Exporting symbols on OSX ?

[David Holmes]

On 8/05/2013 2:55 AM, David DeHaven wrote:
>
>> We just discovered that every symbol is libjvm is being exported on OSX. The OSX/BSD makefiles were copied from the linux ones and so have the options for linker scripts and mapfiles, but as OSX doesn't support linker scripts this is disabled by setting LDNOMAP=true - hence all symbols are exported.
>>
>> However the OSX linker does support -exported_symbols_list
>>
>> https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/ld.1.html
>>
>> so I was wondering why the build wasn't modified to use that so that we only export the JVM symbols we intend to export? (That's a question for the macosx-port-dev folk :) ).
>>
>> Does anyone have experience using this such that we can get the hotspot build fixed?
>
> Exports should be handled by the visibility settings. Until recently the JNIEXPORT macro resolved to nothing, now it sets __attribute__(("visibility" ("default))) (at least in 8, that should be backported to 7u if it hasn't already). The "default" visibility means export that symbol. To hide non-exported symbols you need to provide --fvisibility=hidden to gcc or clang when compiling.
>
> Hotspot has it's own jni_md.h headers with macros to determine JNIEXPORT, but they were wrong last I checked. It appears they attempted to clear the macro if a certain version of gcc was used, but what happens is it disables it for all versions of gcc above a particular release (which hasn't been used on Mac is quite some time). I pointed this out when the JNIEXPORT patch was posted for review to core-libs-dev (I think...), it was CC'd to the hotspot list too so someone has to have seen it. I'm not sure if it's been fixed or not.

The visibility setting changes should aid with the primary exported 
interfaces (JNI_ and JVM_) but looking at the build there are other 
symbols (for Serviceability Agent I think) that get dynamically added to 
the mapfiles - so I'm not sure if the visibility setting alone will help 
here. I think we will still want the "exported_symbol_file" (but it 
seems whatever ld we are using for OSX builds doesn't actually recognize 
that option ?? :(  Or I'm doing something wrong in trying to pass it)

Thanks,
David

> -DrD-
>