+# Emit a header similar to that from diff -u; Print the simulated "diff"
+# command so that the order of arguments is clear. Don't bother with @@ lines.
+emit_diff_u_header_ ()
+{
+ printf '%s\n' "diff -u $*" \
+ "--- $1 1970-01-01" \
+ "+++ $2 1970-01-01"
+}
+
+# Arrange not to let diff or cmp operate on /dev/null,
+# since on some systems (at least OSF/1 5.1), that doesn't work.
+# When there are not two arguments, or no argument is /dev/null, return 2.
+# When one argument is /dev/null and the other is not empty,
+# cat the nonempty file to stderr and return 1.
+# Otherwise, return 0.
+compare_dev_null_ ()
+{
+ test $# = 2 || return 2
+
+ if test "x$1" = x/dev/null; then
+ test -s "$2" || return 0
+ emit_diff_u_header_ "$@"; sed 's/^/+/' "$2"
+ return 1
+ fi
+
+ if test "x$2" = x/dev/null; then
+ test -s "$1" || return 0
+ emit_diff_u_header_ "$@"; sed 's/^/-/' "$1"
+ return 1
+ fi
+
+ return 2
+}
+
+if diff_out_=`exec 2>/dev/null; diff -u "$0" "$0" < /dev/null` \
+ && diff -u Makefile "$0" 2>/dev/null | grep '^[+]#!' >/dev/null; then
+ # diff accepts the -u option and does not (like AIX 7 'diff') produce an
+ # extra space on column 1 of every content line.
+ if test -z "$diff_out_"; then
+ compare_ () { diff -u "$@"; }
+ else
+ compare_ ()
+ {
+ if diff -u "$@" > diff.out; then
+ # No differences were found, but Solaris 'diff' produces output
+ # "No differences encountered". Hide this output.
+ rm -f diff.out
+ true
+ else
+ cat diff.out
+ rm -f diff.out
+ false
+ fi
+ }
+ fi
+elif diff_out_=`exec 2>/dev/null; diff -c "$0" "$0" < /dev/null`; then
+ if test -z "$diff_out_"; then
+ compare_ () { diff -c "$@"; }
+ else
+ compare_ ()
+ {
+ if diff -c "$@" > diff.out; then
+ # No differences were found, but AIX and HP-UX 'diff' produce output
+ # "No differences encountered" or "There are no differences between the
+ # files.". Hide this output.
+ rm -f diff.out
+ true
+ else
+ cat diff.out
+ rm -f diff.out
+ false
+ fi
+ }
+ fi