[PATCH v2 2/2] [gdb] Enable ptype /o for some dynamic types

[Tom de Vries <tdevries@suse.de>]

Printing the offsets of a struct containing a flexible array member using
"ptype /o" currently fails:
...
$ cat test.c
struct s {
  int a;
  int b[];
};
struct s foo;
$ gcc -g test.c -c
$ gdb -q -batch test.o -ex "ptype /o struct s"
warning: ptype/o does not work with dynamic types; disabling '/o'
type = struct s {
    int a;
    int b[];
}
...

This has been the case since gdb 14, containing commit 0c1aa2a0953 ("Disable
ptype/o for dynamic types").

If we revert the commit, we get instead:
...
$ gdb -q -batch test.o -ex "ptype /o struct s"
/* offset      |    size */  type = struct s {
/*      0      |       4 */    int a;
/*      4      |       0 */    int b[];

                               /* total size (bytes):    4 */
                             }
...
which is similar to what pahole prints:
...
struct s {
	int                        a;                    /*     0     4 */
	int                        b[];                  /*     4     0 */

	/* size: 4, cachelines: 1, members: 2 */
	/* last cacheline: 4 bytes */
};
...

The problem is that the commit uses is_dynamic_type:
...
  if (flags.print_offsets && is_dynamic_type (type))
    {
      warning (_("ptype/o does not work with dynamic types; disabling '/o'"));
      flags.print_offsets = 0;
    }
...
which is too restrictive.

Fix this by using a new function cannot_print_offsets instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33966
---
 gdb/gdbtypes.c                             | 39 ++++++++++++--
 gdb/gdbtypes.h                             |  4 ++
 gdb/testsuite/gdb.ada/ptype-o.exp          |  2 +-
 gdb/testsuite/gdb.base/ptype-offsets-c.c   | 37 +++++++++++++
 gdb/testsuite/gdb.base/ptype-offsets-c.exp | 63 ++++++++++++++++++++++
 gdb/typeprint.c                            |  4 +-
 6 files changed, 143 insertions(+), 6 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/ptype-offsets-c.c
 create mode 100644 gdb/testsuite/gdb.base/ptype-offsets-c.exp

diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index 00ef8122978..8e90ded1d8c 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -1910,10 +1910,11 @@ array_type_has_dynamic_stride (struct type *type)
   return prop != nullptr && prop->is_constant ();
 }
 
-/* Worker for is_dynamic_type.  */
+/* Worker for is_dynamic_type/cannot_print_offsets.  */
 
 static bool
-is_dynamic_type_internal_1 (struct type *type)
+is_dynamic_type_internal_1 (struct type *type,
+			    bool cannot_print_offsets_p = false)
 {
   type = check_typedef (type);
 
@@ -1988,7 +1989,24 @@ is_dynamic_type_internal_1 (struct type *type)
 	      continue;
 	    /* If the field has dynamic type, then so does TYPE.  */
 	    if (is_dynamic_type_internal_1 (f.type ()))
-	      return true;
+	      {
+		bool last_struct_field_p
+		  = (type->code () == TYPE_CODE_STRUCT
+		     && i == type->num_fields () - 1);
+		if (cannot_print_offsets_p && last_struct_field_p)
+		  {
+		    if (f.type ()->code () == TYPE_CODE_STRUCT)
+		      /* The last field is a dynamic type and a struct.  Check
+			 if we can print the offsets for the struct.  */
+		      return is_dynamic_type_internal_1 (f.type (), true);
+
+		    /* The last field is a dynamic type, this is ok to print
+		       offsets for.  */
+		    return false;
+		  }
+
+		return true;
+	      }
 	    /* If the field is at a fixed offset, then it is not
 	       dynamic.  */
 	    if (!f.loc_is_dwarf_block ())
@@ -2031,6 +2049,21 @@ is_dynamic_type (struct type *type)
   return is_dynamic_type_internal (type, true);
 }
 
+/* See gdbtypes.h.  */
+
+bool
+cannot_print_offsets (struct type *type)
+{
+  type = check_typedef (type);
+
+  /* We only want to recognize references and pointers at the outermost
+     level.  */
+  if (type->is_pointer_or_reference ())
+    type = check_typedef (type->target_type ());
+
+  return is_dynamic_type_internal_1 (type, true);
+}
+
 static struct type *resolve_dynamic_type_internal
   (struct type *type, const property_addr_info *addr_stack,
    const frame_info_ptr &frame, bool top_level);
diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index 68c272d5fe5..29b03221cb2 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -2644,6 +2644,10 @@ extern struct type *resolve_dynamic_type
    "dynamic".  */
 extern bool is_dynamic_type (struct type *type);
 
+/* Return true if TYPE cannot be printed using ptype /o.  */
+
+extern bool cannot_print_offsets (struct type *type);
+
 /* Resolve any dynamic components of FIELD.  FIELD is updated.
    ADDR_STACK and FRAME are used where necessary to supply information
    for the resolution process; see resolve_dynamic_type.
diff --git a/gdb/testsuite/gdb.ada/ptype-o.exp b/gdb/testsuite/gdb.ada/ptype-o.exp
index 3bac7930762..03de262ecc1 100644
--- a/gdb/testsuite/gdb.ada/ptype-o.exp
+++ b/gdb/testsuite/gdb.ada/ptype-o.exp
@@ -37,7 +37,7 @@ foreach_gnat_encoding scenario flags {all minimal} {
 	"Warning: the current language does not match this frame."
 
     if {$scenario == "minimal"} {
-	set exp "ptype/o does not work with dynamic types.*"
+	set exp "ptype/o does not work with this dynamic type.*"
     } else {
 	# In "all" mode this prints nonsense, but at least does not
 	# crash.
diff --git a/gdb/testsuite/gdb.base/ptype-offsets-c.c b/gdb/testsuite/gdb.base/ptype-offsets-c.c
new file mode 100644
index 00000000000..74ac9ed7b37
--- /dev/null
+++ b/gdb/testsuite/gdb.base/ptype-offsets-c.c
@@ -0,0 +1,37 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2026 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+struct flexible_array_member
+{
+  int an_int;
+  int fam[];
+};
+
+struct nested_flexible_array_member
+{
+  int another_int;
+  struct flexible_array_member sfam;
+};
+
+int
+main (void)
+{
+  struct flexible_array_member fam;
+  struct nested_flexible_array_member nfam;
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/ptype-offsets-c.exp b/gdb/testsuite/gdb.base/ptype-offsets-c.exp
new file mode 100644
index 00000000000..cffcbbeb2f7
--- /dev/null
+++ b/gdb/testsuite/gdb.base/ptype-offsets-c.exp
@@ -0,0 +1,63 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This testcase exercises the "ptype /o" feature, which can be used to
+# print the offsets and sizes of each field of a struct/union.
+#
+# This is similar to ptype-offsets.exp, which uses C++ instead of C.
+
+standard_testfile .c
+
+# Test only works on LP64 targets.  That's how we guarantee that the
+# expected holes will be present in the struct.
+if { ![is_lp64_target] } {
+    untested "test work only on lp64 targets"
+    return 0
+}
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
+    return -1
+}
+
+# Tests handling flexible array member.  Regression tests for PR gdb/33966.
+set l {
+    "ptype /o struct flexible_array_member"
+    "/* offset      |    size */  type = struct flexible_array_member {"
+    "/*      0      |       4 */    int an_int;"
+    "/*      4      |       0 */    int fam[];"
+    ""
+    "                               /* total size (bytes):    4 */"
+    "                             }"
+}
+gdb_test "ptype /o struct flexible_array_member" \
+    [string_to_regexp [multi_line {*}$l]]
+
+set l {
+    "/* offset      |    size */  type = struct nested_flexible_array_member {"
+    "/*      0      |       4 */    int another_int;"
+    "/*      4      |       4 */    struct flexible_array_member {"
+    "/*      4      |       4 */        int an_int;"
+    "/*      8      |       0 */        int fam[];"
+    ""
+    "                                   /* total size (bytes):    4 */"
+    "                               } sfam;"
+    ""
+    "                               /* total size (bytes):    8 */"
+    "                             }"
+}
+gdb_test "ptype /o struct nested_flexible_array_member" \
+    [string_to_regexp [multi_line {*}$l]]
diff --git a/gdb/typeprint.c b/gdb/typeprint.c
index e53a2b21af5..31a9c1814a3 100644
--- a/gdb/typeprint.c
+++ b/gdb/typeprint.c
@@ -451,9 +451,9 @@ whatis_exp (const char *exp, int show)
       type = val->type ();
     }
 
-  if (flags.print_offsets && is_dynamic_type (type))
+  if (flags.print_offsets && cannot_print_offsets (type))
     {
-      warning (_("ptype/o does not work with dynamic types; disabling '/o'"));
+      warning (_("ptype/o does not work with this dynamic type; disabling '/o'"));
       flags.print_offsets = 0;
     }
 
-- 
2.51.0