avahi.c

// Brand new version that just forks and execs the commands 'avahi-browse' or 'avahi-publish'
// Using the Avahi API is a nightmare; it's a twisted mess of callbacks between "callbacks", "services", "resolvers", etc.
// March 2024, Phil Karn KA9Q
#include <stdio.h>
#include <unistd.h>
#include <arpa/inet.h>
#include "avahi.h"

int avahi_start(char const *service_name,char const *service_type,int const service_port,char const *dns_name,int address,char const *description,void *sock,int *socksize){
  if(sock != NULL && socksize != NULL){
    // Return sockaddr structure
    if(*socksize >= sizeof(struct sockaddr_in)){
      struct sockaddr_in *sin = sock;
      sin->sin_family = AF_INET;
      sin->sin_addr.s_addr = htonl(address);
      sin->sin_port = htons(service_port);
      *socksize = sizeof(struct sockaddr_in);
    } else
      *socksize = 0;
  }
  int pid = getpid(); // Advertise the parent's pid, not the child's
  if(fork() == 0){
#if 0
    fprintf(stdout,"avahi-publish-service child pid %d\n",getpid());
#endif
    // run "avahi-publish-service --no-fail --host=dns_name service_name service_type service_port description pid hostname" in subprocess
    // No need to free the asprintf-allocated strings, we're calling exec anyway
    // Turn off warnings about unused return values from asprintf generated by some compilers
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-result" 
    char *port_string = NULL;
    asprintf(&port_string, "%d", service_port);

    char *host_string = NULL;
    asprintf(&host_string, "--host=%s",dns_name);

    char *pid_string = NULL;
    asprintf(&pid_string, "pid=%d", pid);

    char hostname[sysconf(_SC_HOST_NAME_MAX)];
    gethostname(hostname,sizeof(hostname));
    char *source_string = NULL;
    asprintf(&source_string, "source=%s", hostname);
#pragma GCC diagnostic pop

#if 0
    fprintf(stdout,"%s %s %s %s %s %s %s %s %s %s\n",
	    "avahi-publish-service", "avahi-publish-service", "--no-fail", host_string, service_name, service_type, port_string, description, pid_string, hostname);
#endif
    execlp("avahi-publish-service", "avahi-publish-service", "--no-fail", host_string, service_name, service_type, port_string, description, pid_string, source_string, NULL);
    perror("exec avahi publish service");
    return -1;
  }
  if(fork() == 0){
    // run "avahi-publish-address dns_name address"
    char *ip_address_string = NULL;    // No need to free, we're calling exec
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-result" 
    asprintf(&ip_address_string,"%d.%d.%d.%d",(address >> 24) & 0xff, (address >> 16) & 0xff, (address >> 8) & 0xff, address & 0xff);
#pragma GCC diagnostic pop
#if 0
    fprintf(stdout,"avahi start: ip address string = %s\n",ip_address_string);
    fprintf(stdout,"avahi-publish-address child pid %d\n",getpid());
    fprintf(stdout,"%s %s %s %s\n",
	    "avahi-publish-address", "avahi-publish-address",dns_name,ip_address_string);
#endif
    execlp("avahi-publish-address", "avahi-publish-address",dns_name,ip_address_string, NULL);
    perror("exec avahi publish address");
    return -1;
  }

  return 0;
}