Avoiding DLL Hell

This page provides some notes on avoiding DLL hell when creating libraries. Changes to library API's often create a real mess for users.

Always initialize structures by copying from defaults

In the library:

   typedef struct {
      int x;
      int y;
      int z;
      char pad[16];
   } point_3d_t;
   point_3d_t point_3d_defaults = {
      0,0,0
   }

In the application program:

   point_3d_t point;
   point = point_3d_defaults;  // <--- always do this first
   point.x = 100;
   point.y = 200;
   point.z = 300;
   function_using_point(point);

Now consider what happens when we add a member "layer" to point_3d_t.

Use consistent naming conventions. A type ends in "_t", the defaults for that type ends in "_defaults", a pointer ends in "_p", etc.

Add padding

Add some padding as illustrated above. Reduce the padding when you add new members: char pad[16 - sizeof(int)]; Combined with the defaults, this can provide binary compatibilty even though the structure changes. Note, however, that with different sizes for int and different alignment rules on RISC CPUS there may be some more sophistication needed here.

Put versions on functions

Instead of function_using_point(), define function_using_point_v1(). When you change the parameters to the function in the future, change the name to function_using_point_v2() and define a short function_using_point_v1() that calls function_using_point_v2().