tags.c

/*
 * Dummy port of the Amiga tag based extensible api. See:
 * 
 * http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_3._guide/node03D6.html
 *
 *
 * note that the amiga api relied on the fact that the calling convention was
 * very simple so the vararg stub could just take the pointer to the last argument,
 * and get an array of TagItem records.
 *
 * see:
 * http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_3._guide/node03D4.html
 * 
 * we cannot do the same with portable C, so this port uses a slightly different approach,
 * taking some ideas from https://gist.github.com/mmikulicic/9a02109c8f3b66092038 .
 *
 */ 


#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>

typedef int (*tag_t)(va_list argp, void* data);

int TAG_END(va_list argp, void* data) {
  return -1;
}

void process_tags(va_list argp, tag_t tag, void *data) {
  while(1) {
    if (tag(argp, data) == -1) {
      break;
    }
    tag = va_arg(argp, tag_t);
  }
}

// ----- ns_send

typedef struct {
  int deadline;
  int n_headers;
} ns_send_opts_t;


int ns_deadline(va_list argp, void* data) {
  ns_send_opts_t* opts = (ns_send_opts_t*)data;
  opts->deadline = va_arg(argp, int);

  printf("processing ns_deadline: %d\n", opts->deadline);
  return 0;
}

// I'm not saying this should be allowed, but it's technically possible
// to consume multiple arguments instead of bundling them into a struct.
int ns_header(va_list argp, void* data) {
  char *key = va_arg(argp, char*);
  char *value = va_arg(argp, char*);
  printf("processing ns_header: %s:%s\n", key, value);

  ns_send_opts_t* opts = (ns_send_opts_t*)data;
  opts->n_headers++;
  return 0;
}


void ns_send_tags(char* addr, char* body, tag_t tag, ...) {
  // some default values
  ns_send_opts_t opts = {-1, 0};

  va_list argp;  
  va_start(argp, tag);
  process_tags(argp, tag, &opts);
  va_end(argp);

  printf("sending %s to %s, deadline: %d, num headers: %d\n", body, addr, opts.deadline, opts.n_headers);  
}

void ns_send(char* addr, char* body) {
  ns_send_tags(addr, body, TAG_END);
}


int main() {
  ns_send("foo", "bar");
  ns_send_tags("foo", "bar", ns_header, "test", "blah", ns_deadline, 3600, TAG_END);

  return 0;
}