Customizable garbage collectors

Conventional garbage collectors retain components of data structures as long as the structure itself is accessible. In some applications, however, the components of a data structure are useful only if they are referred by external pointers. Recognition and deletion of useless components requires co-operation between the garbage collector and the user program: the analysis of accessibility is carried out by the garbage collector but user-defined routines are necessary to remove obsolete components from the structure. Two different co-operation mechanisms are discussed; the first can be used with garbage collectors based on reference counters, the other with mark-and-sweep collectors.