c standard: Enforce C17 as the minimum required C standard

C99 has been the minimum required C standard version for Zephyr since
its inception. After multiple attempts and discussions, a decision has
been made to upgrade to C17 going forward.
This commits replaces the default C standard from C99 to C17 in the
configuration and build system.

Signed-off-by: Carles Cufi <[email protected]>

Author: carlescufi

CMakeLists.txt

@@ -319,12 +319,6 @@ 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()
@@ -409,7 +403,7 @@ if(CONFIG_COMPILER_FREESTANDING)
   zephyr_compile_options($<$<COMPILE_LANGUAGE:ASM>:$<TARGET_PROPERTY:compiler,freestanding>>)
 endif()
 
-if (CONFIG_PICOLIBC AND NOT CONFIG_PICOLIBC_IO_FLOAT)
+if(CONFIG_PICOLIBC AND NOT CONFIG_PICOLIBC_IO_FLOAT)
   # @Intent: Set compiler specific flag to disable printf-related optimizations
   zephyr_compile_options($<$<COMPILE_LANGUAGE:C>:$<TARGET_PROPERTY:compiler,no_printf_return_value>>)
 endif()
@@ -1157,7 +1151,7 @@ endif()
 
 get_property(OUTPUT_FORMAT        GLOBAL PROPERTY PROPERTY_OUTPUT_FORMAT)
 
-if (CONFIG_CODE_DATA_RELOCATION)
+if(CONFIG_CODE_DATA_RELOCATION)
   set(CODE_RELOCATION_DEP code_relocation_source_lib)
 endif() # CONFIG_CODE_DATA_RELOCATION
 
@@ -2134,7 +2128,7 @@ if(CONFIG_BUILD_OUTPUT_INFO_HEADER)
     )
 endif()
 
-if (CONFIG_LLEXT AND CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID)
+if(CONFIG_LLEXT AND CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID)
   #slidgen must be the first post-build command to be executed
   #on the Zephyr ELF to ensure that all other commands, such as
   #binary file generation, are operating on a preparated ELF.
@@ -2200,7 +2194,7 @@ if(signing_script)
 endif()
 
 # Generate USB-C VIF policies in XML format
-if (CONFIG_BUILD_OUTPUT_VIF)
+if(CONFIG_BUILD_OUTPUT_VIF)
   include(${CMAKE_CURRENT_LIST_DIR}/cmake/vif.cmake)
 endif()
 
@@ -2251,7 +2245,7 @@ if(LOG_DICT_DB_NAME_ARG)
       --build-header ${PROJECT_BINARY_DIR}/include/generated/zephyr/version.h
   )
 
-  if (NOT CONFIG_LOG_DICTIONARY_DB_TARGET)
+  if(NOT CONFIG_LOG_DICTIONARY_DB_TARGET)
     # If not using a separate target for generating logging dictionary
     # database, add the generation to post build command to make sure
     # the database is actually being generated.

Kconfig.zephyr

@@ -355,20 +355,8 @@ 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.
 
@@ -381,31 +369,10 @@ config REQUIRES_STD_C23
 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
+	default STD_C17
 	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

arch/Kconfig

@@ -152,12 +152,6 @@ 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
 

cmake/compiler/compiler_features.cmake

@@ -1,18 +1,15 @@
-set(c23id c2x gnu2x)
+set(c23id c2x gnu2x "iso9899:2023" "iso9899:2024")
 set(c17id c17 c18 gnu17 gnu18 "iso9899:2017" "iso9899:2018")
-set(c11id c11 gnu11 "iso9899:2011")
-set(c99id c99 gnu99 "iso9899:1999")
-set(c90id c89 c90 gnu89 gnu90 "iso9899:1990" "iso9899:199409")
 
 set(compile_features_list)
 
 # For each id value above a compile_features_${idval} with a list of supported
 # `c_std_XX` values are created for easy lookup.
 # For example, the settings
-# - `compile_feature_c99` will contain `c_std_90;c_std_99`
-# - `compile_feature_iso9899:2011` will contain `c_std_90;c_std_99;c_std_11`
+# - `compile_feature_c23` will contain `c_std_17;c_std_23`
+# - `compile_feature_iso9899:2023` will contain `c_std_17;c_std_23`
 # that can then be used to set CMAKE_C_COMPILE_FEATURES accordingly.
-foreach(standard 90 99 11 17 23)
+foreach(standard 17 23)
   list(APPEND compile_features_list c_std_${standard})
   foreach(id ${c${standard}id})
     set(compile_features_${id} ${compile_features_list})

doc/develop/languages/c/index.rst

@@ -24,27 +24,21 @@ Language Standards
 
 Zephyr does not target a specific version of the C standards; however, the
 Zephyr codebase makes extensive use of the features newly introduced in the
-1999 release of the ISO C standard (ISO/IEC 9899:1999, hereinafter referred to
-as C99) such as those listed below, effectively requiring the use of a compiler
-toolchain that supports the C99 standard and above:
+2017 release of the ISO C standard (ISO/IEC 9899:2017, hereinafter referred to
+as C17) such as those listed below, effectively requiring the use of a compiler
+toolchain that supports the C17 standard and above:
 
 * inline functions
 * standard boolean types (``bool`` in ``<stdbool.h>``)
 * fixed-width integer types (``[u]intN_t`` in ``<stdint.h>``)
 * designated initializers
 * variadic macros
 * ``restrict`` qualification
-
-Some Zephyr components make use of the features newly introduced in the 2011
-release of the ISO C standard (ISO/IEC 9899:2011, hereinafter referred to as
-C11) such as the type-generic expressions using the ``_Generic`` keyword. For
-example, the :c:func:`cbprintf` component, used as the default formatted output
-processor for Zephyr, makes use of the C11 type-generic expressions, and this
-effectively requires most Zephyr applications to be compiled using a compiler
-toolchain that supports the C11 standard and above.
+* _Generic keyword
+* _Static_assert keyword
 
 In summary, it is recommended to use a compiler toolchain that supports at
-least the C11 standard for developing with Zephyr. It is, however, important to
+least the C17 standard for developing with Zephyr. It is, however, important to
 note that some optional Zephyr components and external modules may make use of
 the C language features that have been introduced in more recent versions of
 the standards, in which case it will be necessary to use a more up-to-date

modules/nanopb/Kconfig

@@ -6,8 +6,6 @@ config ZEPHYR_NANOPB_MODULE
 
 menuconfig NANOPB
 	bool "Nanopb Support"
-	# Nanopb requires c_std_11 compiler features like _Static_assert
-	select REQUIRES_STD_C11
 	help
 	  This option enables the Nanopb library and generator.
 

tests/subsys/bindesc/definition/testcase.yaml

@@ -19,23 +19,9 @@ tests:
       - qemu_riscv32
       - qemu_riscv32e
       - qemu_riscv64
-  bindesc.define.c99:
-    extra_configs:
-      - CONFIG_STD_C99=y
-  bindesc.define.c11:
-    extra_configs:
-      - CONFIG_STD_C11=y
   bindesc.define.c17:
     extra_configs:
       - CONFIG_STD_C17=y
-  bindesc.define.gnu99:
-    extra_configs:
-      - CONFIG_STD_C99=y
-      - CONFIG_GNU_C_EXTENSIONS=y
-  bindesc.define.gnu11:
-    extra_configs:
-      - CONFIG_STD_C11=y
-      - CONFIG_GNU_C_EXTENSIONS=y
   bindesc.define.gnu17:
     extra_configs:
       - CONFIG_STD_C17=y

tests/subsys/bindesc/reading/testcase.yaml

@@ -8,23 +8,9 @@ tests:
   bindesc.read:
     platform_allow:
       - native_sim
-  bindesc.read.c99:
-    extra_configs:
-      - CONFIG_STD_C99=y
-  bindesc.read.c11:
-    extra_configs:
-      - CONFIG_STD_C11=y
   bindesc.read.c17:
     extra_configs:
       - CONFIG_STD_C17=y
-  bindesc.read.gnu99:
-    extra_configs:
-      - CONFIG_STD_C99=y
-      - CONFIG_GNU_C_EXTENSIONS=y
-  bindesc.read.gnu11:
-    extra_configs:
-      - CONFIG_STD_C11=y
-      - CONFIG_GNU_C_EXTENSIONS=y
   bindesc.read.gnu17:
     extra_configs:
       - CONFIG_STD_C17=y