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Harbour code blocks

Compilation of a codeblock.

During compile time the codeblock is stored in the following form:
  1. the header
  2. the stream of pcode bytes

The header

The header stores information about referenced local variables.
  • +0: the pcode byte for _PUSHBLOCK
  • +1: the number of bytes that defines a codeblock
  • +3: number of codeblock parameters (declared between || in a codeblock)
  • +5: number of used local variables declared in procedure/function where the codeblock is created
  • +7: the list of procedure/function local variables positions on the eval stack of procedure/function. Every local variable used in a codeblock occupies 2 bytes in this list. When nested codeblocks are used then this list is created for the outermost codeblock only.
  • +x: The stream of pcode bytes follows the header.
  • +y: the pcode byte for _ENDBLOCK

Creation of a codeblock.

When HB_P_PUSHBLOCK opcode is executed then the HB_ITEM structure is created and placed on the eval stack.

The type of item is IT_BLOCK. The value of this item is a pointer to HB_CODEBLOCK structure.

Additionally this item stores the base of static variables defined for the current function/procedure - this is used during a codeblock evaluation when the evaluation is called from a codefrom other PRG module. Also the number of expected parameters is stored.

The HB_CODEBLOCK structure stores a pointer to the pcodes stream that is executed during a codeblock evaluation.

It stores also the pointer to a table with local variables references. Values of all local variables defined in a procedure and used in a codeblock are replaced with a reference to a value stored in a global memory variables pool.

This allows the correct access to detached local variables in a codeblock returned from this function (either directly in RETURN statement or indirectly by assigning it to a static or memvar variable.

This automatic and unconditional replace is required because there is no safe method to find if a codeblock will be accessed from an outside of a function where it is created.

Reference

  • bazirano na "The Harbour implementation of codeblocks, Ryszard Glab <> (harbour doc/codebloc.txt)