Re: [PATCH v3 2/7] gdb: fail configure if Python version is too old for limited API

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

On 24/03/2026 18:53, Tom Tromey wrote:
>>>>>> Matthieu Longo <matthieu.longo@arm.com> writes:
> 
>> GDB can be built against the Python limited API using the configure
>> flag '--enable-py-limited-api=yes'. This flag is currently experimental,
>> and the build is not yet fully successful. Today, the minimum required
>> Python version for this option is 3.11. This requirement is not final
>> and will be raised to a later version as the migration progresses.
> 
>> However, the configure script does not currently report an error if an
>> older version of Python is used. Instead, the build fails later with
>> numerous errors that are difficult to relate to Python limited API
>> compatiblity.
> 
>> This patch adds a version check when '--enable-py-limited-api=yes' is
>> specified, ensuring that the provided Python version meets the minimum
>> requirements for the limited API support. If it does not, configure will
>> now fail with a clear error message.
> 
> Sorry about the delay on this.
> 
> I'm a bit worried about adding a bunch of new python-related configury.
> Like are we sure it will agree with what's already in there?  As in,
> picking up the same version from the same place, etc?  For instance does
> this new code respect --with-python-libdir?
> 
> I wonder if instead there could just be a new compile test that is run
> when --enable-py-limited-api is used.  This test could just check
> PY_VERSION_HEX and #error on failure.
> 
> Wouldn't that have the same effect but without adding a ton of new code?
> 
> TBH I'd even be happy with not doing this check in configure at all and
> saying that if you specify --enable-py-limited-api then you should know
> what you're doing and if you mess up you'll just get compile-time
> errors.  That is, stick the check in python-internal.h.
> 
> Tom

So I followed your advice and added a compile test checking PY_VERSION_HEX and #erroring on failure.
Here below is the new patch that I posted here to avoid a new revision.

Matthieu


diff --git a/gdb/configure b/gdb/configure
index 12c54521682..16580e4af7d 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -28507,6 +28507,7 @@ fi
      # do except assume that the compiler will be able to find those files.
      python_includes=
      python_libs=
+    python_prefix=
      have_python_config=no
    fi

@@ -28743,6 +28744,7 @@ else
  fi


+
  # Check whether to build GDB against Python limited C API.
  # Check whether --enable-py-limited-api was given.
  if test "${enable_py_limited_api+set}" = set; then :
@@ -28763,11 +28765,46 @@ fi
  if test "$enable_py_limited_api" = yes; then
    # The minimal Python limited API version is currently set to 3.11 for the
    # support of PyBuffer_FillInfo and PyBuffer_Release.
-  # The choice of the minimal version for the Python limited API won't be frozen
-  # until the end of the migration.
+  # The choice of the minimal version for the Python limited API won't be
+  # frozen until the end of the migration.
+  old_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $PYTHON_CFLAGS"
+  old_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $PYTHON_CPPFLAGS"
+  old_LIBS="$LIBS"
+  LIBS="$LIBS $PYTHON_LIBS"
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <Python.h>
+#ifndef PY_VERSION_HEX
+#error "PY_VERSION_HEX is not defined"
+#endif
+#if PY_VERSION_HEX < 0x030b0000
+#error "Python limited API support requires at least Python version 3.11"
+#endif
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :

  $as_echo "#define Py_LIMITED_API 0x030b0000" >>confdefs.h

+
+else
+
+      as_fn_error $? "Python limited API support requires at least Python version 3.11" "$LINENO" 5
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  CFLAGS="$old_CFLAGS"
+  CPPFLAGS="$old_CPPFLAGS"
+  LIBS="$old_LIBS"
  fi

  # -------------------- #
diff --git a/gdb/configure.ac b/gdb/configure.ac
index cf8078e1d89..c6b0653e3b0 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 +1080,31 @@ AC_ARG_ENABLE([py-limited-api],
  if test "$enable_py_limited_api" = yes; then
    # The minimal Python limited API version is currently set to 3.11 for the
    # support of PyBuffer_FillInfo and PyBuffer_Release.
-  # The choice of the minimal version for the Python limited API won't be frozen
-  # until the end of the migration.
-  AC_DEFINE(Py_LIMITED_API, 0x030b0000,
-           [Define if GDB should be built against the Python limited C API.])
+  # The choice of the minimal version for the Python limited API won't be
+  # frozen until the end of the migration.
+  old_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $PYTHON_CFLAGS"
+  old_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $PYTHON_CPPFLAGS"
+  old_LIBS="$LIBS"
+  LIBS="$LIBS $PYTHON_LIBS"
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+    [[#include <Python.h>
+#ifndef PY_VERSION_HEX
+#error "PY_VERSION_HEX is not defined"
+#endif
+#if PY_VERSION_HEX < 0x030b0000
+#error "Python limited API support requires at least Python version 3.11"
+#endif
+    ]],[[]])],
+    [AC_DEFINE(Py_LIMITED_API, 0x030b0000,
+      [Define if GDB should be built against the Python limited C API.])
+    ],[
+      AC_MSG_ERROR([Python limited API support requires at least Python version 3.11])
+    ])
+  CFLAGS="$old_CFLAGS"
+  CPPFLAGS="$old_CPPFLAGS"
+  LIBS="$old_LIBS"
  fi

  # -------------------- #