Optimizing shared library size

c0d1f1ed · 08-01-2012, 09:48 PM

Hi all, I've ported code for a Windows DLL to Linux, but the shared library is over four times bigger than the DLL!

It's a fairly big code base but only a small number of functions is exported from the DLL (using a .def file). I found out that the static library on the other hand exports all functions. So what I believe is happening is that on Windows all functions which are unused by the exported functions get stripped from the DLL.

So my question is, how can I specify the functions that should be exported from the shared library, and how do I get the linker to minimize the binary size based on that limited set?

NevemTeve · 08-01-2012, 10:30 PM

strip(1) might help to reduce the size

NevemTeve · 08-02-2012, 03:22 AM

Here are some macros I've hacked to control exorting symbols from a shared object:

Code:

#ifndef DLL_PUBLIC
  #if defined (_Windows)
    #if defined (__GNUC__)
      #define DLL_PUBLIC(type,name) __attribute__ ((dllexport)) type name
      #define DLL_LOCAL(type,name) type name
    #else
      #define DLL_PUBLIC(type,name) type _export name
      #define DLL_LOCAL(type,name) type name
    #endif

  #elif defined (__unix__) || defined (__UNIX__)
    #define DLL_PUBLIC(type,name) __attribute__ ((visibility ("default"))) type name
    #define DLL_LOCAL(type,name)  __attribute__ ((visibility ("hidden"))) type name
  #endif
#endif

usage example:

Code:

DLL_LOCAL (long, HttpValue) (const ConstBuffData *b, int base) {...}
DLL_PUBLIC (int, Http_Mime) (const ConstBuffData *b, HTTP_MIME *ret) {...}

Here HttpValue won't be exported from the shared object, but HttpMime will.

kauuttt · 08-02-2012, 03:32 AM

Some browsing landed me to the below links. Please check, if it helps you.

http://www.rt-embedded.com/blog/arch...zation-part-1/
http://www.rt-embedded.com/blog/arch...zation-part-2/
http://www.rt-embedded.com/blog/arch...zation-part-3/

c0d1f1ed · 08-02-2012, 04:34 PM

Quote:

Originally Posted by NevemTeve

strip(1) might help to reduce the size

I'm already doing that. The shared library is still much, much bigger than the DLL.

c0d1f1ed · 08-03-2012, 10:55 AM

Quote:

Originally Posted by NevemTeve

Here are some macros I've hacked to control exorting symbols from a shared object:

Code:

#ifndef DLL_PUBLIC
  #if defined (_Windows)
    #if defined (__GNUC__)
      #define DLL_PUBLIC(type,name) __attribute__ ((dllexport)) type name
      #define DLL_LOCAL(type,name) type name
    #else
      #define DLL_PUBLIC(type,name) type _export name
      #define DLL_LOCAL(type,name) type name
    #endif

  #elif defined (__unix__) || defined (__UNIX__)
    #define DLL_PUBLIC(type,name) __attribute__ ((visibility ("default"))) type name
    #define DLL_LOCAL(type,name)  __attribute__ ((visibility ("hidden"))) type name
  #endif
#endif

usage example:

Code:

DLL_LOCAL (long, HttpValue) (const ConstBuffData *b, int base) {...}
DLL_PUBLIC (int, Http_Mime) (const ConstBuffData *b, HTTP_MIME *ret) {...}

Here HttpValue won't be exported from the shared object, but HttpMime will.

Unfortunately that's not practical since there are many thousands of functions, and there's lots of third party source code. However, this did lead me to a GCC option to hide all functions by default: http://gcc.gnu.org/wiki/Visibility I have high hopes that this will reduce the library size to something more acceptable. So thanks for pointing me in this direction!