Re: [PATCH v2 4/9] gdb: add new helpers for retrieving a type's fully qualified name

[Matthieu Longo <matthieu.longo@arm.com>]

On 03/03/2026 18:59, Tom Tromey wrote:
>>>>>> Matthieu Longo <matthieu.longo@arm.com> writes:
> 
>> +/* Return the type's fully qualified name from a PyTypeObject.  */
>> +const char *
>> +gdb_py_tp_name (PyTypeObject *py_type) noexcept
>> +{
>> +#if PY_VERSION_HEX >= 0x030d0000
> ...
>> +#elif ! defined (Py_LIMITED_API)
> ...
>> +#else
>> +  #error "The version of Python you are using does not expose " \
>> +	 "PyType_GetFullyQualifiedName() as a part of the limited C API."
>> +#endif
> 
> I think this case can't actually be hit... can it?
> 

It currently can when you build GDB with python 3.11 for instance with --enable-py-limited-api=yes.
Unfortunately, I haven't added the logic to error when someone requests to build GDB against the limited API, but does not have a suitable version of Python.
This #error were a workaround to provide a meaningful message to the developer in this case.

> My reasoning is that the very first case is taken if either the limited
> API is in use (which requires 0x030e0000 as of this patch) or if Python
> is new enough.
> 
> So, all other cases fall into the non-limited case.
> 
> It would be better IMO to remove it if it cannot ever happen.
> 

I agree with you, all other cases should fall into the non-limited case.
To do so, it requires me to fix the configure script to detect the issue before the build (see below).

>> +#else
>> +  #error "The version of Python you are using does not expose Py_TYPE() "\
>> +	 "as a part of the limited C API."
> 
> I'd prefer that #error not be indented here.
> 

Removed in the patch below.

>> +++ b/gdb/python/py-obj-type.h
>> @@ -0,0 +1,31 @@
>> +/* Helpers related to Python object type
>> +
>> +/* Return the type's fully qualified name from a PyTypeObject.  */
>> +extern const char *
>> +gdb_py_tp_name (PyTypeObject *py_type) noexcept;
>> +
>> +/* Return the type's fully qualified name from a PyObject.  */
>> +extern const char *
>> +gdbpy_py_obj_tp_name (PyObject *self) noexcept;
> 
> Declarations in gdb don't have a newline after the return type.
> 

Fixed in the next revision.

> thanks,
> Tom

Here is the fix to remove #error macros, and detect the issue at configure time.
I will split the configure fix from the removal of the #error macros in the next revision.

Matthieu


diff --git a/gdb/configure.ac b/gdb/configure.ac
index a3343dec118..b7ae2f3bb3c 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -933,6 +933,7 @@ else
      # do except assume that the compiler will be able to find those files.
      python_includes=
      python_libs=
+    python_prefix=
      have_python_config=no
    fi

@@ -1062,6 +1063,13 @@ AC_SUBST(PYTHON_CPPFLAGS)
  AC_SUBST(PYTHON_LIBS)
  AM_CONDITIONAL(HAVE_PYTHON, test "${have_libpython}" != no)

+dnl Use --enable-py-limited-api to enable the build of GDB against the Python
+dnl limited API.
+dnl
+dnl no -   Disable the Python limited API.
+dnl yes -  Use the Python limited API to build GDB, error if the selected
+dnl        version of Python is not compatible with the Python limited API.
+
  # Check whether to build GDB against Python limited C API.
  AC_ARG_ENABLE([py-limited-api],
               [AS_HELP_STRING([--enable-py-limited-api],
@@ -1072,10 +1080,14 @@ AC_ARG_ENABLE([py-limited-api],
  if test "$enable_py_limited_api" = yes; then
    # The minimal Python limited API version is currently set to 3.14 for the
    # support of Py_TYPE().
-  # The choice of the minimal version for the Python limited API won't be frozen
-  # until the end of the migration.
-  AC_DEFINE(Py_LIMITED_API, 0x030e0000,
-           [Define if GDB should be built against the Python limited C API.])
+  # The choice of the minimal version for the Python limited API won't be
+  # frozen until the end of the migration.
+  AM_PYTHON_CHECK_VERSION([$python_prog], [3.14], [
+    AC_DEFINE(Py_LIMITED_API, 0x030e0000,
+      [Define if GDB should be built against the Python limited C API.])
+    ],[
+      AC_MSG_ERROR([Python limited API support requires at least Python version 3.14])
+    ])
  fi

  # -------------------- #
diff --git a/gdb/python/py-obj-type.c b/gdb/python/py-obj-type.c
index 034ebd26b8d..ea0b59a8447 100644
--- a/gdb/python/py-obj-type.c
+++ b/gdb/python/py-obj-type.c
@@ -33,7 +33,7 @@ gdb_py_tp_name (PyTypeObject *py_type) noexcept

    return PyUnicode_AsUTF8AndSize (fully_qualified_name, nullptr);

-#elif ! defined (Py_LIMITED_API)
+#else /* PY_VERSION_HEX < 0x030d0000 && ! defined (Py_LIMITED_API)  */
    /* For non-heap types, the fully qualified name corresponds to tp_name.  */
    if (! (PyType_GetFlags (py_type) & Py_TPFLAGS_HEAPTYPE))
      return py_type->tp_name;
@@ -62,10 +62,6 @@ gdb_py_tp_name (PyTypeObject *py_type) noexcept

    return PyUnicode_AsUTF8AndSize (ht->ht_qualname, nullptr);
  # endif
-
-#else
-  #error "The version of Python you are using does not expose " \
-        "PyType_GetFullyQualifiedName() as a part of the limited C API."
  #endif
  }

@@ -74,11 +70,5 @@ const char *
  gdbpy_py_obj_tp_name (PyObject *self) noexcept
  {
    /* Note: Py_TYPE () is part of the stable ABI since version 3.14.  */
-#if ! defined (Py_LIMITED_API) \
-    || Py_LIMITED_API >= 0x030e0000
    return gdb_py_tp_name (Py_TYPE (self));
-#else
-  #error "The version of Python you are using does not expose Py_TYPE() "\
-        "as a part of the limited C API."
-#endif
  }