UDP relay - for implementing other programs working with UDP

udprelay.c

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

#ifndef WIN32

#include <netdb.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>

#else

#define WINVER 0x0501
#include <winsock2.h>
#include <windows.h>
#include <ws2tcpip.h>
#include <fcntl.h>


#endif

#include <errno.h>
#include <signal.h>
#include <sys/time.h>

#define BUFSIZE 4096

int s_recv = -1;
int s_send = -1;

int start = 0;
int end = 0;
    
unsigned char buf[BUFSIZE];

int main(int argc, char* argv[]) {
    if (argc<5 || !strcmp(argv[1], "--help") || !strcmp(argv[1], "-?")) {
        fprintf (stderr, 
        "Usage:\n"
        "\tudprelay recv_host recv_port send_host send_port"
        "\tCreated by Vitaly \"_Vi\" Shukela\n"
        );
        return 1;
    }
    
    struct addrinfo hints;
    struct addrinfo *recv_addr;
    struct addrinfo *send_addr;
    
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
    hints.ai_flags = AI_PASSIVE;
    
    char* recv_host = argv[1];
    char* recv_port = argv[2];
    char* send_host = argv[3];
    char* send_port = argv[4];
    
    if (!strcmp(recv_host, "NULL")) recv_host=NULL;
    if (!strcmp(recv_port, "NULL")) recv_port=NULL; 
    if (!strcmp(send_host, "NULL")) send_host=NULL;
    if (!strcmp(send_port, "NULL")) send_port=NULL;     
    
    
    int gai_error;
    gai_error=getaddrinfo(recv_host,recv_port, &hints, &recv_addr);
    if (gai_error) { fprintf(stderr, "getaddrinfo 1: %s\n",gai_strerror(gai_error)); return 4; }
    if (!recv_addr) { fprintf(stderr, "getaddrinfo returned no addresses\n");   return 6;  }
    if (recv_addr->ai_next) {
        fprintf(stderr, "Warning: using only one of addresses retuned by getaddrinfo\n");
    }
    
    hints.ai_flags &= !AI_PASSIVE;
    gai_error=getaddrinfo(send_host, send_port, &hints, &send_addr);
    if (gai_error) { fprintf(stderr, "getaddrinfo 2: %s\n",gai_strerror(gai_error));   return 13;  }
    if (!send_addr) { fprintf(stderr, "getaddrinfo 2 returned no addresses\n");  return 15;  }
    if (send_addr->ai_next) {
        fprintf(stderr, "Warning: using only one of addresses retuned by getaddrinfo 2\n");
    }
    
    s_recv = socket(recv_addr->ai_family, recv_addr->ai_socktype, recv_addr->ai_protocol);
    if (s_recv == -1) { perror("socket"); return 7; }
    {
        int one = 1;
        setsockopt(s_recv, SOL_SOCKET, SO_REUSEADDR, &one, sizeof one);
    }
    if (bind(s_recv, recv_addr->ai_addr, recv_addr->ai_addrlen)) {  perror("bind");  return 5;  }
    
    s_send = socket(send_addr->ai_family, send_addr->ai_socktype, send_addr->ai_protocol);
    if (s_send == -1) { perror("socket 2"); return 12; }
        
    //char should_be_enough[1024];
    struct sockaddr peeraddr ;
    socklen_t peeraddrlen = sizeof peeraddr;
    
    for (;;) {
        fd_set rfds;
        FD_ZERO(&rfds);
        
        int maxfd = 0;
        
         
        FD_SET(s_recv, &rfds);
        FD_SET(s_send, &rfds);
        if (s_recv >= maxfd) maxfd = s_recv+1;
        if (s_send >= maxfd) maxfd = s_send+1;
        
        struct timeval *timeout = NULL;
        struct timeval tv;
            
        
        int select_ret = select(maxfd, &rfds, NULL, NULL, timeout);
        
        if (select_ret==-1) {
            if (errno==EAGAIN || errno==EINTR) continue;
            perror("select");
            return 91;
        }
        
        if (FD_ISSET(s_recv, &rfds)) {
            int ret = recvfrom(s_recv, buf, BUFSIZE, 0, &peeraddr, &peeraddrlen);
            sendto(s_send, buf, ret, 0,  send_addr->ai_addr,  send_addr->ai_addrlen);            
        }
        if (FD_ISSET(s_send, &rfds)) {
            int ret = recv(s_send, buf, BUFSIZE, 0);
            sendto(s_recv, buf, ret, 0,  &peeraddr,  peeraddrlen);            
        }
    }
}