Advanced Perl Programming

Stacks and Messaging Protocol

Whew! We have now finished a reasonably in-depth look at all the value types offered by Perl. The next half of this chapter is devoted to understanding the data structures, API, and protocol used between caller and called subroutines.

We mentioned earlier that the argument stack is the data structure used for passing parameters and results between functions. Figure 20.11 shows the stack after calling foo(10,20), which in turn has called bar("hello", 30.2, 100).

Figure 20-11. Argument and mark stack after foo has been called and foo has just called bar

How does bar know how many parameters it should pick up from the top of stack? Well, Perl keeps track of the stretches of the argument stack using another stack called a markstack (a stack of bookmarks, in a sense). bar knows the parameters meant for it by simply computing the difference between the current top of stack and the bookmark stored at the top of markstack. This stretch of the stack corresponds to bar’s @_ array. Conversely, when bar is ready to return, it dumps one or more results in its stretch of stack, and foo knows how many scalars have been returned by looking at the markstack.

All these manipulations happen transparently when you are in script space. But if you write C routines that are called by Perl (extending Perl) or call Perl functions from C (embedding Perl), there are some details ...