[Ffmpeg-devel] [patch] gcc4 visibility support

matthieu castet castet.matthieu
Fri Jun 2 18:40:37 CEST 2006

Mike Melanson wrote:
> Luca Barbato wrote:
>> Diego Petten? started adding support for gcc visibility on every
>> application he could touch, including ffmpeg.
>> Looks ok and helped already to find application playing with private
>> symbols. (should also trim a bit the binary size)
> Could you explain this a bit? What is meant by "gcc visibility"? A 
> cursory review of the patch seems to indicate that public symbols in the 
> C files are now qualified with the keyword EXPORTED. What's this all 
> about? I haven't heard of this before.

in sumary it allow to hide global variable to others libraries and allow 
to avoid symbol clash :

visibility ("visibility_type")
     The visibility attribute on ELF targets causes the declaration to 
be emitted with default, hidden, protected or internal visibility.

           void __attribute__ ((visibility ("protected")))
           f () { /* Do something. */; }
           int i __attribute__ ((visibility ("hidden")));

     See the ELF gABI for complete details, but the short story is:

         Default visibility is the normal case for ELF. This value is 
available for the visibility attribute to override other options that 
may change the assumed visibility of symbols.
         Hidden visibility indicates that the symbol will not be placed 
into the dynamic symbol table, so no other module (executable or shared 
library) can reference it directly.
         Internal visibility is like hidden visibility, but with 
additional processor specific semantics. Unless otherwise specified by 
the psABI, GCC defines internal visibility to mean that the function is 
never called from another module. Note that hidden symbols, while they 
cannot be referenced directly by other modules, can be referenced 
indirectly via function pointers. By indicating that a symbol cannot be 
called from outside the module, GCC may for instance omit the load of a 
PIC register since it is known that the calling function loaded the 
correct value.
         Protected visibility indicates that the symbol will be placed 
in the dynamic symbol table, but that references within the defining 
module will bind to the local symbol. That is, the symbol cannot be 
overridden by another module.

     Not all ELF targets support this attribute.

More information about the ffmpeg-devel mailing list