[FFmpeg-devel] [PATCH 2/2] Require compilers to support C17.

[Anton Khirnov <anton@khirnov.net>]

It should be available in all relevant modern compilers and will allow
us to use features like anonymous unions.

Note that stdatomic.h is still emulated on MSVC, as current versions
require the /experimental:c11atomics, and do not support
ATOMIC_VAR_INIT() anyway.
---
Now moving to C17 rather than C11, as the former contains important
fixes and its support across the compilers we care about should be
similar.

Now also tested with MSVC in wine, thanks to Martin for pointing me at
https://github.com/mstorsjo/msvc-wine
---
 configure          | 23 +++++++++++------------
 doc/developer.texi | 10 ++--------
 2 files changed, 13 insertions(+), 20 deletions(-)

diff --git a/configure b/configure
index f72533b7d2..1bb9e23f19 100755
--- a/configure
+++ b/configure
@@ -4705,7 +4705,7 @@ msvc_common_flags(){
             # generic catch all at the bottom will print the original flag.
             -Wall)                ;;
             -Wextra)              ;;
-            -std=c*)              ;;
+            -std=c*)              echo /std:${flag#-std=};;
             # Common flags
             -fomit-frame-pointer) ;;
             -g)                   echo -Z7 ;;
@@ -4750,7 +4750,7 @@ icl_flags(){
             # Despite what Intel's documentation says -Wall, which is supported
             # on Windows, does enable remarks so disable them here.
             -Wall)                echo $flag -Qdiag-disable:remark ;;
-            -std=c99)             echo -Qstd=c99 ;;
+            -std=c17)             echo -Qstd=c17 ;;
             -flto*)               echo -ipo ;;
         esac
     done
@@ -4798,7 +4798,7 @@ suncc_flags(){
                     athlon*)                   echo -xarch=pentium_proa  ;;
                 esac
                 ;;
-            -std=c99)             echo -xc99              ;;
+            -std=c17)             echo -xc17              ;;
             -fomit-frame-pointer) echo -xregs=frameptr    ;;
             -fPIC)                echo -KPIC -xcode=pic32 ;;
             -W*,*)                echo $flag              ;;
@@ -4887,8 +4887,8 @@ probe_cc(){
         _type=suncc
         _ident=$($_cc -V 2>&1 | head -n1 | cut -d' ' -f 2-)
         _DEPCMD='$(DEP$(1)) $(DEP$(1)FLAGS) $($(1)DEP_FLAGS) $< | sed -e "1s,^.*: ,$@: ," -e "\$$!s,\$$, \\\," -e "1!s,^.*: , ," > $(@:.o=.d)'
-        _DEPFLAGS='-xM1 -xc99'
-        _ldflags='-std=c99'
+        _DEPFLAGS='-xM1 -xc17'
+        _ldflags='-std=c17'
         _cflags_speed='-O5'
         _cflags_size='-O5 -xspace'
         _flags_filter=suncc_flags
@@ -5517,21 +5517,20 @@ if test "$?" != 0; then
     die "C compiler test failed."
 fi
 
-add_cppflags -D_ISOC99_SOURCE
+add_cppflags -D_ISOC11_SOURCE
 add_cxxflags -D__STDC_CONSTANT_MACROS
 check_cxxflags -std=c++11 || check_cxxflags -std=c++0x
 
-# some compilers silently accept -std=c11, so we also need to check that the
+# some compilers silently accept -std=c17, so we also need to check that the
 # version macro is defined properly
-test_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" &&
-    add_cflags -std=c11 ||
-    check_cflags -std=c99
+test_cflags_cc -std=c17 ctype.h "__STDC_VERSION__ >= 201710L" &&
+    add_cflags -std=c17 || die "Compiler lacks C17 support"
 
 check_cppflags -D_FILE_OFFSET_BITS=64
 check_cppflags -D_LARGEFILE_SOURCE
 
-add_host_cppflags -D_ISOC99_SOURCE
-check_host_cflags -std=c99
+add_host_cppflags -D_ISOC11_SOURCE
+check_host_cflags -std=c17
 check_host_cflags -Wall
 check_host_cflags $host_cflags_speed
 
diff --git a/doc/developer.texi b/doc/developer.texi
index eed0ee4915..6e9807aa06 100644
--- a/doc/developer.texi
+++ b/doc/developer.texi
@@ -56,14 +56,8 @@ and should try to fix issues their commit causes.
 
 @section Language
 
-FFmpeg is mainly programmed in the ISO C99 language, extended with:
-@itemize @bullet
-@item
-Atomic operations from C11 @file{stdatomic.h}. They are emulated on
-architectures/compilers that do not support them, so all FFmpeg-internal code
-may use atomics without any extra checks. However, @file{stdatomic.h} must not
-be included in public headers, so they stay C99-compatible.
-@end itemize
+FFmpeg is mainly programmed in the ISO C17 language, except for the public
+headers which must stay C99 compatible.
 
 Compiler-specific extensions may be used with good reason, but must not be
 depended on, i.e. the code must still compile and work with compilers lacking
-- 
2.42.0

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