# memchr.m4 serial 13

dnl Copyright (C) 2002-2004, 2009-2018 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.

AC_DEFUN_ONCE([gl_FUNC_MEMCHR],

[

  AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles

  dnl Check for prerequisites for memory fence checks.

  gl_FUNC_MMAP_ANON

  AC_CHECK_HEADERS_ONCE([sys/mman.h])

  AC_CHECK_FUNCS_ONCE([mprotect])

  AC_REQUIRE([gl_HEADER_STRING_H_DEFAULTS])

  m4_ifdef([gl_FUNC_MEMCHR_OBSOLETE], [

    dnl These days, we assume memchr is present.  But if support for old

    dnl platforms is desired:

    AC_CHECK_FUNCS_ONCE([memchr])

    if test $ac_cv_func_memchr = no; then

      HAVE_MEMCHR=0

    fi

  ])

  if test $HAVE_MEMCHR = 1; then

    # Detect platform-specific bugs in some versions of glibc:

    # memchr should not dereference anything with length 0

    #   https://bugzilla.redhat.com/show_bug.cgi?id=499689

    # 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

    # 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([[

#include <string.h>

#if HAVE_SYS_MMAN_H

# include <fcntl.h>

# include <unistd.h>

# include <sys/types.h>

# include <sys/mman.h>

# ifndef MAP_FILE

#  define MAP_FILE 0

# endif

#endif

]], [[

  int result = 0;

  char *fence = NULL;

#if HAVE_SYS_MMAN_H && HAVE_MPROTECT

# if HAVE_MAP_ANONYMOUS

  const int flags = MAP_ANONYMOUS | MAP_PRIVATE;

  const int fd = -1;

# else /* !HAVE_MAP_ANONYMOUS */

  const int flags = MAP_FILE | MAP_PRIVATE;

  int fd = open ("/dev/zero", O_RDONLY, 0666);

  if (fd >= 0)

# endif

    {

      int pagesize = getpagesize ();

      char *two_pages =

        (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,

                       flags, fd, 0);

      if (two_pages != (char *)(-1)

          && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)

        fence = two_pages + pagesize;

    }

#endif

  if (fence)

    {

      if (memchr (fence, 0, 0))

        result |= 1;

      strcpy (fence - 9, "12345678");

      if (memchr (fence - 9, 0, 79) != fence - 1)

        result |= 2;

      if (memchr (fence - 1, 0, 3) != fence - 1)

        result |= 4;

    }

  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" ;;

          esac

         ])

      ])

    case "$gl_cv_func_memchr_works" in

      *yes) ;;

      *) REPLACE_MEMCHR=1 ;;

    esac

  fi

])

# Prerequisites of lib/memchr.c.

AC_DEFUN([gl_PREREQ_MEMCHR], [

  AC_CHECK_HEADERS([bp-sym.h])

])