Noob's attempt: Cross Platform UDP (Linux + Windows)

udpClient.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#ifdef _WIN32
#define WIN32_LEAN_AND_MEAN // prevent import of WinSock ? (conflict with WinSock2)
#include <windows.h> // sleep()
#define sleep(n) Sleep((n) * 1000)
#include <winsock2.h>
#include <ws2tcpip.h>
# define close closesocket
#else
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h> // sleep()
#endif


void main(int argc, char **argv){
  if(argc != 2){
    printf("Usage: %s <port>\n", argv[0]);
    exit(0);
  }



#ifdef _WIN32
  WSADATA wsaData;

  if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
      fprintf(stderr, "WSAStartup failed.\n");
      exit(1);
  }

  if (LOBYTE(wsaData.wVersion) != 2 ||
      HIBYTE(wsaData.wVersion) != 2)
  {
      fprintf(stderr,"Version 2.2 of Winsock is not available.\n");
      WSACleanup();
      exit(2);
  }
#endif





  int port = atoi(argv[1]);
  int sockfd;
  struct sockaddr_in serverAddr;
  char buffer[1024];
  socklen_t addr_size;

  sockfd = socket(PF_INET, SOCK_DGRAM, 0);
  memset(&serverAddr, '\0', sizeof(serverAddr));

  serverAddr.sin_family = AF_INET;
  serverAddr.sin_port = htons(port);
  serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");

  strcpy(buffer, "Hello Server\n");
  while(1){
    // sendto(sockfd, buffer, 1024, 0, (struct sockaddr*)&serverAddr, sizeof(serverAddr));
    sendto(sockfd, buffer, 1024, 0, (struct sockaddr*)&serverAddr, sizeof(serverAddr));
    printf("[+]Data Send: %s", buffer);
    sleep(1);
  }

  close(sockfd);


}

udpServer.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#ifdef _WIN32
# define WIN32_LEAN_AND_MEAN
# include <windows.h>
# define sleep(n) Sleep((n) * 1000)
# include <winsock2.h>
# include <ws2tcpip.h>
# define close closesocket
#else
# include <sys/socket.h>
# include <netinet/in.h>
# include <arpa/inet.h>
# include <unistd.h> // sleep()
#endif

void net_init() {
  printf("Server starting.\n");
#ifdef _WIN32
  WSADATA wsaData;

  if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
      fprintf(stderr, "WSAStartup failed.\n");
      exit(1);
  }

  if (LOBYTE(wsaData.wVersion) != 2 ||
      HIBYTE(wsaData.wVersion) != 2)
  {
      fprintf(stderr,"Version 2.2 of Winsock is not available.\n");
      WSACleanup();
      exit(2);
  }
#endif
}

void net_exit() {
  #ifdef _WIN32
  WSACleanup();
#endif
}

void main(int argc, char **argv){
  // if(argc != 2){
  //   printf("Usage: %s <port>\n", argv[0]);
  //   exit(0);
  // }


  char* address = "127.0.0.1";
  char* port_str = "1111";

  net_init();

  // int port = atoi(argv[1]);
  int port = atoi(port_str);

  // TODO: make global; move to net_init()?
  int sockfd;
  struct sockaddr_in si_me, si_other;
  char buffer[1024]; // TODO: how to queue messages in and out?

  socklen_t addr_size;

  sockfd = socket(AF_INET, SOCK_DGRAM, 0);

  memset(&si_me, '\0', sizeof(si_me));
  si_me.sin_family = AF_INET;
  si_me.sin_port = htons(port);
  // si_me.sin_addr.s_addr = inet_addr("127.0.0.1");
  // inet_aton("127.0.0.1", si_me.sin_addr); // doesn't work on windows.. sigh
  inet_pton(AF_INET, address, &si_me.sin_addr.s_addr);
  // NOTE: Can also use AF_INET6

  // https://stackoverflow.com/questions/23187496/what-is-the-meaning-of-sin-addr-s-addr-and-inet-addr
  // inet_addr was deprecated in favour of inet_aton even before IPv6 come into play because it's a poorly designed API call. It signals an error by returning INADDR_NONE (-1) but its return type in_addr_t is actually an unsigned integer, which makes passing the signed -1 very compiler-dependent. Also on machines with two's complement representation of negative numbers (that's most computer architectures nowadays) INADDR_NONE equals INADDR_BROADCAST (255.255.255.255).

  ///////////
  // kept becaue funny

//   int bindstatus = bind(sockfd, (struct sockaddr*)&si_me, sizeof(si_me));
//   if (bindstatus !=0 ) {
// #ifdef _WIN32 // sigh
//     int WS_err = WSAGetLastError();
//     if (WS_err == WSAEADDRINUSE) {
// #else
//     if (errno == EADDRINUSE) {
// #endif
//       // for local usage. If port used, try next port
//       printf("port %d taken... trying %d", port, port+1);
//       port++;
//       si_me.sin_port = htons(port);
//       int bindstatus2 = bind(sockfd, (struct sockaddr*)&si_me, sizeof(si_me));
//       if (bindstatus2 != 0) {
// #ifdef _WIN32
//         int WS_err = WSAGetLastError(); // note: shadowed in nested block
//         printf("Could not bind to port. WS_err: %d", WS_err);
// #else
//         printf("Could not bind to port. errno: %d", errno);
// #endif
//         exit(1);
//       }
//     } else {
// #ifdef _WIN32
//         int WS_err = WSAGetLastError(); // note: shadowed in nested block
//         printf("Could not bind to port. WS_err: %d", WS_err);
// #else
//         printf("Could not bind to port. errno: %d", errno);
// #endif
//       exit(1);
//     }
//   }


#ifdef _WIN32

  int bindstatus = bind(sockfd, (struct sockaddr*)&si_me, sizeof(si_me));
  if (bindstatus !=0 ) {
    int WS_err = WSAGetLastError();
    if (WS_err == WSAEADDRINUSE) {

      // for local usage. If port used, try next port
      printf("port %d taken... trying %d", port, port+1);
      port++;
      si_me.sin_port = htons(port);
      int bindstatus2 = bind(sockfd, (struct sockaddr*)&si_me, sizeof(si_me));
      if (bindstatus2 != 0) {
        int WS_err = WSAGetLastError(); // note: shadowed in nested block
        printf("Could not bind to port. WS_err: %d", WS_err);

        exit(1);
      }
    } else {
        int WS_err = WSAGetLastError(); // note: shadowed in nested block
        printf("Could not bind to port. WS_err: %d", WS_err);

      exit(1);
    }
  }


#else



  int bindstatus = bind(sockfd, (struct sockaddr*)&si_me, sizeof(si_me));
  if (bindstatus !=0 ) {

    if (errno == EADDRINUSE) {
      // for local usage. If port used, try next port
      printf("port %d taken... trying %d", port, port+1);
      port++;
      si_me.sin_port = htons(port);
      int bindstatus2 = bind(sockfd, (struct sockaddr*)&si_me, sizeof(si_me));
      if (bindstatus2 != 0) {
        printf("Could not bind to port. errno: %d", errno);
        exit(1);
      }
    } else {
        printf("Could not bind to port. errno: %d", errno);
      exit(1);
    }
  }


#endif


  addr_size = sizeof(si_other);

  printf("Server started. Listening on %s:%d", address, port);
  while(1){
    int recvstat = recvfrom(sockfd, buffer, 1024, 0, (struct sockaddr*)(struct sockaddr*)& si_other, &addr_size);
    if (recvstat < 0)
    {
      printf("*** ERROR - recvfrom() failed \n");
      exit(-1);
    } else {
      printf("[+]Data Received: %s", buffer);
      printf("   FROM: %s:%d", inet_ntoa(si_other.sin_addr), si_other.sin_port);
    }
  }

  close(sockfd);
  net_exit();

}