Looking for runtime functions

I've written some code that uses these functions from the Apple runtime:

    /* get/set iVar values */
    object_getInstanceVariable
    object_setInstanceVariable

    /* fetch the objc_ivar struct for a named ivar */
    class_getInstanceVariable

I can't seem to find the equivalents in the GNU runtime. The information
is all there, in the various structs, there's just no ready-made function
to retrieve it.

Do these or similar functions exist already somewhere I've missed, or do
I need to write them?

Thanks,

sherm--