Send and receive in UDP from server and client in C

问题

I am trying to implement a server and a client that both send and receive (in different order) a datagram. I am trying on the following way but the datagram from client is not sent/received.

Here the client:

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

#define DATA "Este es el mensaje ...."

int main(int argc, char *argv[])
{
 int sock, lon2;
 struct sockaddr_in name, name2;
 struct hostent *hp;
 char buf[1024];

 sock=socket(PF_INET,SOCK_DGRAM,0);

 if (sock<0)
    {
     perror("Abriendo socket de datagramas");
     exit(1);
    }


 /*devuelve una estructura hostent para el host especificado en argv[1]*/
 /*para obtener la direccion IP a partir del nombre de la maquina*/
 hp=gethostbyname(argv[1]);

 if (hp == 0)
    {
     fprintf(stderr,"%s: host desconocido",argv[1]);
     exit(2);
    }

 /*Copiamos en la estructura name la direccion del ordenador al que */
 /*vamos a conectarnos.*/
 memcpy((char *)&name.sin_addr, (char *)hp->h_addr, hp->h_length);

 name.sin_family = AF_INET;
 name.sin_port   = htons(atoi(argv[2]));

 if (sendto(sock,DATA,strlen(DATA)+1,0,(struct sockaddr *)&name,sizeof(name))<0)
    perror("Enviando un datagrama");



 if (read(sock,buf,1024)<0)
    perror("Recibiendo el datagrama");   

 printf("-->%s\n",buf);   
 close(sock);
 exit(0);       
}

Here the server:

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#define DATA "Este es el mensaje ....bvbvbvbv"
int main()
{
 int sock, length, lon2;
 struct sockaddr_in name;
 char buf[1024];

 sock=socket(PF_INET,SOCK_DGRAM,0);

 if (sock<0)
    {
     perror("Abriendo socket de datagramas");
     exit(1);
    }

 /*Formato de la direccion*/
 name.sin_family=AF_INET;   

 /*Direccion IP, INADDR_ANY -> que cuando se use bind para asociar una*/
 /*direccion a un socket, la direccion IP es la de la maquina donde esta*/
 /*ejecutandose el programa.*/                        
 name.sin_addr.s_addr=htonl(INADDR_ANY);

 /*0-> cuando se utilice bind(), el puerto que se va a asociar al socket es */
 /*uno libre asignado por el SO.*/
 name.sin_port=htons(0);

 if (bind(sock,(struct sockaddr *)&name, sizeof(name))<0)
    {
     perror("Asociando nombre al socket");
     exit(1);
    } 

 /*Hasta despues del bind no sabremos la direccion del socket asignada.*/
 /*=> usar getsockname()*/    

 length=sizeof(name);

 /*sock   = socket del que queremos saber la direccion*/
 /*name   = estructura en la que se va a dejar la direccion*/
 /*length = tamano ocupado por la estructura.*/ 
 if (getsockname(sock,(struct sockaddr *)&name,&length)<0)
    {
     perror("Averiguando el nombre del socket");
     exit(1);
    }   

/*Imprimimos el puerto para que el emisor mande a ese puerto.*/    
 printf("puerto del socket -->%d\n", ntohs(name.sin_port));

 if (recvfrom(sock,buf,1024,0,(struct sockaddr *)NULL,&lon2)<0)
    perror("Recibiendo el datagrama"); 

 printf("-->%s\n",buf);

 if (sendto(sock,DATA,strlen(DATA)+1,0,(struct sockaddr *)&name,sizeof(name))<0)
    perror("Enviando un datagrama");

 close(sock);
 exit(0);       
}

回答1:

Your client code is fine, but you really should "bind" the client socket. (Specifying sin_port=0 is fine to let the OS select a random port number). I think some operating systems will pick a random port for each sendto if the socket is not been "binded" to a port and not actually listen on a port. I may be confusing that with something else, but in any case, calling bind for the client socket is a good thing to do. Binding to port 0 will at least make the socket listen on the same port it sends data with.

sockaddr_in localAddr = {}; // zero's out sin_port and sets sin_addr to INADDR_ANY (0)
int result;

localAddr.sin_family = PF_INET;
sock=socket(PF_INET,SOCK_DGRAM,0);
if (sock < 0) {perror("socket error"); exit(1);}

result = bind(sock, (sockaddr*)&localAddr, sizeof(localAddr));
if (result < 0) {perror("bind error"); exit(1);}

But that's not your only problem. In your server code:

if (recvfrom(sock,buf,1024,0,(struct sockaddr *)NULL,&lon2)<0)
    perror("Recibiendo el datagrama"); 

 printf("-->%s\n",buf);

if (sendto(sock,DATA,strlen(DATA)+1,0,(struct sockaddr *)&name,sizeof(name))<0)
    perror("Enviando un datagrama");

You aren't actually sending back to the remote IP/port that you received data on. This will more likely work:

sockaddr_in remoteClient = {};
socklen_t remoteClientSize = sizeof(remoteClient);

if (recvfrom(sock,buf,1024,0,(struct sockaddr *)&remoteClient,&remoteClientSize)<0)
    perror("Recibiendo el datagrama"); 

 printf("-->%s\n",buf);

if (sendto(sock,DATA,strlen(DATA)+1,0,(struct sockaddr *)&remoteClient,remoteClientSize)<0)
    perror("Enviando un datagrama");

来源：https://stackoverflow.com/questions/13979511/send-and-receive-in-udp-from-server-and-client-in-c

标签

sockets

datagram