[FFmpeg-devel] [PATCH] ffbuild: read library linker objects from a file

[Gyan Doshi]

On 2025-03-12 01:29 pm, Martin Storsjö wrote:
> On Wed, 12 Mar 2025, Gyan Doshi wrote:
>
>> The linker command can exceed the maximum argument limit on MinGW,
>> especially for libavcodec.
>>
>> The objects list is now stored in a file and passed to the linker.
>> ---
>> ffbuild/library.mak | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/ffbuild/library.mak b/ffbuild/library.mak
>> index 793e9d41fa..781b018e00 100644
>> --- a/ffbuild/library.mak
>> +++ b/ffbuild/library.mak
>> @@ -66,8 +66,10 @@ $(SUBDIR)$(SLIBNAME): $(SUBDIR)$(SLIBNAME_WITH_MAJOR)
>>
>> $(SUBDIR)$(SLIBNAME_WITH_MAJOR): $(OBJS) $(SHLIBOBJS) $(SLIBOBJS) 
>> $(SUBDIR)lib$(NAME).ver
>>     $(SLIB_CREATE_DEF_CMD)
>> -    $$(LD) $(SHFLAGS) $(LDFLAGS) $(LDSOFLAGS) $$(LD_O) $$(filter 
>> %.o,$$^) $(FFEXTRALIBS)
>> +    $(Q)echo $$(filter %.o,$$^) > $$@.objs
>> +    $$(LD) $(SHFLAGS) $(LDFLAGS) $(LDSOFLAGS) $$(LD_O) @$$@.objs 
>> $(FFEXTRALIBS)
>>     $(SLIB_EXTRA_CMD)
>> +    -$(RM) $$@.objs
>
> Don't we need do the same for static libraries too?
>
> On first glance, this looks quite reasonable... However, the limit 
> that is an issue is the length of a command line. The .objs file is 
> written with an "echo" command - doesn't that fundamentally also hit 
> the same limit (just a little bit later, as there are fewer command 
> line flags in this command)?
>
> Or do we assume that make executes it with a shell, where the shell 
> handles "echo" as a shell builtin so that it doesn't actually spawn a 
> subprocess for this? Can you test it, if you could extend the list of 
> object files to the point where the .objs file is clearly over 32 KB, 
> and verify that it did include all the files you intended?

The specific error I got when building a shared build of 7.1.1 (with ~90 
external libs) was

/bin/sh: line 1: /mingw64/bin/ccache: Argument list too long

Got same error without ccache.

The static build of the same config succeeded without any patching.
The .objs file generated for libavcodec shared build is 29KB.

How do I inflate the size to above 32K? I've enabled pretty much 
everything I can.


> Secondly, less fundamental - even if we try to remove the .objs file 
> here at the same time it's quite possible for it to be left behind 
> (e.g. if the linking command fails) - so it would be good to make sure 
> that it is removed on "make clean" too.
>
> Plus we probably should include it in .gitignore, if someone does 
> in-tree builds.

Will add both.

Regards,
Gyan