Archives

• An archive (or static library) is simply a collection of object files stored as a single file.
• When you provide an archive to the linker, the linker searches the archive for the object files it needs, extracts them, and links them into your program much as if you had provided those object files directly.
• You can create an archive using the ar command.

  $ ar cr libtest.a test1.o test2.o
  

  The cr flags tell ar to create the archive. Now you can link with this archive using the -ltest option with gcc or g++
• When the linker encounters an archive on the command line, it searches the archive for all definitions of symbols (functions or variables) that are referenced from the object files that it has already processed but not yet defined.
• The object files that define those symbols are extracted from the archive and included in the final executable. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/test.ctest.c

  int f () 
  {
    return 3;
  }
  

  http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/app.capp.c

  int main ()
  {
  	return f ();
  }
  

  The following command line will not work:

  $ gcc -o app -L. -ltest app.o
  app.o: In function 'main':
  app.o(.text+0x4): undefined reference to 'f'
  collect2: ld returned 1 exit status
  

  On the other hand, if we use this line, no error messages are issued:

  $ gcc -o app app.o -L. -ltest
  

  The reason is that the reference to f in app.o causes the linker to include the test.o object file from the libtest.a archive.