text: #define _GNU_SOURCE
  
  #include <sys/socket.h>
  #include <netdb.h>
  #include <string.h>
  #include <netinet/in.h>
  #include <errno.h>
  #include <inttypes.h>
  
  int gethostbyaddr_r(const void *a, socklen_t l, int af,
  	struct hostent *h, char *buf, size_t buflen,
  	struct hostent **res, int *err)
  {
  	union {
  		struct sockaddr_in sin;
  		struct sockaddr_in6 sin6;
  	} sa = { .sin.sin_family = af };
  	socklen_t sl = af==AF_INET6 ? sizeof sa.sin6 : sizeof sa.sin;
  	int i;
  
  	/* Load address argument into sockaddr structure */
  	if (af==AF_INET6 && l==16) memcpy(&sa.sin6.sin6_addr, a, 16);
  	else if (af==AF_INET && l==4) memcpy(&sa.sin.sin_addr, a, 4);
  	else {
  		*err = NO_RECOVERY;
  		return -1;
  	}
  
  	/* Align buffer and check for space for pointers and ip address */
  	i = (uintptr_t)buf & sizeof(char *)-1;
  	if (!i) i = sizeof(char *);
  	if (buflen <= 5*sizeof(char *)-i + l) {
  		errno = ERANGE;
  		return -1;
  	}
  	buf += sizeof(char *)-i;
  	buflen -= 5*sizeof(char *)-i + l;
  
  	h->h_addr_list = (void *)buf;
  	buf += 2*sizeof(char *);
  	h->h_aliases = (void *)buf;
  	buf += 2*sizeof(char *);
  
  	h->h_addr_list[0] = buf;
  	memcpy(h->h_addr_list[0], a, l);
  	buf += l;
  	h->h_addr_list[1] = 0;
  	h->h_aliases[0] = buf;
  	h->h_aliases[1] = 0;
  
  	switch (getnameinfo((void *)&sa, sl, buf, buflen, 0, 0, 0)) {
  	case EAI_AGAIN:
  		*err = TRY_AGAIN;
  		return -1;
  	case EAI_OVERFLOW:
  		errno = ERANGE;
  	default:
  	case EAI_MEMORY:
  	case EAI_SYSTEM:
  	case EAI_FAIL:
  		*err = NO_RECOVERY;
  		return -1;
  	case 0:
  		break;
  	}
  
  	h->h_addrtype = af;
  	h->h_name = h->h_aliases[0];
  	*res = h;
  	return 0;
  }
  