kconfig: Add symbols for C standard

CMakeLists.txt

@@ -200,7 +200,8 @@ elseif(CONFIG_SPEED_OPTIMIZATIONS)
 elseif(CONFIG_SIZE_OPTIMIZATIONS)
   set(OPTIMIZATION_FLAG ${OPTIMIZE_FOR_SIZE_FLAG}) # Default in kconfig
 else()
-  assert(0 "Unreachable code. Expected optimization level to have been chosen. See Kconfig.zephyr")
+  message(FATAL_ERROR
+    "Unreachable code. Expected optimization level to have been chosen. See Kconfig.zephyr")
 endif()
 
 if(NOT CONFIG_ARCH_IS_SET)
@@ -247,7 +248,8 @@ if(CONFIG_CPP)
     set(STD_CPP_DIALECT_FLAGS $<TARGET_PROPERTY:compiler-cpp,dialect_cpp2b>)
     list(APPEND CMAKE_CXX_COMPILE_FEATURES ${compile_features_cpp20})
   else()
-    assert(0 "Unreachable code. Expected C++ standard to have been chosen. See Kconfig.zephyr.")
+    message(FATAL_ERROR
+      "Unreachable code. Expected C++ standard to have been chosen. See Kconfig.zephyr.")
   endif()
   set(CMAKE_CXX_COMPILE_FEATURES ${CMAKE_CXX_COMPILE_FEATURES} PARENT_SCOPE)
 
@@ -955,7 +957,28 @@ if(CONFIG_USERSPACE)
 endif()
 
 get_property(CSTD GLOBAL PROPERTY CSTD)
-set_ifndef(CSTD c99)
+if(NOT DEFINED CSTD)
+  if(CONFIG_STD_C23)
+    set(CSTD c2x)
+  elseif(CONFIG_STD_C17)
+    set(CSTD c17)
+  elseif(CONFIG_STD_C11)
+    set(CSTD c11)
+  elseif(CONFIG_STD_C99)
+    set(CSTD c99)
+  elseif(CONFIG_STD_C90)
+    set(CSTD c90)
+  else()
+    message(FATAL_ERROR "Unreachable code. Expected C standard to have been chosen.")
+  endif()
+
+  if(CONFIG_GNU_C_EXTENSIONS)
+    string(REPLACE "c" "gnu" CSTD "${CSTD}")
+  endif()
+else()
+  message(DEPRECATION
+    "Global CSTD property is deprecated, see Kconfig.zephyr for C Standard options.")
+endif()
 
 # @Intent: Obtain compiler specific flag for specifying the c standard
 zephyr_compile_options(

Kconfig.zephyr

@@ -338,6 +338,84 @@ endmenu
 
 menu "Compiler Options"
 
+config REQUIRES_STD_C99
+	bool
+	help
+	  Hidden option to select compiler support C99 standard or higher.
+
+config REQUIRES_STD_C11
+	bool
+	select REQUIRES_STD_C99
+	help
+	  Hidden option to select compiler support C11 standard or higher.
+
+config REQUIRES_STD_C17
+	bool
+	select REQUIRES_STD_C11
+	help
+	  Hidden option to select compiler support C17 standard or higher.
+
+config REQUIRES_STD_C23
+	bool
+	select REQUIRES_STD_C17
+	help
+	  Hidden option to select compiler support C23 standard or higher.
+
+choice STD_C
+	prompt "C Standard"
+	default STD_C23 if REQUIRES_STD_C23
+	default STD_C17 if REQUIRES_STD_C17
+	default STD_C11 if REQUIRES_STD_C11
+	default STD_C99
+	help
+	  C Standards.
+
+config STD_C90
+	bool "C90"
+	depends on !REQUIRES_STD_C99
+	help
+	  1989 C standard as completed in 1989 and ratified by ISO/IEC
+	  as ISO/IEC 9899:1990. This version is known as "ANSI C".
+
+config STD_C99
+	bool "C99"
+	depends on !REQUIRES_STD_C11
+	help
+	  1999 C standard.
+
+config STD_C11
+	bool "C11"
+	depends on !REQUIRES_STD_C17
+	help
+	  2011 C standard.
+
+config STD_C17
+	bool "C17"
+	depends on !REQUIRES_STD_C23
+	help
+	  2017 C standard, addresses defects in C11 without introducing
+	  new language features.
+
+config STD_C23
+	bool "C23"
+	help
+	  2023 C standard.
+
+endchoice
+
+config TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS
+	bool
+	default y if "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr"
+	help
+	  Hidden option to signal that toolchain supports GNU Extensions.
+
+config GNU_C_EXTENSIONS
+	bool "GNU C Extensions"
+	depends on TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS
+	help
+	  Enable GNU C Extensions. GNU C provides several language features
+	  not found in ISO standard C.
+
 config CODING_GUIDELINE_CHECK
 	bool "Enforce coding guideline rules"
 	help

arch/Kconfig

@@ -144,6 +144,12 @@ config ARCH_POSIX
 	select BARRIER_OPERATIONS_BUILTIN
 	# POSIX arch based targets get their memory cleared on entry by the host OS
 	select SKIP_BSS_CLEAR
+	# Override the C standard used for compilation to C 2011
+	# This is due to some tests using _Static_assert which is a 2011 feature, but
+	# otherwise relying on compilers supporting it also when set to C99.
+	# This was in general ok, but with some host compilers and C library versions
+	# it led to problems. So we override it to 2011 for the native targets.
+	select REQUIRES_STD_C11
 	help
 	  POSIX (native) architecture
 

arch/posix/CMakeLists.txt

@@ -196,11 +196,4 @@ if(NOT ${LLVM_SANITIZERS_ARG} STREQUAL "")
   target_compile_options(native_simulator INTERFACE ${LLVM_SANITIZERS_ARG})
 endif()
 
-# Override the C standard used for compilation to C 2011
-# This is due to some tests using _Static_assert which is a 2011 feature, but
-# otherwise relying on compilers supporting it also when set to C99.
-# This was in general ok, but with some host compilers and C library versions
-# it led to problems. So we override it to 2011 for the native targets.
-set_property(GLOBAL PROPERTY CSTD c11)
-
 add_subdirectory(core)

cmake/compiler/arcmwdt/compiler_flags.cmake

@@ -1,5 +1,3 @@
-set_property(GLOBAL PROPERTY CSTD gnu99)
-
 # List the warnings that are not supported for C++ compilations
 list(APPEND CXX_EXCLUDED_OPTIONS
   -Werror=implicit-int

cmake/toolchain/arcmwdt/Kconfig

@@ -4,3 +4,7 @@
 config TOOLCHAIN_ARCMWDT_SUPPORTS_THREAD_LOCAL_STORAGE
 	def_bool y
 	select TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
+
+config TOOLCHAIN_ARCMWDT_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/armclang/Kconfig

@@ -23,3 +23,7 @@ config ARMCLANG_STD_LIBC
 	  not provided by ARM Compiler standard libraries.
 
 endchoice
+
+config TOOLCHAIN_ARMCLANG_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/cross-compile/Kconfig

@@ -7,3 +7,10 @@ config TOOLCHAIN_CROSS_COMPILE_SUPPORTS_THREAD_LOCAL_STORAGE
 	help
 	  Set this if the cross-compile toolchain being used for the build
 	  supports thread local storage.
+
+config TOOLCHAIN_CROSS_COMPILE_SUPPORTS_GNU_EXTENSIONS
+	bool "Cross-compile toolchain supports GNU Extensions"
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS
+	help
+	  Set this if the cross-compile toolchain being used for the build
+	  supports GNU Extensions.

cmake/toolchain/gnuarmemb/Kconfig

@@ -7,3 +7,7 @@ config TOOLCHAIN_GNUARMEMB
 	def_bool y
 	select HAS_NEWLIB_LIBC_NANO
 	select NEWLIB_LIBC_SUPPORTED
+
+config TOOLCHAIN_GNUARMEMB_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/host/Kconfig

@@ -0,0 +1,6 @@
+# Copyright (c) 2024 Basalte bv
+# SPDX-License-Identifier: Apache-2.0
+
+config TOOLCHAIN_HOST_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

cmake/toolchain/llvm/Kconfig

@@ -22,3 +22,7 @@ config TOOLCHAIN_LLVM_SUPPORTS_THREAD_LOCAL_STORAGE
 	depends on RISCV
 	def_bool y
 	select TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
+
+config TOOLCHAIN_LLVM_SUPPORTS_GNU_EXTENSIONS
+	def_bool y
+	select TOOLCHAIN_SUPPORTS_GNU_EXTENSIONS

doc/releases/release-notes-3.7.rst

@@ -93,6 +93,12 @@ Build system and Infrastructure
 
 * CI-enabled blackbox tests were added in order to verify correctness of the vast majority of Twister flags.
 
+* Compiler
+
+  * Deprecated the global CSTD cmake property in favor of the :kconfig:option:`CONFIG_STD_C`
+    choice to select the C Standard version. Additionally subsystems can select a minimum
+    required C Standard version, with for example :kconfig:option:`CONFIG_REQUIRES_STD_C11`.
+
 Drivers and Sensors
 *******************
 

tests/subsys/bindesc/definition/testcase.yaml

@@ -22,14 +22,23 @@ tests:
       - qemu_riscv32e
       - qemu_riscv64
   bindesc.define.c99:
-    extra_args: CSTD="c99"
+    extra_configs:
+      - CONFIG_STD_C99=y
   bindesc.define.c11:
-    extra_args: CSTD="c11"
+    extra_configs:
+      - CONFIG_STD_C11=y
   bindesc.define.c17:
-    extra_args: CSTD="c17"
+    extra_configs:
+      - CONFIG_STD_C17=y
   bindesc.define.gnu99:
-    extra_args: CSTD="gnu99"
+    extra_configs:
+      - CONFIG_STD_C99=y
+      - CONFIG_GNU_C_EXTENSIONS=y
   bindesc.define.gnu11:
-    extra_args: CSTD="gnu11"
+    extra_configs:
+      - CONFIG_STD_C11=y
+      - CONFIG_GNU_C_EXTENSIONS=y
   bindesc.define.gnu17:
-    extra_args: CSTD="gnu17"
+    extra_configs:
+      - CONFIG_STD_C17=y
+      - CONFIG_GNU_C_EXTENSIONS=y