What is the correct way to link against shared libraries?

Question

I'm cross compiling a C application and am linking against the alsa library with -lasound

My newest cross compiler said it can't find the library, so I went exploring in the compiler's library directories and found.

libasound.la libasound.so.2 libasound.so.2.0.0

I did not find a libasound.so , so to work around my problem i created a sym link

ln -s libasound.so.2.0.0 libasound.so

and everything appears to be okay now. I am positive this is not the right way to do this though. Am I supposed to use different linker options to link against this? And what do each of the different libasound.* files mean?

Answer 1

The most usual name of libraries in Linux is:

• The real library is libasound.so.2.0.0 . The last 3 numbers is the library version (major.minor.revision).
• libasound.so.2 is a symbolic link to the preferred (latest) 2.* version of the library, should you have more than one installed in the system. It is assumed that all the 2.* version are backwards binary compatible.
• libasound.la is a text file with a lot of information about the library to be used with libtool . Useful if you use libtool and the other autotools.
• libasound.so is a symbolic link to the library to be used by the toolchain. This is the file looked for when you link with -lasound .

You are missing the last one, maybe because in debian based systems it is installed only with the libasound-dev package. You can simply create it manually. It is not needed during runtime because the library has a SONAME entry in the header:

$ objdump -x /usr/lib/libasound.so | grep SONAME
  SONAME               libasound.so.2

That makes the dynamic linker look for that name at runtime, no matter what compiler options you used.

I hope I made some sense of this, because it is a bit complicated...