Require compilers to support C11.

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.

Changelog

@@ -24,6 +24,9 @@ version <next>:
 - ffmpeg CLI options may now be used as -/opt <path>, which is equivalent
   to -opt <contents of file <path>>
 - showinfo bitstream filter
+- a C11-compliant compiler is now required; note that this requirement
+  will be bumped to C17 in the near future, so consider updating your
+  build environment if it lacks C17 support
 
 version 6.1:
 - libaribcaption decoder

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=c11)             echo -Qstd=c11 ;;
             -flto*)               echo -ipo ;;
         esac
     done
@@ -4798,7 +4798,7 @@ suncc_flags(){
                     athlon*)                   echo -xarch=pentium_proa  ;;
                 esac
                 ;;
-            -std=c99)             echo -xc99              ;;
+            -std=c11)             echo -xc11              ;;
             -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,^.*: , ," > $(@:.o=.d)'
-        _DEPFLAGS='-xM1 -xc99'
+        _DEPFLAGS='-xM1 -xc11'
-        _ldflags='-std=c99'
+        _ldflags='-std=c11'
         _cflags_speed='-O5'
         _cflags_size='-O5 -xspace'
         _flags_filter=suncc_flags
@@ -5517,21 +5517,20 @@ if test "$?" != 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
 # version macro is defined properly
 test_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" &&
-    add_cflags -std=c11 ||
+    add_cflags -std=c11 || die "Compiler lacks C11 support"
-    check_cflags -std=c99
 
 check_cppflags -D_FILE_OFFSET_BITS=64
 check_cppflags -D_LARGEFILE_SOURCE
 
-add_host_cppflags -D_ISOC99_SOURCE
+add_host_cppflags -D_ISOC11_SOURCE
-check_host_cflags -std=c99
+check_host_cflags -std=c11
 check_host_cflags -Wall
 check_host_cflags $host_cflags_speed
 

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:
+FFmpeg is mainly programmed in the ISO C11 language, except for the public
-@itemize @bullet
+headers which must stay C99 compatible.
-@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
 
 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