Question

i’m trying to implement a little UDP-Server/Client Application in C and got two errors on Server-side: recvfrom: Bad address && sendto: Address family not supported by protocol. I searched for the mistake and googled for answers but, unfortunately, they wasn’t really helpfully… maybe i’m casting a parameter in a wrong way and don’t get it. I hope you can give me a hint :).

#include <unistd.h>
#include <ctype.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <errno.h>

#define BUFFSIZE 256
#define IP "127.0.0.1"
#define PORT 7755

int main(void){

  int socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
  char buffer[1] = "s";

  struct sockaddr_in src_addr;
  struct sockaddr_in dest_addr;

  src_addr.sin_family = AF_INET;
  src_addr.sin_port = htons(PORT);
  src_addr.sin_addr.s_addr = inet_addr(IP);

  if(socket_fd==-1)
    perror("socket");

  if(bind(socket_fd, (struct sockaddr*)&src_addr, sizeof(src_addr))==-1)
    perror("bind");

  if(recvfrom(socket_fd, buffer, 2, 0, (struct sockaddr*)&dest_addr, (unsigned int *)sizeof(struct sockaddr_in))==-1)
    perror("recvfrom");

  if(sendto(socket_fd, buffer, 2, 0,(struct sockaddr*)&dest_addr, sizeof(dest_addr))==-1)
    perror("sendto");

  if(close(socket_fd)==-1)
    perror("close");

  return 0;

}

Answer 1

You need to pass a valid pointer to recvfrom. (unsigned int *)sizeof(struct sockaddr_in) is not a valid pointer.

Change

 if(recvfrom(socket_fd, buffer, 2, 0, (struct sockaddr*)&dest_addr, 
             (unsigned int *)sizeof(struct sockaddr_in) == -1)

to e.g.

 socklen_t len = sizeof dest_addr;
 if(recvfrom(socket_fd, buffer, 2, 0, 
             (struct sockaddr*)&dest_addr, &len) == -1)

You are also constructing an invalid array that you send, Your array has a length of 1, but you tell sendto/recvfrom that it has a length of 2. So change

char buffer[1] = "s";

to

char buffer[] = "s";

(The string “s” is the character ‘s’ and a 0 byte, so buffer will have length 2 in the latter case.)