text: #define _GNU_SOURCE
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <netdb.h>
  #include <inttypes.h>
  #include <errno.h>
  #include <string.h>
  #include <stdlib.h>
  
  int getservbyport_r(int port, const char *prots,
  	struct servent *se, char *buf, size_t buflen, struct servent **res)
  {
  	int i;
  	struct sockaddr_in sin = {
  		.sin_family = AF_INET,
  		.sin_port = port,
  	};
  
  	if (!prots) {
  		int r = getservbyport_r(port, "tcp", se, buf, buflen, res);
  		if (r) r = getservbyport_r(port, "udp", se, buf, buflen, res);
  		return r;
  	}
  	*res = 0;
  
  	/* Align buffer */
  	i = (uintptr_t)buf & sizeof(char *)-1;
  	if (!i) i = sizeof(char *);
  	if (buflen < 3*sizeof(char *)-i)
  		return ERANGE;
  	buf += sizeof(char *)-i;
  	buflen -= sizeof(char *)-i;
  
  	if (strcmp(prots, "tcp") && strcmp(prots, "udp")) return EINVAL;
  
  	se->s_port = port;
  	se->s_proto = (char *)prots;
  	se->s_aliases = (void *)buf;
  	buf += 2*sizeof(char *);
  	buflen -= 2*sizeof(char *);
  	se->s_aliases[1] = 0;
  	se->s_aliases[0] = se->s_name = buf;
  
  	switch (getnameinfo((void *)&sin, sizeof sin, 0, 0, buf, buflen,
  		strcmp(prots, "udp") ? 0 : NI_DGRAM)) {
  	case EAI_MEMORY:
  	case EAI_SYSTEM:
  		return ENOMEM;
  	default:
  		return ENOENT;
  	case 0:
  		break;
  	}
  
  	/* A numeric port string is not a service record. */
  	if (strtol(buf, 0, 10)==ntohs(port)) return ENOENT;
  
  	*res = se;
  	return 0;
  }
  