C sockaddr function call for sendto?

Question

I'm currently setting up a UDP socket for a school assignment but I can't figure out how to properly send sockaddr as a parameter into a function. The input arguments for SendData is defined the same way they are in the sendto function.

void SendData(int fileDescriptor, const struct sockaddr_in *destAddrUdp, unsigned char buffer[MAXMSG])
{
  /* crc */
  int strlength = strlen(buffer);
  unsigned char SCRC = CRC(buffer, strlength-1);
  buffer[strlength-1] = SCRC;
  buffer[strlength] = '\0'; 
  /* send */
  if (sendto(fileDescriptor, buffer, strlen(buffer), 0, (struct sockaddr*) &destAddrUdp,sizeof(destAddrUdp)) < 0) {
    fprintf(stderr, "Could not send data\n");
  }
} `

When the below code is called from main it works however when it's called from SendData the return value from sendto is -1.

if (sendto(fileDescriptor, buffer, strlen(buffer), 0, (struct sockaddr*) &destAddrUdp,sizeof(destAddrUdp)) < 0) {
    fprintf(stderr, "Could not send data\n");
  }

If the program is compiled I get warning: passing argument 5 of ‘recvfrom’ from incompatible pointer type. on the sendto functioncall in SendData. The function call for SendData in my main program is: SendData(sockfd, (struct sockaddr *) &destAddrUdp, buffer);

The main program in case it is relevant:

unsigned char SCRC;
  unsigned char strlength;
  unsigned char buffer[MAXMSG-5];
  unsigned char header[MAXMSG];
  struct rtp_struct *sendstruct; 
  /* option 2 */
  struct sockaddr_in destAddrUdp;
  int sockfd; 
  char dstHost[15]; 
  printf("input host IP:\n>");
  fgets(dstHost, 15, stdin);

  /* Create socket */ 
  if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) 
    fprintf(stderr, "Can't create UDP socket\n");

  /* set configurations */
  memset (&destAddrUdp, 0, sizeof(destAddrUdp));  //set zero
  destAddrUdp.sin_family = AF_INET; //internet use
  destAddrUdp.sin_addr.s_addr = inet_addr(dstHost); //set so anyone can connect
  destAddrUdp.sin_port = htons(dstUdpPort); //set the port to use

  /* Generate CRC table */
  GenerateCRCTable();

/* get string and send */
while(1) {
    printf("\n>");
    fgets(buffer, MAXMSG, stdin);
    if(strncmp(buffer,"quit\n",MAXMSG) != 0){ //if read string == quit, see else
      strlength = strlen(buffer);
      SCRC = CRC(buffer, strlength-1);
      buffer[strlength-1] = SCRC;
      buffer[strlength] = '\0';

      SendData(sockfd, (struct sockaddr *) &destAddrUdp, buffer);
      /*if (sendto(sockfd, buffer, strlength, 0, (struct sockaddr *) &destAddrUdp,sizeof(destAddrUdp)) < 0){
        fprintf(stderr, "Could not send data\n");}*/
    }  
    else {   
      close(sockfd); 
      exit(EXIT_SUCCESS);
    }
  }
}

Answer 1

The problem is the sizeof(destAddrUdp) and the (struct sockaddr*) &destAddrUdp.

In your main, destAddrUdp is an struct sockaddr_in, in your function however, its a sockaddr_in* so you can't use them equally. First, sizeof(destAddrUdp) in your function will give you the size of a pointer, second, &destAddrUdp in your function will give you a destAddrUdp**. Thats not what you want.

Try sendto(fileDescriptor, buffer, strlen(buffer), 0, (struct sockaddr*) destAddrUdp,sizeof(*destAddrUdp))

Answer 2

In your SendData function, the parameter destAddrUdp has type const struct sockaddr_in *. When you then make this call:

sendto(fileDescriptor, buffer, strlen(buffer), 0, 
       (struct sockaddr*) &destAddrUdp, sizeof(destAddrUdp)

The expression &destAddrUdp has type const struct sockaddr_in **, so the pointer type is incompatible. Also, sizeof(destAddrUdp) is returning the size of a pointer, not the size of the struct.

You failed to account for the differing types of destAddrUdp in SendData and main. The correct call to sendto would be:

sendto(fileDescriptor, buffer, strlen(buffer), 0, 
       (struct sockaddr*) destAddrUdp, sizeof(*destAddrUdp)