From e7e9318a9efa5ba9660abae8aab4372c9c91e351 Mon Sep 17 00:00:00 2001
From: Zdenek Zambersky <zzambers@redhat.com>
Date: Tue, 2 Sep 2025 15:18:15 +0200
Subject: [PATCH] backport 03428d73fc3618202627bbe9d9880a63221c1811

---
 doc/building.html             |  4 ++++
 doc/building.md               |  5 +++++
 make/autoconf/flags-cflags.m4 | 26 +++++++++++++++++++++++++-
 make/lib/Awt2dLibraries.gmk   |  1 -
 4 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/doc/building.html b/doc/building.html
index 13ee9b7b6fc..6433786904f 100644
--- a/doc/building.html
+++ b/doc/building.html
@@ -296,6 +296,10 @@ <h2 id="native-compiler-toolchain-requirements">Native Compiler (Toolchain) Requ
 </tr>
 </tbody>
 </table>
+<p>All compilers are expected to be able to compile to the C99 language standard,
+as some C99 features are used in the source code. Microsoft Visual Studio
+doesn't fully support C99 so in practice shared code is limited to using C99
+features that it does support.</p>
 <h3 id="gcc">gcc</h3>
 <p>The minimum accepted version of gcc is 4.8. Older versions will generate a warning by <code>configure</code> and are unlikely to work.</p>
 <p>The JDK is currently known to be able to compile with at least version 7.4 of gcc.</p>
diff --git a/doc/building.md b/doc/building.md
index 648be30b4af..d7eb913995f 100644
--- a/doc/building.md
+++ b/doc/building.md
@@ -313,6 +313,11 @@ issues.
 | Solaris            | Oracle Solaris Studio 12.4 (with compiler version 5.13) |
 | Windows            | Microsoft Visual Studio 2017 update 15.9.16             |
 
+All compilers are expected to be able to compile to the C99 language standard,
+as some C99 features are used in the source code. Microsoft Visual Studio
+doesn't fully support C99 so in practice shared code is limited to using C99
+features that it does support.
+
 ### gcc
 
 The minimum accepted version of gcc is 4.8. Older versions will generate a warning
diff --git a/make/autoconf/flags-cflags.m4 b/make/autoconf/flags-cflags.m4
index 8ead0e22d64..cb86f3acf09 100644
--- a/make/autoconf/flags-cflags.m4
+++ b/make/autoconf/flags-cflags.m4
@@ -563,7 +563,7 @@ AC_DEFUN([FLAGS_SETUP_CFLAGS_HELPER],
     TOOLCHAIN_CFLAGS="-errshort=tags"
 
     TOOLCHAIN_CFLAGS_JDK="-mt $TOOLCHAIN_FLAGS"
-    TOOLCHAIN_CFLAGS_JDK_CONLY="-xCC -Xa -W0,-noglobal $TOOLCHAIN_CFLAGS" # C only
+    TOOLCHAIN_CFLAGS_JDK_CONLY="-W0,-noglobal $TOOLCHAIN_CFLAGS" # C only
     TOOLCHAIN_CFLAGS_JDK_CXXONLY="-features=no%except -norunpath -xnolib" # CXX only
     TOOLCHAIN_CFLAGS_JVM="-template=no%extdef -features=no%split_init \
         -library=stlport4 -mt -features=no%except $TOOLCHAIN_FLAGS"
@@ -584,6 +584,30 @@ AC_DEFUN([FLAGS_SETUP_CFLAGS_HELPER],
     TOOLCHAIN_CFLAGS_JDK="-nologo -MD -Zc:wchar_t-"
   fi
 
+  # CFLAGS C language level for JDK sources (hotspot only uses C++)
+  # Ideally we would have a common level across all toolchains so that all sources
+  # are sure to conform to the same standard. Unfortunately neither our sources nor
+  # our toolchains are in a condition to support that. But what we loosely aim for is
+  # C99 level.
+  if test "x$TOOLCHAIN_TYPE" = xgcc || test "x$TOOLCHAIN_TYPE" = xclang || test "x$TOOLCHAIN_TYPE" = xxlc; then
+    # This raises the language level for older 4.8 gcc, while lowering it for later
+    # versions. clang and xlclang support the same flag.
+    LANGSTD_CFLAGS="-std=c99"
+  elif test "x$TOOLCHAIN_TYPE" = xsolstudio; then
+    # We can't turn on -std=c99 without breaking compilation of the splashscreen/png
+    # utilities. But we can enable c99 as below (previously achieved by using -Xa).
+    # It is the no_lib that makes the difference.
+    LANGSTD_CFLAGS="-xc99=all,no_lib"
+  elif test "x$TOOLCHAIN_TYPE" = xmicrosoft; then
+    # MSVC doesn't support C99/C11 explicitly, unless you compile as C++:
+    # LANGSTD_CFLAGS="/TP"
+    # but that requires numerous changes to the sources files. So we are limited
+    # to C89/C90 plus whatever extensions Visual Studio has decided to implement.
+    # This is the lowest bar for shared code.
+    LANGSTD_CFLAGS=""
+  fi
+  TOOLCHAIN_CFLAGS_JDK_CONLY="$LANGSTD_CFLAGS $TOOLCHAIN_CFLAGS_JDK_CONLY"
+
   # CFLAGS WARNINGS STUFF
   # Set JVM_CFLAGS warning handling
   if test "x$TOOLCHAIN_TYPE" = xgcc; then
diff --git a/make/lib/Awt2dLibraries.gmk b/make/lib/Awt2dLibraries.gmk
index 7eaf5119a0f..4ab8c9a6e11 100644
--- a/make/lib/Awt2dLibraries.gmk
+++ b/make/lib/Awt2dLibraries.gmk
@@ -382,7 +382,6 @@ $(eval $(call SetupJdkLibrary, BUILD_LIBLCMS, \
     OPTIMIZATION := HIGHEST, \
     CFLAGS := $(CFLAGS_JDKLIB) \
         $(LCMS_CFLAGS), \
-    CFLAGS_solaris := -xc99=no_lib, \
     CFLAGS_windows := -DCMS_IS_WINDOWS_, \
     EXTRA_HEADER_DIRS := \
         common/awt/debug \