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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug exp/26875] New: Incorrect value printed for address of first
 element of zero-length array
Date: Fri, 13 Nov 2020 09:32:33 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D26875

            Bug ID: 26875
           Summary: Incorrect value printed for address of first element
                    of zero-length array
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: normal
          Priority: P2
         Component: exp
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider test-case gdb.base/arrayidx.exp, augmented with fedora patch
https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-archer-vla-tests.p=
atch:
...
diff --git a/gdb/testsuite/gdb.base/arrayidx.c
b/gdb/testsuite/gdb.base/arrayidx.c
--- a/gdb/testsuite/gdb.base/arrayidx.c
+++ b/gdb/testsuite/gdb.base/arrayidx.c
@@ -17,6 +17,13 @@

 int array[] =3D {1, 2, 3, 4};

+#ifdef __GNUC__
+struct
+  {
+    int a[0];
+  } unbound;
+#endif
+
 int
 main (void)
 {
diff --git a/gdb/testsuite/gdb.base/arrayidx.exp
b/gdb/testsuite/gdb.base/arrayidx.exp
--- a/gdb/testsuite/gdb.base/arrayidx.exp
+++ b/gdb/testsuite/gdb.base/arrayidx.exp
@@ -49,4 +49,12 @@ gdb_test "print array" \
          "\\{\\\[0\\\] =3D 1, \\\[1\\\] =3D 2, \\\[2\\\] =3D 3, \\\[3\\\] =
=3D 4\\}" \
          "print array with array-indexes on"

-
+set test "p unbound.a =3D=3D &unbound.a\[0\]"
+gdb_test_multiple $test $test {
+    -re " =3D 1\r\n$gdb_prompt $" {
+       pass $test
+    }
+    -re "No symbol \"unbound\" in current context.\r\n$gdb_prompt $" {
+       unsupported "$test (no GCC)"
+    }
+}
...

We run into this FAIL:
...
p unbound.a =3D=3D &unbound.a[0]^M
$7 =3D 0^M
(gdb) FAIL: gdb.base/arrayidx.exp: p unbound.a =3D=3D &unbound.a[0]
...

A bit more probing:
...
(gdb) p unbound
$3 =3D {a =3D 0x601044}
(gdb) p &unbound
$4 =3D (struct {...} *) 0x601044
(gdb) p unbound.a
$5 =3D 0x601044
(gdb) p &unbound.a[0]
$6 =3D (int *) 0xffffffffefc93d84^M
...
reveals that we have a problem printing the address of the first element of=
 a
zero length array.

--=20
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