// simple ping
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <assert.h>
#include <arpa/inet.h>
#include <netinet/ip_icmp.h>

typedef unsigned char byte;
#define warn(a...) do{fprintf(stderr, a); }while(0)
#define die(a...) do{fprintf(stderr, a); exit(1);}while(0)
#define diesys(a...) do{fprintf(stderr, a); perror(" sys"); exit(1);}while(0)

static struct{
  unsigned seqno;
  uint64_t cur_us;
  struct sockaddr_in pingaddr;
  int myid;
}ping;

static uint16_t
inet_cksum(void const* ptr, unsigned sz){
  byte const* p = ptr;
  uint32_t sum = 0;
  uint16_t u16;
  for(;sz>=2; sz-=2,p+=2){
    memcpy(&u16, p, sizeof(u16));
    sum += u16;
  }
  if(1==sz){
    memcpy(&u16, (byte[]){*p, 0}, sizeof(u16));
    sum += u16;
  }
  sum = (sum >> 16) + (sum & 0xffff);
  sum += (sum >> 16);
  return (uint16_t)~sum;
}

static int
create_icmp_sock(void){
  enum{icmp_protocol=1};
  int sock = socket(AF_INET, SOCK_RAW, icmp_protocol);
  if(sock<1) diesys("socket");

  int sockopt=1;
  if(setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &sockopt, sizeof(sockopt))) die("setsockopt SO_BROADCAST");
  return sock;
}

static uint64_t
nowus(void){
  struct timespec ts;
  if(clock_gettime(CLOCK_MONOTONIC, &ts)) diesys("clock_gettime");
  uint64_t const us = ts.tv_sec * 1000000ULL + ts.tv_nsec/1000;
  return us;
}

static void show(struct sockaddr_in* from, unsigned sz, struct icmp *icmp){
  assert(sz>=sizeof(struct icmp));
  static char ipaddrstr[INET_ADDRSTRLEN];
  inet_ntop(AF_INET, &(from->sin_addr), ipaddrstr, INET_ADDRSTRLEN);
  fprintf(stderr, "%u bytes from from (%s)", sz, ipaddrstr);
  uint64_t tus;
  memcpy(&tus, icmp->icmp_data, sizeof(tus));
  fprintf(stderr, " elapsed time %0.3fus\n", (double)(nowus()-tus)/1000);
}

static void send_ping(int sock){
  enum{noflags=0};
  struct icmp icmp;
  memset(&icmp, 0xab, sizeof(icmp));
  icmp.icmp_type = ICMP_ECHO;
  icmp.icmp_code = 0;
  icmp.icmp_cksum = 0; // must=zero for checksum calc
  icmp.icmp_seq = htons(ping.seqno);
  icmp.icmp_id = ping.myid = getpid();
  ++ping.seqno;

  uint64_t tus  = nowus();
  memcpy(icmp.icmp_data, &tus, sizeof(tus)); // the icmp_dun{} data union is big enough for the timestamp

  size_t const size_pkt = sizeof(struct icmp);
  icmp.icmp_cksum = inet_cksum(&icmp, size_pkt);

  ssize_t sz = sendto(sock, &icmp, size_pkt, noflags, (struct sockaddr*)&ping.pingaddr, sizeof(ping.pingaddr));
  if(size_pkt != sz) diesys("sendto");
}

static void
await_pingback(int sock){
  enum{noflags=0};
  struct sockaddr_in from;
  socklen_t fromlen = (socklen_t)sizeof(from);
  byte pkt[sizeof(struct iphdr) + sizeof(struct icmp)];
  for(;;){
    int const sz = recvfrom(sock, pkt, sizeof(pkt), noflags, (struct sockaddr *)&from, &fromlen);
    if(sz<0){
      if(EINTR==errno) continue;
      diesys("recvfrom");
    }
    if(sz < (int)(sizeof(struct icmp))){
      warn("received packet is too small size:%d\n", sz);
      continue;
    }
    struct iphdr *iphdr = (struct iphdr *)pkt;
    struct icmp *icmp = (struct icmp *)(pkt + (iphdr->ihl * 4));
    if(icmp->icmp_id != ping.myid){
      warn("not our ping id:%d\n", icmp->icmp_id);
      continue;
    }
    switch(icmp->icmp_type){
      case ICMP_ECHOREPLY: show(&from, sz, icmp); return; break;
      case ICMP_ECHO: /*ignore, echo from us*/; break;
      default:
        warn("received not ICMP_ECHOREPLY/ICMP_ECHO type:icmp->icmp_type:%d\n", icmp->icmp_type);
        break;
    }
  }
}

int main(int ac, char *av[]){
  if(2!=ac) die("usage: %s <numeric ipaddr>\n", av[0]);
  switch(inet_pton(AF_INET, av[1], &(ping.pingaddr.sin_addr))){
    default: diesys("inet_pton");
    case 0: die("expected a numeric ip address e.g. 127.0.0.1\n");
    case 1: break;
  }
  int sock = create_icmp_sock();
  send_ping(sock);
  await_pingback(sock);

  if(close(sock)<0) diesys("close socket");
  return 0;
}