From 753d5eee46d1d9c2c7f28192ae62d5da9d7d1408 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 25 Feb 2026 11:57:26 -0500
Subject: [PATCH] Allow PG_PRINTF_ATTRIBUTE to be different in C and C++ code.

Although clang claims to be compatible with gcc's printf format
archetypes, this appears to be a falsehood: it likes __syslog__
(which gcc does not, on most platforms) and doesn't accept
gnu_printf.  This means that if you try to use gcc with clang++
or clang with g++, you get compiler warnings when compiling
printf-like calls in our C++ code.  This has been true for quite
awhile, but it's gotten more annoying with the recent appearance
of several buildfarm members that are configured like this.

To fix, run separate probes for the format archetype to use with the
C and C++ compilers, and conditionally define PG_PRINTF_ATTRIBUTE
depending on __cplusplus.

(We could alternatively insist that you not mix-and-match C and
C++ compilers; but if the case works otherwise, this is a poor
reason to insist on that.)

This commit back-patches 0909380e4 into supported branches.

Discussion: https://postgr.es/m/986485.1764825548@sss.pgh.pa.us
Discussion: https://postgr.es/m/3988414.1771950285@sss.pgh.pa.us
Backpatch-through: 14-18
---
 config/c-compiler.m4       | 46 ++++++++++++++++--
 configure                  | 96 ++++++++++++++++++++++++++++++++++++--
 configure.ac               |  1 +
 meson.build                | 17 ++++++-
 src/include/c.h            | 10 ++++
 src/include/pg_config.h.in | 11 +++--
 6 files changed, 169 insertions(+), 12 deletions(-)

diff --git a/config/c-compiler.m4 b/config/c-compiler.m4
index da40bd6a647..8de232ec050 100644
--- a/config/c-compiler.m4
+++ b/config/c-compiler.m4
@@ -7,10 +7,10 @@
 # Select the format archetype to be used by gcc to check printf-type functions.
 # We prefer "gnu_printf", as that most closely matches the features supported
 # by src/port/snprintf.c (particularly the %m conversion spec).  However,
-# on some NetBSD versions, that doesn't work while "__syslog__" does.
-# If all else fails, use "printf".
+# on clang and on some NetBSD versions, that doesn't work while "__syslog__"
+# does.  If all else fails, use "printf".
 AC_DEFUN([PGAC_PRINTF_ARCHETYPE],
-[AC_CACHE_CHECK([for printf format archetype], pgac_cv_printf_archetype,
+[AC_CACHE_CHECK([for C printf format archetype], pgac_cv_printf_archetype,
 [pgac_cv_printf_archetype=gnu_printf
 PGAC_TEST_PRINTF_ARCHETYPE
 if [[ "$ac_archetype_ok" = no ]]; then
@@ -20,8 +20,8 @@ if [[ "$ac_archetype_ok" = no ]]; then
     pgac_cv_printf_archetype=printf
   fi
 fi])
-AC_DEFINE_UNQUOTED([PG_PRINTF_ATTRIBUTE], [$pgac_cv_printf_archetype],
-[Define to best printf format archetype, usually gnu_printf if available.])
+AC_DEFINE_UNQUOTED([PG_C_PRINTF_ATTRIBUTE], [$pgac_cv_printf_archetype],
+[Define to best C printf format archetype, usually gnu_printf if available.])
 ])# PGAC_PRINTF_ARCHETYPE
 
 # Subroutine: test $pgac_cv_printf_archetype, set $ac_archetype_ok to yes or no
@@ -38,6 +38,42 @@ ac_c_werror_flag=$ac_save_c_werror_flag
 ])# PGAC_TEST_PRINTF_ARCHETYPE
 
 
+# PGAC_CXX_PRINTF_ARCHETYPE
+# -------------------------
+# Because we support using gcc as C compiler with clang as C++ compiler,
+# we have to be prepared to use different printf archetypes in C++ code.
+# So, do the above test all over in C++.
+AC_DEFUN([PGAC_CXX_PRINTF_ARCHETYPE],
+[AC_CACHE_CHECK([for C++ printf format archetype], pgac_cv_cxx_printf_archetype,
+[pgac_cv_cxx_printf_archetype=gnu_printf
+PGAC_TEST_CXX_PRINTF_ARCHETYPE
+if [[ "$ac_archetype_ok" = no ]]; then
+  pgac_cv_cxx_printf_archetype=__syslog__
+  PGAC_TEST_CXX_PRINTF_ARCHETYPE
+  if [[ "$ac_archetype_ok" = no ]]; then
+    pgac_cv_cxx_printf_archetype=printf
+  fi
+fi])
+AC_DEFINE_UNQUOTED([PG_CXX_PRINTF_ATTRIBUTE], [$pgac_cv_cxx_printf_archetype],
+[Define to best C++ printf format archetype, usually gnu_printf if available.])
+])# PGAC_CXX_PRINTF_ARCHETYPE
+
+# Subroutine: test $pgac_cv_cxx_printf_archetype, set $ac_archetype_ok to yes or no
+AC_DEFUN([PGAC_TEST_CXX_PRINTF_ARCHETYPE],
+[ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
+AC_LANG_PUSH(C++)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+[extern void pgac_write(int ignore, const char *fmt,...)
+__attribute__((format($pgac_cv_cxx_printf_archetype, 2, 3)));],
+[pgac_write(0, "error %s: %m", "foo");])],
+                  [ac_archetype_ok=yes],
+                  [ac_archetype_ok=no])
+AC_LANG_POP([])
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
+])# PGAC_TEST_CXX_PRINTF_ARCHETYPE
+
+
 # PGAC_TYPE_128BIT_INT
 # --------------------
 # Check if __int128 is a working 128 bit integer type, and if so
diff --git a/configure b/configure
index 540f911863f..2f1377b7ee0 100755
--- a/configure
+++ b/configure
@@ -14837,8 +14837,8 @@ _ACEOF
     ;;
 esac
 
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for printf format archetype" >&5
-$as_echo_n "checking for printf format archetype... " >&6; }
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for C printf format archetype" >&5
+$as_echo_n "checking for C printf format archetype... " >&6; }
 if ${pgac_cv_printf_archetype+:} false; then :
   $as_echo_n "(cached) " >&6
 else
@@ -14898,7 +14898,97 @@ fi
 $as_echo "$pgac_cv_printf_archetype" >&6; }
 
 cat >>confdefs.h <<_ACEOF
-#define PG_PRINTF_ATTRIBUTE $pgac_cv_printf_archetype
+#define PG_C_PRINTF_ATTRIBUTE $pgac_cv_printf_archetype
+_ACEOF
+
+
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for C++ printf format archetype" >&5
+$as_echo_n "checking for C++ printf format archetype... " >&6; }
+if ${pgac_cv_cxx_printf_archetype+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_cv_cxx_printf_archetype=gnu_printf
+ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
+ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+extern void pgac_write(int ignore, const char *fmt,...)
+__attribute__((format($pgac_cv_cxx_printf_archetype, 2, 3)));
+int
+main ()
+{
+pgac_write(0, "error %s: %m", "foo");
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_archetype_ok=yes
+else
+  ac_archetype_ok=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
+
+if [ "$ac_archetype_ok" = no ]; then
+  pgac_cv_cxx_printf_archetype=__syslog__
+  ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
+ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+extern void pgac_write(int ignore, const char *fmt,...)
+__attribute__((format($pgac_cv_cxx_printf_archetype, 2, 3)));
+int
+main ()
+{
+pgac_write(0, "error %s: %m", "foo");
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_archetype_ok=yes
+else
+  ac_archetype_ok=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
+
+  if [ "$ac_archetype_ok" = no ]; then
+    pgac_cv_cxx_printf_archetype=printf
+  fi
+fi
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_cxx_printf_archetype" >&5
+$as_echo "$pgac_cv_cxx_printf_archetype" >&6; }
+
+cat >>confdefs.h <<_ACEOF
+#define PG_CXX_PRINTF_ATTRIBUTE $pgac_cv_cxx_printf_archetype
 _ACEOF
 
 
diff --git a/configure.ac b/configure.ac
index 6007f573561..2335947522d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1681,6 +1681,7 @@ m4_defun([AC_PROG_CC_STDC], []) dnl We don't want that.
 AC_C_BIGENDIAN
 AC_C_INLINE
 PGAC_PRINTF_ARCHETYPE
+PGAC_CXX_PRINTF_ARCHETYPE
 PGAC_C_STATIC_ASSERT
 PGAC_C_TYPEOF
 PGAC_C_TYPES_COMPATIBLE
diff --git a/meson.build b/meson.build
index 34456b36bbf..8218cee9b4e 100644
--- a/meson.build
+++ b/meson.build
@@ -1909,6 +1909,8 @@ if cc.compiles('''
 endif
 
 
+# Select the format archetype to be used to check printf-type functions.
+#
 # Need to check a call with %m because netbsd supports gnu_printf but emits a
 # warning for each use of %m.
 printf_attributes = ['gnu_printf', '__syslog__', 'printf']
@@ -1923,11 +1925,24 @@ attrib_error_args = cc.get_supported_arguments('-Werror=format', '-Werror=ignore
 foreach a : printf_attributes
   if cc.compiles(testsrc.format(a),
       args: test_c_args + attrib_error_args, name: 'format ' + a)
-    cdata.set('PG_PRINTF_ATTRIBUTE', a)
+    cdata.set('PG_C_PRINTF_ATTRIBUTE', a)
     break
   endif
 endforeach
 
+# We need to repeat the test for C++ because gcc and clang prefer different
+# format archetypes.
+if llvm.found()
+  attrib_error_args = cpp.get_supported_arguments('-Werror=format', '-Werror=ignored-attributes')
+  foreach a : printf_attributes
+    if cpp.compiles(testsrc.format(a),
+        args: attrib_error_args, name: 'cppformat ' + a)
+      cdata.set('PG_CXX_PRINTF_ATTRIBUTE', a)
+      break
+    endif
+  endforeach
+endif
+
 
 if cc.has_function_attribute('visibility:default') and \
     cc.has_function_attribute('visibility:hidden')
diff --git a/src/include/c.h b/src/include/c.h
index 6dc44fa0129..508c007cedd 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -224,6 +224,16 @@
 #define PG_USED_FOR_ASSERTS_ONLY pg_attribute_unused()
 #endif
 
+/*
+ * Our C and C++ compilers may have different ideas about which printf
+ * archetype best represents what src/port/snprintf.c can do.
+ */
+#ifndef __cplusplus
+#define PG_PRINTF_ATTRIBUTE PG_C_PRINTF_ATTRIBUTE
+#else
+#define PG_PRINTF_ATTRIBUTE PG_CXX_PRINTF_ATTRIBUTE
+#endif
+
 /* GCC supports format attributes */
 #if defined(__GNUC__)
 #define pg_attribute_format_arg(a) __attribute__((format_arg(a)))
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index c4dc5d72bdb..14833363dd9 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -593,6 +593,14 @@
 /* Define to the version of this package. */
 #undef PACKAGE_VERSION
 
+/* Define to best C++ printf format archetype, usually gnu_printf if
+   available. */
+#undef PG_CXX_PRINTF_ATTRIBUTE
+
+/* Define to best C printf format archetype, usually gnu_printf if available.
+   */
+#undef PG_C_PRINTF_ATTRIBUTE
+
 /* Define to the name of a signed 128-bit integer type. */
 #undef PG_INT128_TYPE
 
@@ -609,9 +617,6 @@
 /* PostgreSQL minor version number */
 #undef PG_MINORVERSION_NUM
 
-/* Define to best printf format archetype, usually gnu_printf if available. */
-#undef PG_PRINTF_ATTRIBUTE
-
 /* PostgreSQL version as a string */
 #undef PG_VERSION
 
-- 
2.47.3