memchr: Work around bug on Android <= 5.0

[Bruno Haible]

On Android 4.3, I'm seeing this test failure from a testdir:

../../gltests/test-memchr.c:60: assertion 'MEMCHR (input + 1, 0x789abc00 | 'e', 
n - 1) == input + n - 2' failed
FAIL test-memchr (exit status: 139)


2019-01-24  Bruno Haible  <address@hidden>

        memchr: Work around bug on Android <= 5.0.
        * m4/memchr.m4 (gl_FUNC_MEMCHR): Add test against the Android bug.
        * doc/posix-functions/memchr.texi: Mention the Android bug.

diff --git a/doc/posix-functions/memchr.texi b/doc/posix-functions/memchr.texi
index 68a15a2..c818ab9 100644
--- a/doc/posix-functions/memchr.texi
+++ b/doc/posix-functions/memchr.texi
@@ -11,6 +11,10 @@ Portability problems fixed by either Gnulib module 
@code{memchr} or @code{memchr
 @item
 This function dereferences too much memory on some platforms:
 glibc 2.10 on x86_64, IA-64; glibc 2.11 on Alpha.
address@hidden
+This function returns NULL if the character argument is not in the range
+of an @code{unsigned char} on some platforms:
+Android 5.0.
 @end itemize
 
 Portability problems fixed by Gnulib module @code{memchr-obsolete}:
diff --git a/m4/memchr.m4 b/m4/memchr.m4
index db227e7..0be394d 100644
--- a/m4/memchr.m4
+++ b/m4/memchr.m4
@@ -1,4 +1,4 @@
-# memchr.m4 serial 13
+# memchr.m4 serial 14
 dnl Copyright (C) 2002-2004, 2009-2019 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -29,6 +29,8 @@ AC_DEFUN_ONCE([gl_FUNC_MEMCHR],
     # memchr should not dereference overestimated length after a match
     #   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=521737
     #   https://sourceware.org/bugzilla/show_bug.cgi?id=10162
+    # memchr should cast the second argument to 'unsigned char'.
+    #   This bug exists in Android 4.3.
     # Assume that memchr works on platforms that lack mprotect.
     AC_CACHE_CHECK([whether memchr works], [gl_cv_func_memchr_works],
       [AC_RUN_IFELSE([AC_LANG_PROGRAM([[
@@ -74,15 +76,26 @@ AC_DEFUN_ONCE([gl_FUNC_MEMCHR],
       if (memchr (fence - 1, 0, 3) != fence - 1)
         result |= 4;
     }
+  /* Test against bug on Android 4.3.  */
+  {
+    char input[3];
+    input[0] = 'a';
+    input[1] = 'b';
+    input[2] = 'c';
+    if (memchr (input, 0x789abc00 | 'b', 3) != input + 1)
+      result |= 8;
+  }
   return result;
 ]])],
          [gl_cv_func_memchr_works=yes],
          [gl_cv_func_memchr_works=no],
          [case "$host_os" in
-                    # Guess yes on native Windows.
-            mingw*) gl_cv_func_memchr_works="guessing yes" ;;
-                    # Be pessimistic for now.
-            *)      gl_cv_func_memchr_works="guessing no" ;;
+                             # Guess no on Android.
+            linux*-android*) gl_cv_func_memchr_works="guessing no" ;;
+                             # Guess yes on native Windows.
+            mingw*)          gl_cv_func_memchr_works="guessing yes" ;;
+                             # Be pessimistic for now.
+            *)               gl_cv_func_memchr_works="guessing no" ;;
           esac
          ])
       ])