2.05: How do I make my own shared library?

To make your own shared object or library of shared objects, you should
know that a shared object cannot have undefined symbols.  Thus, if your
code uses any externals from /lib/libc.a, the latter MUST be linked with
your code to make a shared object.  Mike Heath (mike@pencom.com) said it
is possible to split code into more than one shared object when externals
in one object refer to another one.  You must be very good at
import/export files.  Perhaps he or someone can provide an example. 

Assume you have one file, sub1.c, containing a routine with no external
references, and another one, sub2.c, calling stuff in /lib/libc.a.  You
will also need two export files, sub1.exp, sub2.exp.  Read the example
below together with the examples on the ld man page. 

---- sub1.c ----
    int addint(int a, int b)
    {
      return a + b;
    }
---- sub2.c ----
    #include <stdio.h>

    void printint(int a)
    {
      printf("The integer is: %d\n", a);
    }
---- sub1.exp ----
    #!
    addint
---- sub2.exp ----
    #!
    printint
---- usesub.c ----
    main()
    {
      printint( addint(5,8) );
    }

The following commands will build your libshr.a, and compile/link the
program usesub to use it.

  $ cc  -c sub1.c
  $ cc -bM:SRE -bnoentry -bE:sub1.exp -o sub1shr.o sub1.o
  $ cc  -c sub2.c
  $ cc -bM:SRE -bnoentry -bE:sub2.exp -o sub2shr.o sub2.o
  $ ar r libshr.a sub1shr.o sub2shr.o
  $ cc -o usesub usesub.c -L: libshr.a
  $ usesub
  The integer is: 13
  $