about recvfrom()?

Obviously you are using an incorrectly formated socket or "from address". Check the following:

1. Create the socket using "socket call".
2. Do the Bind(). Make sure you using functions like "htonl()"
   and "htons()" to translate "endianness", especially if you
   have an Intel machine (or any other little endian CPU). This
   is a typical mistake.
3. Construct the "from" structure (for recvfrom) taking care
   of endianness, too. You can receive from any IP using
   ADDR_ANY.

Here you are.

I include a small example, using TCP:

        soc=socket(AF_INET,SOCK_STREAM,0);
        if(soc==-1) {
                perror("No se puede abrir el socket");
                exit(errno);
        }
        servidor=gethostbyname(IRCop_servidor);
        if(servidor==NULL) {
                perror("No se puede resolver el nombre del servidor");
                exit(errno);
        }
       
        conexion.sin_family=AF_INET;
        conexion.sin_addr=*(struct in_addr *)servidor->h_addr;
^^^^ Here I don't use htonl() because servidor->h_addr has
already the correct endianness
        conexion.sin_port=htons(IRCop_puerto);
^^^^ Look at this
        estado=connect(soc,(struct sockaddr *)&conexion,sizeof(conexion));
        if(estado==-1) {
                perror("Error en la conexión con el server. Reintentando...");
                close(soc);
                sleep(5);
                goto inicio;
        }

Hi:
   jcea.
   I do it by your idael,but it is still.here is my app:

//*******************///////

/* This is a server for chat            */
/*    1st edtion: Taiyuan 1998/02/23    */
/*    <Bill Sun>RedSun@Rocketmail.com   */

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
#include <time.h>
#include <errno.h>
#define PORT 1234
#define HOST_ADDR "127.0.0.1"
main()
  {
     int s;         /* socket for send / receive info */
     static int width;  /* select  bit mask */
     int fromlen;
     fd_set rset;   /* for read  */    
/*   fd_set wset;   for write    */
     struct sockaddr_in sour,dest;
/*     struct sockaddr  dest; */
     static struct hostent *host;
     char *buf="111";    
     int ret=10;
     unsigned long addr;
     
    s=socket(AF_INET,SOCK_DGRAM,0);
    if(s<0)
     {
       perror("socket\n");
       exit(1);
        }
   
   width=s+1;
     FD_ZERO(&rset);
   /*  FD_ZERO(&wset);  */
     FD_SET(s,&rset);
   /*  FD_SET(s,&wset);  */

   bzero(&sour,sizeof(sour));
/*   addr=inet_addr(HOST_ADDR);
   host=gethostbyaddr((char *)&addr,4,AF_INET);
*/
   host=gethostbyname("linux");
   sour.sin_family=AF_INET;
   sour.sin_port=htons(PORT);
   bcopy(host->h_addr,(char *)&sour.sin_addr,host->h_length);
 
  if((ret=bind(s,(struct sockaddr *)&sour,sizeof(sour))<0))
     {
       perror("bind");
       exit(1);
    }
   fromlen=sizeof(dest);
   ret=recvfrom(s,buf,strlen(buf),0,(struct sockaddr *)&dest,&fromlen);
   if(ret<0)      
    {
     switch(errno){
  case  EINTR : ret=0; break;
  case  EFAULT: ret=1;break;
 
    }
    perror("recvfrom");
    exit(1);
    }
   else
     {
   printf("%s\n",buf);
   printf("%d",ret);
      }
   /*   while(1) */
   {
/*    if (select(width,&rset,NULL,NULL,NULL)<0)
     {
      perror("select");
      exit(1);
     }
*/
 /* watch if we can get some from the socket? */  
/*  if(FD_ISSET(s,&rset))
    {
    recvfrom(s,buf,strlen(buf),0,\
            (struct sockaddr *)&dest, &fromlen);
    printf("%s",buf);    
    exit(0);
    } */
  /* watch if we need send some data ?
  if(FD_ISSET(s,&wset))
    {

    }
   
   */
 


 };   /* while (1)*/    
    close(s);
     
}









/////******************/////



  the errno=EFAULT.



   thanx!

Before the bind, try to dup "host" onto "sour". Entire, not only the address. Use, also, "ANY_ADDR" instead "linux". Is "linux" the name of your machine?. Has a correct IP Translation?

Hi:
  jcea,yes,linux is my machine name and I can "ping linux" successfully.would you help me correct my error?

   thanx!