Features: Extend concept to C language.

Add properties and variables corresponding to CXX equivalents.

Add features for c_function_prototypes (C90), c_restrict (C99),
c_variadic_macros (C99) and c_static_assert (C11). This feature
set can be extended later.

Add a <PREFIX>_RESTRICT symbol define to WriteCompilerDetectionHeader
to conditionally represent the c_restrict feature.

Modules/CMakeCCompiler.cmake.in

@@ -2,6 +2,11 @@ set(CMAKE_C_COMPILER "@CMAKE_C_COMPILER@")
 set(CMAKE_C_COMPILER_ARG1 "@CMAKE_C_COMPILER_ARG1@")
 set(CMAKE_C_COMPILER_ID "@CMAKE_C_COMPILER_ID@")
 set(CMAKE_C_COMPILER_VERSION "@CMAKE_C_COMPILER_VERSION@")
+set(CMAKE_C_COMPILE_FEATURES "@CMAKE_C_COMPILE_FEATURES@")
+set(CMAKE_C90_COMPILE_FEATURES "@CMAKE_C90_COMPILE_FEATURES@")
+set(CMAKE_C99_COMPILE_FEATURES "@CMAKE_C99_COMPILE_FEATURES@")
+set(CMAKE_C11_COMPILE_FEATURES "@CMAKE_C11_COMPILE_FEATURES@")
+
 set(CMAKE_C_PLATFORM_ID "@CMAKE_C_PLATFORM_ID@")
 set(CMAKE_C_SIMULATE_ID "@CMAKE_C_SIMULATE_ID@")
 set(CMAKE_C_SIMULATE_VERSION "@CMAKE_C_SIMULATE_VERSION@")

Modules/CMakeDetermineCompileFeatures.cmake

@@ -14,7 +14,45 @@
 
 function(cmake_determine_compile_features lang)
 
-  if(lang STREQUAL CXX AND COMMAND cmake_record_cxx_compile_features)
+  if(lang STREQUAL C AND COMMAND cmake_record_c_compile_features)
+    message(STATUS "Detecting ${lang} compile features")
+
+    set(CMAKE_C90_COMPILE_FEATURES)
+    set(CMAKE_C99_COMPILE_FEATURES)
+    set(CMAKE_C11_COMPILE_FEATURES)
+
+    include("${CMAKE_ROOT}/Modules/Internal/FeatureTesting.cmake")
+
+    cmake_record_c_compile_features()
+
+    if(NOT _result EQUAL 0)
+      message(STATUS "Detecting ${lang} compile features - failed")
+      return()
+    endif()
+
+    if (CMAKE_C99_COMPILE_FEATURES AND CMAKE_C11_COMPILE_FEATURES)
+      list(REMOVE_ITEM CMAKE_C11_COMPILE_FEATURES ${CMAKE_C99_COMPILE_FEATURES})
+    endif()
+    if (CMAKE_C90_COMPILE_FEATURES AND CMAKE_C99_COMPILE_FEATURES)
+      list(REMOVE_ITEM CMAKE_C99_COMPILE_FEATURES ${CMAKE_C90_COMPILE_FEATURES})
+    endif()
+
+    if(NOT CMAKE_C_COMPILE_FEATURES)
+      set(CMAKE_C_COMPILE_FEATURES
+        ${CMAKE_C90_COMPILE_FEATURES}
+        ${CMAKE_C99_COMPILE_FEATURES}
+        ${CMAKE_C11_COMPILE_FEATURES}
+      )
+    endif()
+
+    set(CMAKE_C_COMPILE_FEATURES ${CMAKE_C_COMPILE_FEATURES} PARENT_SCOPE)
+    set(CMAKE_C90_COMPILE_FEATURES ${CMAKE_C90_COMPILE_FEATURES} PARENT_SCOPE)
+    set(CMAKE_C99_COMPILE_FEATURES ${CMAKE_C99_COMPILE_FEATURES} PARENT_SCOPE)
+    set(CMAKE_C11_COMPILE_FEATURES ${CMAKE_C11_COMPILE_FEATURES} PARENT_SCOPE)
+
+    message(STATUS "Detecting ${lang} compile features - done")
+
+  elseif(lang STREQUAL CXX AND COMMAND cmake_record_cxx_compile_features)
     message(STATUS "Detecting ${lang} compile features")
 
     set(CMAKE_CXX98_COMPILE_FEATURES)

Modules/CMakeTestCCompiler.cmake

@@ -73,6 +73,9 @@ else()
   # Try to identify the ABI and configure it into CMakeCCompiler.cmake
   include(${CMAKE_ROOT}/Modules/CMakeDetermineCompilerABI.cmake)
   CMAKE_DETERMINE_COMPILER_ABI(C ${CMAKE_ROOT}/Modules/CMakeCCompilerABI.c)
+  # Try to identify the compiler features
+  include(${CMAKE_ROOT}/Modules/CMakeDetermineCompileFeatures.cmake)
+  CMAKE_DETERMINE_COMPILE_FEATURES(C)
 
   # Re-configure to save learned information.
   configure_file(

Modules/Compiler/GNU-C-FeatureTests.cmake

@@ -0,0 +1,12 @@
+
+set(_cmake_oldestSupported "(__GNUC__ * 100 + __GNUC_MINOR__) >= 407")
+
+set(GNU46_C11 "${_cmake_oldestSupported} && __STDC_VERSION__ >= 201112L")
+set(_cmake_feature_test_c_static_assert "${GNU46_C11}")
+# Since 4.4 at least:
+set(GNU44_C99 "${_cmake_oldestSupported} && __STDC_VERSION__ >= 199901L")
+set(_cmake_feature_test_c_restrict "${GNU44_C99}")
+set(_cmake_feature_test_c_variadic_macros "${GNU44_C99}")
+
+set(GNU_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(_cmake_feature_test_c_function_prototypes "${GNU_C90}")

Modules/Compiler/GNU-C.cmake

@@ -1,2 +1,34 @@
 include(Compiler/GNU)
 __compiler_gnu(C)
+
+if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.7)
+  set(CMAKE_C90_STANDARD_COMPILE_OPTION "-std=c90")
+  set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-std=gnu90")
+
+  set(CMAKE_C99_STANDARD_COMPILE_OPTION "-std=c99")
+  set(CMAKE_C99_EXTENSION_COMPILE_OPTION "-std=gnu99")
+
+  set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
+  set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
+endif()
+
+# This may change in a future GNU version.
+set(CMAKE_C_STANDARD_DEFAULT 90)
+
+macro(cmake_record_c_compile_features)
+  macro(_get_gcc_features std_version list)
+    record_compiler_features(C "-std=${std_version}" ${list})
+  endmacro()
+
+  if (UNIX AND NOT APPLE AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.7)
+    _get_gcc_features(c90 CMAKE_C90_COMPILE_FEATURES)
+    if (_result EQUAL 0)
+      _get_gcc_features(c99 CMAKE_C99_COMPILE_FEATURES)
+    endif()
+    if (_result EQUAL 0)
+      _get_gcc_features(c11 CMAKE_C11_COMPILE_FEATURES)
+    endif()
+  else()
+    set(_result 0)
+  endif()
+endmacro()

Modules/WriteCompilerDetectionHeader.cmake

@@ -43,7 +43,8 @@
 # Possible compiler identifiers are documented with the
 # :variable:`CMAKE_<LANG>_COMPILER_ID` variable.
 # Available features in this version of CMake are listed in the
-# :prop_gbl:`CMAKE_CXX_KNOWN_FEATURES` global property.
+# :prop_gbl:`CMAKE_C_KNOWN_FEATURES` and
+# :prop_gbl:`CMAKE_CXX_KNOWN_FEATURES` global properties.
 #
 # Feature Test Macros
 # ===================
@@ -102,6 +103,7 @@
 # ========================== =================================== =================
 #         Feature                          Define                      Symbol
 # ========================== =================================== =================
+# ``c_restrict``              ``<PREFIX>_RESTRICT``               ``restrict``
 # ``cxx_constexpr``           ``<PREFIX>_CONSTEXPR``              ``constexpr``
 # ``cxx_deleted_functions``   ``<PREFIX>_DELETED_FUNCTION``       ``= delete``
 # ``cxx_extern_templates``    ``<PREFIX>_EXTERN_TEMPLATE``        ``extern``
@@ -221,6 +223,9 @@ function(write_compiler_detection_header
     if (feature MATCHES "^cxx_")
       list(APPEND _langs CXX)
       list(APPEND CXX_features ${feature})
+    elseif (feature MATCHES "^c_")
+      list(APPEND _langs C)
+      list(APPEND C_features ${feature})
     else()
       message(FATAL_ERROR "Unsupported feature ${feature}.")
     endif()
@@ -239,6 +244,8 @@ function(write_compiler_detection_header
 
     if(_lang STREQUAL CXX)
       set(file_content "${file_content}\n#ifdef __cplusplus\n")
+    else()
+      set(file_content "${file_content}\n#ifndef __cplusplus\n")
     endif()
 
     compiler_id_detection(ID_CONTENT ${_lang} PREFIX ${prefix_arg}_
@@ -279,6 +286,16 @@ function(write_compiler_detection_header
       string(TOUPPER ${feature} feature_upper)
       set(feature_PP "COMPILER_${feature_upper}")
       set(def_name ${prefix_arg}_${feature_PP})
+      if (feature STREQUAL c_restrict)
+        set(def_value "${prefix_arg}_RESTRICT")
+        set(file_content "${file_content}
+#  if ${def_name}
+#    define ${def_value} restrict
+#  else
+#    define ${def_value}
+#  endif
+\n")
+      endif()
       if (feature STREQUAL cxx_constexpr)
         set(def_value "${prefix_arg}_DECL_${feature_upper}")
         set(file_content "${file_content}
@@ -382,9 +399,8 @@ function(write_compiler_detection_header
 \n")
       endif()
     endforeach()
-    if(_lang STREQUAL CXX)
-      set(file_content "${file_content}#endif\n")
-    endif()
+
+    set(file_content "${file_content}#endif\n")
 
   endforeach()