],
+    [gl_cv_func_fflush_stdin],
+    [echo hello world > conftest.txt
+     AC_RUN_IFELSE([AC_LANG_PROGRAM(
+       [[
+#include <stdio.h>
+#if HAVE_UNISTD_H
+# include <unistd.h>
+#else /* on Windows with MSVC */
+# include <io.h>
+#endif
+       ]GL_MDA_DEFINES],
+       [[FILE *f = fopen ("conftest.txt", "r");
+         char buffer[10];
+         int fd;
+         int c;
+         if (f == NULL)
+           return 1;
+         fd = fileno (f);
+         if (fd < 0 || fread (buffer, 1, 5, f) != 5)
+           { fclose (f); return 2; }
+         /* For deterministic results, ensure f read a bigger buffer.  */
+         if (lseek (fd, 0, SEEK_CUR) == 5)
+           { fclose (f); return 3; }
+         /* POSIX requires fflush-fseek to set file offset of fd.  This fails
+            on BSD systems and on mingw.  */
+         if (fflush (f) != 0 || fseek (f, 0, SEEK_CUR) != 0)
+           { fclose (f); return 4; }
+         if (lseek (fd, 0, SEEK_CUR) != 5)
+           { fclose (f); return 5; }
+         /* Verify behaviour of fflush after ungetc. See
+            <https://www.opengroup.org/austin/aardvark/latest/xshbug3.txt>  */
+         /* Verify behaviour of fflush after a backup ungetc.  This fails on
+            mingw.  */
+         c = fgetc (f);
+         ungetc (c, f);
+         fflush (f);
+         if (fgetc (f) != c)
+           { fclose (f); return 6; }
+         /* Verify behaviour of fflush after a non-backup ungetc.  This fails
+            on glibc 2.8 and on BSD systems.  */
+         c = fgetc (f);
+         ungetc ('@', f);
+         fflush (f);
+         if (fgetc (f) != c)
+           { fclose (f); return 7; }
+         fclose (f);
+         return 0;
+       ]])],
+       [gl_cv_func_fflush_stdin=yes],
+       [gl_cv_func_fflush_stdin=no],
+       [case "$host_os" in
+                  # Guess no on native Windows.
+          mingw*) gl_cv_func_fflush_stdin="guessing no" ;;
+          *)      gl_cv_func_fflush_stdin=cross ;;
+        esac
+       ])
+     rm conftest.txt
+    ])
+  case "$gl_cv_func_fflush_stdin" in
+    *yes) gl_func_fflush_stdin=1 ;;
+    *no)  gl_func_fflush_stdin=0 ;;
+    *)    gl_func_fflush_stdin='(-1)' ;;
+  esac
+  AC_DEFINE_UNQUOTED([FUNC_FFLUSH_STDIN], [$gl_func_fflush_stdin],
+    [Define to 1 if fflush is known to work on stdin as per POSIX.1-2008,
+     0 if fflush is known to not work, -1 if unknown.])
+])
+
+# Prerequisites of lib/fflush.c.
+AC_DEFUN([gl_PREREQ_FFLUSH], [:])