C sockaddr function call for sendto? - c

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

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)

Answer 2

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))