[FFmpeg-devel] [PATCH] configure: link to libatomic when it's present

[James Almer]

On 1/19/2022 8:54 AM, Anton Khirnov wrote:
> C11 atomics in some configurations (e.g. 64bit operations on ppc64 with
> GCC) require linking to libatomic.
> ---
> Testing welcome, especially in configurations where
> * libatomic is not present
> * libatomic is actually needed
> ---
>   configure | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 1413122d87..1ff5dbee5b 100755
> --- a/configure
> +++ b/configure
> @@ -6324,7 +6324,14 @@ check_headers asm/types.h
>   # it seems there are versions of clang in some distros that try to use the
>   # gcc headers, which explodes for stdatomic
>   # so we also check that atomics actually work here
> -check_builtin stdatomic stdatomic.h "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0); foo += bar"
> +#
> +# some configurations also require linking to libatomic, so try
> +# both with -latomic and without
> +for LATOMIC in "-latomic" ""; do

Shouldn't you try without it first? On my toolchain libatomic is 
present, but libraries compile without linking to it just fine. That 
changes after this patch, where it starts linking to it explicitly.

> +    check_builtin stdatomic stdatomic.h                                                 \
> +        "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0); foo += bar"  \
> +        $LATOMIC && add_extralibs $LATOMIC && break

You should probably add it to the required libraries' extralibs only.
Just replace the add_extralibs part with setting stdatomic_extralibs to 
$LATOMIC, and then add stdatomic to all the libraries' _suggest lists, 
same as we do for libm.

> +done
>   
>   check_lib advapi32 "windows.h"            RegCloseKey          -ladvapi32
>   check_lib bcrypt   "windows.h bcrypt.h"   BCryptGenRandom      -lbcrypt &&