🚀 go-pugleaf

jek@cs.duke.edu (James E. Kittock) writes:

> While working on this, I have come to the conclusion that the C
> interface to PVM has a serious "want", namely, a routine to
> automagically put a structure and all of its contents, *including* the
> contents of any arrays/structures/unions pointed to by members of that
> structure.

  This is a standard problem in distributed systems. To recursively
pass a data structure between machines, you in general have
to pack recursively all the objects to which the first object points into a
message, then unpack the objects on the other side and reconstruct
the pointers. To pack an object given a pointer to that object, 
you need to know the type and size of the object. In general, 
it is impossible for a C program to get type and size information 
given only a pointer to an object. If you want to recursively pass
data structures between machines, you need to augment either the 
pointers or the data structures to which they point with type and 
size information. 

  Here at Stanford, we have developed a language called Jade that
supports the concept of an object which consists of several 
allocation units. The Jade frontend generates routines that 
recursively pack and unpack such objects, reconstructing the
pointers as necessary.  When the Jade implementation determines that
such an object needs to be passed from one machine to another, 
the sender calls the packing routines to put the data into a message,
then sends the message off to the receiver. The receiver then 
invokes the routines that unpack the object and reconstruct the
pointers. To support this functionality, the Jade memory allocators
augment allocated data with a type tag and a size tag. The packing
routines use these tags to determine how to pack and unpack the objects.

  So, such a scheme (in general) requires programming language support
beyond that which C provides. But, you can put the type and size tags
in by hand, and generate your own routines to pack and unpack the
data structures. 


  Martin