From e916ccf0892407602ca1436bd50ad48684215d4f Mon Sep 17 00:00:00 2001
From: Jakob Blomer <jblomer@cern.ch>
Date: Thu, 5 Nov 2026 14:00:18 +0100
Subject: [PATCH 1/2] add missing includes

Found by compiling with clang/libc++ on Linux.
---
 interpreter/cling/include/cling/Interpreter/RuntimeUniverse.h | 1 +
 main/src/rmain.cxx                                            | 1 +
 tree/treeplayer/src/TTreeFormulaManager.cxx                   | 2 ++
 3 files changed, 4 insertions(+)

diff --git a/interpreter/cling/include/cling/Interpreter/RuntimeUniverse.h b/interpreter/cling/include/cling/Interpreter/RuntimeUniverse.h
index c6f0f2eab0972..72b39090b9658 100644
--- a/interpreter/cling/include/cling/Interpreter/RuntimeUniverse.h
+++ b/interpreter/cling/include/cling/Interpreter/RuntimeUniverse.h
@@ -25,6 +25,7 @@
 
 #include <cling/Interpreter/RuntimeOptions.h>
 #include "cling/Interpreter/Visibility.h"
+#include <cstddef>
 #include <new>
 
 namespace cling {
diff --git a/main/src/rmain.cxx b/main/src/rmain.cxx
index 3e0e0fe5c2dea..f5c108c772232 100644
--- a/main/src/rmain.cxx
+++ b/main/src/rmain.cxx
@@ -21,6 +21,7 @@
 #include "RConfigure.h"
 
 #include <cstdio>
+#include <cstdlib>
 #ifdef _MSC_VER
 #include <process.h>
 #define execv _execv
diff --git a/tree/treeplayer/src/TTreeFormulaManager.cxx b/tree/treeplayer/src/TTreeFormulaManager.cxx
index 8ef1fad85302d..55cd9d400a0ca 100644
--- a/tree/treeplayer/src/TTreeFormulaManager.cxx
+++ b/tree/treeplayer/src/TTreeFormulaManager.cxx
@@ -15,6 +15,8 @@
 #include "TError.h"
 #include "TLeafElement.h"
 
+#include <cmath>
+
 
    /** \class TTreeFormulaManager
    Used to coordinate one or more TTreeFormula objects.

From c9e90353c61b19e56ff4acdcc54fd720c71d299e Mon Sep 17 00:00:00 2001
From: Jakob Blomer <jblomer@cern.ch>
Date: Thu, 5 Nov 2026 13:57:47 +0100
Subject: [PATCH 2/2] [cmake] check std::vector layout

The std::vector layout, currently, uses three pointers (begin, end, end
of capacity). This is used by the ROOT I/O to work with vectors of
anything by treating any vector as a vector<char>.  The approach breaks,
however, with the std::vector layout in the libc++ unstable ABI that
implements vector with one pointer and two integers.
---
 cmake/modules/CheckCompiler.cmake     | 18 ++++++++++++++++++
 cmake/modules/check_libcpp_vector.cpp | 27 +++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 cmake/modules/check_libcpp_vector.cpp

diff --git a/cmake/modules/CheckCompiler.cmake b/cmake/modules/CheckCompiler.cmake
index bab91fd76e5f3..524ce7bbc8bb7 100644
--- a/cmake/modules/CheckCompiler.cmake
+++ b/cmake/modules/CheckCompiler.cmake
@@ -261,3 +261,21 @@ endif()
 if(gnuinstall)
   set(R__HAVE_CONFIG 1)
 endif()
+
+
+#---Check if the std::vector implementation is using three pointers or one pointer and two integers
+message(STATUS "Checking std::vector layout")
+try_run(RUN_CHECK_LIBCPP_VECTOR_RESULT COMPILE_CHECK_LIBCPP_VECTOR_RESULT
+      ${CMAKE_BINARY_DIR}
+      SOURCES "${CMAKE_SOURCE_DIR}/cmake/modules/check_libcpp_vector.cpp"
+      COMPILE_OUTPUT_VARIABLE COMPILE_CHECK_LIBCPP_VECTOR_OUTPUT)
+
+if(NOT COMPILE_CHECK_LIBCPP_VECTOR_RESULT)
+   message(FATAL_ERROR "Failed to compile vector layout check, "
+           "likely an issue with the compiler installation: ${COMPILE_CHECK_LIBCPP_VECTOR_OUTPUT}")
+endif()
+
+if(NOT RUN_CHECK_LIBCPP_VECTOR_RESULT EQUAL 0)
+  message(FATAL_ERROR "Size based std::vector layout is unsupported; "
+          "run result='${RUN_CHECK_LIBCPP_VECTOR_RESULT}' (1 = size based, 2 = unexpected).")
+endif()
diff --git a/cmake/modules/check_libcpp_vector.cpp b/cmake/modules/check_libcpp_vector.cpp
new file mode 100644
index 0000000000000..7d123aa2cbbc6
--- /dev/null
+++ b/cmake/modules/check_libcpp_vector.cpp
@@ -0,0 +1,27 @@
+#include <cstddef>
+#include <vector>
+
+#if defined(_MSC_VER)
+  #define NOINLINE __declspec(noinline)
+#else
+  #define NOINLINE __attribute__((noinline))
+#endif
+
+// Don't inline to prevent the optimizer from interfering with the test
+NOINLINE static std::size_t GetTypeErasedSize(const void *vec)
+{
+   return reinterpret_cast<const std::vector<char> *>(vec)->size();
+}
+
+int main()
+{
+   std::vector<int> v(1);
+   const auto size = GetTypeErasedSize(reinterpret_cast<const void *>(&v));
+   if (size == sizeof(int)) {
+      return 0;
+   } else if (size == 1) {
+      return 1; // New ABI
+   } else {
+      return 2; // Unexpected
+   }
+}