17.3.1 Request-processing Instruction

Request-processing program consists of instructions. There are seven basic instruction types:

grad_instr_conditional_t

This instruction marks a branch point within the program.

grad_instr_call_t

Represents a call of a subprogram

grad_instr_action_t

Invokes a Rewrite <FIXME> or Scheme? </> function

grad_instr_proxy_t

Proxies a request to the remote server

grad_instr_forward_t

Forwards a request to the remote server

grad_instr_reply_t

Replies back to the requesting NAS.

Consequently, an instruction is defined as a union of the above node types:

Instruction: grad_instr_t

enum grad_instr_type { grad_instr_conditional, grad_instr_call, grad_instr_return, grad_instr_action, grad_instr_reply, grad_instr_proxy, grad_instr_forward }; typedef struct grad_instr grad_instr_t; struct grad_instr { enum grad_instr_type type; grad_instr_t *next; union { grad_instr_conditional_t cond; grad_instr_call_t call; grad_instr_action_t action; grad_instr_reply_t reply; grad_instr_proxy_t proxy; grad_instr_forward_t forward; } v; };

Type member contains type of the instruction. The evaluator uses type to determine which part of union v, holds instruction-specific data.

Next points to the next instruction. The evaluator will go to this instruction unless the present one changes the control flow.

Finally, v contains instruction-specific data. These will be discussed in the following subsections.