[Git][ghc/ghc][master] ghc-toolchain: add C11 check

12 Feb 2026


      Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
9e58b8a1 by Cheng Shao at 2026-02-11T16:08:10-05:00
ghc-toolchain: add C11 check

This patch partially reverts commit
b8307eab80c5809df5405d76c822bf86877f5960 that removed C99 check in
autoconf/ghc-toolchain. Now we:

- No longer re-implement `FP_SET_CFLAGS_C11` similar to
  `FP_SET_CFLAGS_C99` in the past, since autoconf doesn't provide a
  convenient `AC_PROG_CC_C11` function. ghc-toolchain will handle it
  anyway.
- The Cmm CPP C99 check is relanded and repurposed for C11.
- The C99 logic in ghc-toolchain is relanded and repurposed for C11.
- The C99 check in Stg.h is corrected to check for C11. The obsolete
  _ISOC99_SOURCE trick is dropped.
- Usages of `-std=gnu99` in the testsuite are corrected to use
  `-std=gnu11`.

Closes #26908.

- - - - -


7 changed files:

- configure.ac
- distrib/configure.ac.in
- m4/fp_cmm_cpp_cmd_with_args.m4
- rts/include/Stg.h
- testsuite/tests/ffi/should_run/all.T
- utils/ghc-toolchain/src/GHC/Toolchain/Tools/Cc.hs
- utils/ghc-toolchain/src/GHC/Toolchain/Tools/Cpp.hs


Changes:

=====================================
configure.ac
=====================================
@@ -671,8 +671,8 @@ FP_CC_IGNORE_UNUSED_ARGS([$CC], [CONF_CC_OPTS_STAGE2])
 
 # CPP, CPPFLAGS
 # --with-cpp/-with-cpp-flags
-dnl Note that we must do this after setting and using the C99 CPPFLAGS, or
-dnl otherwise risk trying to configure the C99 and LD flags using -E as a CPPFLAG
+dnl Note that we must do this after setting and using the C11 CPPFLAGS, or
+dnl otherwise risk trying to configure the C11 and LD flags using -E as a CPPFLAG
 FP_CPP_CMD_WITH_ARGS([$CC_STAGE0],[CPPCmd_STAGE0],[CONF_CPP_OPTS_STAGE0])
 FP_CPP_CMD_WITH_ARGS([$CC],[CPPCmd],[CONF_CPP_OPTS_STAGE1])
 FP_CPP_CMD_WITH_ARGS([$CC],[CPPCmd],[CONF_CPP_OPTS_STAGE2])


=====================================
distrib/configure.ac.in
=====================================
@@ -313,8 +313,8 @@ FP_CC_IGNORE_UNUSED_ARGS([$CC], [CONF_CC_OPTS_STAGE2])
 
 # CPP, CPPFLAGS
 # --with-cpp/-with-cpp-flags
-dnl Note that we must do this after setting and using the C99 CPPFLAGS, or
-dnl otherwise risk trying to configure the C99 and LD flags using -E as a CPPFLAG
+dnl Note that we must do this after setting and using the C11 CPPFLAGS, or
+dnl otherwise risk trying to configure the C11 and LD flags using -E as a CPPFLAG
 FP_CPP_CMD_WITH_ARGS([$CC_STAGE0],[CPPCmd_STAGE0],[CONF_CPP_OPTS_STAGE0])
 FP_CPP_CMD_WITH_ARGS([$CC],[CPPCmd],[CONF_CPP_OPTS_STAGE1])
 FP_CPP_CMD_WITH_ARGS([$CC],[CPPCmd],[CONF_CPP_OPTS_STAGE2])


=====================================
m4/fp_cmm_cpp_cmd_with_args.m4
=====================================
@@ -56,6 +56,27 @@ else
   AC_MSG_RESULT([no])
 fi
 
+AC_MSG_CHECKING([the C-- preprocessor for C11 support])
+cat > conftest.c <<EOF
+#include <stdio.h>
+#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112L
+# error "Compiler does not advertise C11 conformance"
+#endif
+EOF
+if "$CMM_CPP_CMD" $CMM_CPP_ARGS conftest.c -o conftest -g0 >/dev/null 2>&1; then
+  AC_MSG_RESULT([yes])
+else
+    # Try -std=gnu11
+    if "$CMM_CPP_CMD" -std=gnu11 $CMM_CPP_ARGS conftest.c -o conftest -g0 >/dev/null 2>&1; then
+      $3="-std=gnu11 $$3"
+      AC_MSG_RESULT([needs -std=gnu11])
+    else
+      AC_MSG_ERROR([C11-compatible compiler needed])
+    fi
+fi
+rm -f conftest.c conftest.o conftest
+
+
 $2="$CMM_CPP_CMD"
 $3="$$3 $CMM_CPP_ARGS"
 


=====================================
rts/include/Stg.h
=====================================
@@ -31,8 +31,8 @@
 #define __STDC_VERSION__ 0
 #endif
 
-#if !(__STDC_VERSION__ >= 199901L) && !(__cplusplus >= 201103L)
-# error __STDC_VERSION__ does not advertise C99, C++11 or later
+#if !(__STDC_VERSION__ >= 201112L) && !(__cplusplus >= 201103L)
+# error __STDC_VERSION__ does not advertise C11, C++11 or later
 #endif
 
 /*
@@ -49,10 +49,6 @@
 #if !defined(IN_STG_CODE)
 # define IN_STG_CODE 1
 
-// Turn on C99 for .hc code.  This gives us the INFINITY and NAN
-// constants from math.h, which we occasionally need to use in .hc (#1861)
-# define _ISOC99_SOURCE
-
 // We need _BSD_SOURCE so that math.h defines things like gamma
 // on Linux
 # define _BSD_SOURCE


=====================================
testsuite/tests/ffi/should_run/all.T
=====================================
@@ -103,7 +103,7 @@ test('T2276_ghci', [ only_ghci,
                      pre_cmd('$MAKE -s --no-print-directory T2276_ghci_setup ghciWayFlags=' + config.ghci_way_flags) ],
                    compile_and_run, ['-fobject-code T2276_ghci_c.o'])
 
-test('T2469', normal, compile_and_run, ['-optc-std=gnu99'])
+test('T2469', normal, compile_and_run, ['-optc-std=gnu11'])
 
 test('T2594', [req_c], compile_and_run, ['T2594_c.c'])
 


=====================================
utils/ghc-toolchain/src/GHC/Toolchain/Tools/Cc.hs
=====================================
@@ -11,6 +11,7 @@ module GHC.Toolchain.Tools.Cc
     , compileC
     , compileAsm
     , addPlatformDepCcFlags
+    , checkC11Support
     ) where
 
 import Control.Monad
@@ -50,8 +51,12 @@ findCc archOs llvmTarget progOpt = do
     cc1 <- ignoreUnusedArgs cc0
     cc2 <- ccSupportsTarget archOs llvmTarget cc1
     checking "whether Cc works" $ checkCcWorks cc2
-    checkCcSupportsExtraViaCFlags cc2
-    return cc2
+    cc3 <- oneOf "cc doesn't support C11" $ map checkC11Support
+        [ cc2
+        , cc2 & _ccFlags %++ "-std=gnu11"
+        ]
+    checkCcSupportsExtraViaCFlags cc3
+    return cc3
 
 checkCcWorks :: Cc -> M ()
 checkCcWorks cc = withTempDir $ \dir -> do
@@ -83,6 +88,17 @@ ccSupportsTarget archOs target cc =
     checking "whether Cc supports --target" $
     supportsTarget archOs _ccProgram checkCcWorks target cc
 
+checkC11Support :: Cc -> M Cc
+checkC11Support cc = checking "for C11 support" $ withTempDir $ \dir -> do
+    let test_o = dir </> "test.o"
+    compileC cc test_o $ unlines
+        [ "#include <stdio.h>"
+        , "#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112L"
+        , "# error \"Compiler does not advertise C11 conformance\""
+        , "#endif"
+        ]
+    return cc
+
 checkCcSupportsExtraViaCFlags :: Cc -> M ()
 checkCcSupportsExtraViaCFlags cc = checking "whether cc supports extra via-c flags" $ withTempDir $ \dir -> do
   let test_o = dir </> "test.o"


=====================================
utils/ghc-toolchain/src/GHC/Toolchain/Tools/Cpp.hs
=====================================
@@ -19,7 +19,7 @@ import GHC.Toolchain.Prelude
 import GHC.Toolchain.Program
 
 import GHC.Toolchain.Tools.Cc
-import GHC.Toolchain.Utils (withTempDir, expectFileExists)
+import GHC.Toolchain.Utils (withTempDir, oneOf, expectFileExists)
 
 newtype Cpp = Cpp { cppProgram :: Program
                     }
@@ -160,7 +160,13 @@ findJsCpp progOpt cc = checking "for JavaScript C preprocessor" $ do
 findCmmCpp :: ProgOpt -> Cc -> M CmmCpp
 findCmmCpp progOpt cc = checking "for a Cmm preprocessor" $ do
   -- Use the specified CPP or try to use the c compiler
-  cpp <- findProgram "Cmm preprocessor" progOpt [] <|> pure (programFromOpt progOpt (prgPath $ ccProgram cc) [])
+  foundCppProg <- findProgram "Cmm preprocessor" progOpt [] <|> pure (programFromOpt progOpt (prgPath $ ccProgram cc) [])
+  -- Check whether the C preprocessor needs -std=gnu11 (only very old toolchains need this)
+  Cc cpp <- oneOf "cc doesn't support C11" $ map checkC11Support
+        [ Cc foundCppProg
+        , Cc (foundCppProg & _prgFlags %++ "-std=gnu11")
+        ]
+
   cmmCppSupportsG0 <- withTempDir $ \dir -> do
     let conftest = dir </> "conftest.c"
     writeFile conftest "int main(void) {}"
@@ -175,9 +181,14 @@ findCmmCpp progOpt cc = checking "for a Cmm preprocessor" $ do
 findCpp :: ProgOpt -> Cc -> M Cpp
 findCpp progOpt cc = checking "for C preprocessor" $ do
   -- Use the specified CPP or try to use the c compiler
-  cpp <- findProgram "C preprocessor" progOpt [] <|> pure (programFromOpt progOpt (prgPath $ ccProgram cc) [])
+  foundCppProg <- findProgram "C preprocessor" progOpt [] <|> pure (programFromOpt progOpt (prgPath $ ccProgram cc) [])
+  -- Check whether the C preprocessor needs -std=gnu11 (only very old toolchains need this)
+  Cc cpp2 <- oneOf "cc doesn't support C11" $ map checkC11Support
+        [ Cc foundCppProg
+        , Cc (foundCppProg & _prgFlags %++ "-std=gnu11")
+        ]
   -- Always add the -E flag to the CPP, regardless of the user options
-  let cppProgram = addFlagIfNew "-E" cpp
+  let cppProgram = addFlagIfNew "-E" cpp2
   return Cpp{cppProgram}
 
 --------------------------------------------------------------------------------



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/9e58b8a14b7c2190dbd4c72e26fd47bc...

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