Unable to receive full integer array through UDP socket in C [closed] - c++

Question

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 7 years ago.
Improve this question
Here's my sender side code
void send_data(int port, int coords[4]){
struct sockaddr_in si_me, si_other;
int s, i, slen = sizeof(si_other), recv_len;
char buf[BUFLEN];
//create a UDP socket
if((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1){
err_print("socket");
}
//zero out the structure
memset((char*)&si_me, 0, sizeof(si_me));
si_me.sin_family = AF_INET;
si_me.sin_port = htons(port);
si_me.sin_addr.s_addr = htonl(INADDR_ANY);
//bind socket to the port
if(bind(s, (struct sockaddr *)&si_me, sizeof(si_me) ) == -1){
err_print("bind");
}
int size = sizeof(coords);
printf("Size: %d\n", size);
//Keep listening for data
while(1){
printf("Waiting for Data\n");
fflush(stdout);
memset(buf, 0, BUFLEN);
//try to receive some data, this is a blocking call
if((recv_len = recvfrom(s, buf, BUFLEN, 0, (struct sockaddr*) &si_other, &slen)) == -1){
err_print("recvfrom()");
}
//print details of the client/peer and the data received
printf("Received packet from %s:%d\n", inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));
printf("%s\n", buf);
if(!strcmp(buf, "coords")) {
printf("Sending Coordinates: \n");
//now reply the client with the same data
int siz = sendto(s, coords, sizeof(coords), 0, (struct sockaddr*) &si_other, slen);
if(siz == -1){
err_print("sendto()");
}
printf("Siz: %d\n", siz);
}
break;
}
}
close(s);
}
The value returned from sendto is 8 which is the correct size of the int array but at the receiver side recvfrom returns 0 although the array receives the first two integer elements and the rest is garbage values when I print the array. Here's the receiver side code:
void get_data(int port, int coords[]){
struct sockaddr_in si_other;
int s, i;
char message[BUFLEN];
if((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1){
err_print("socket");
}
memset((char*)&si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(port);
si_other.sin_addr.s_addr = inet_addr(SERVER);
if(inet_aton(SERVER, &si_other.sin_addr) == 0){
fprintf(stderr, "inet_aton() failed\n");
exit(1);
}
socklen_t slen = sizeof(si_other);
int size = sizeof(coords);
printf("Size: %d\n", size);
int siz = 0;
while(1)
{
printf("Enter message : ");
strcpy(message, "coords");
//send the message
if (sendto(s, message, strlen(message) , 0 , (struct sockaddr *) &si_other, slen)==-1)
{
err_print("sendto()");
}
//try to receive some data, this is a blocking call
if (siz = (recvfrom(s, coords, 8, 0, (struct sockaddr *) &si_other, &slen)) == -1)
{
err_print("recvfrom()");
}
printf("Siz: %d\n", siz);
break;
}
for(int j = 0; j<4; j++){
printf("C: %d\n", coords[j]);
}
close(s);
return;
}
And the output of coords array is on the receiver side : 1 2 4196592 0
Now, i thought perhaps, the whole array wasnt getting sent, but the size values returned by sendto is 8 and that of recvfrom is 0 even if it gets the first two elements right. Also, do note that on compilation, I get warning for sizeof:
‘sizeof’ on array function parameter ‘coords’ will return size of ‘int *’
What could be wrong here and what's the right way to send the array? And if possible, could a double array too be sent?

Answer 1

The clue here is the claim in the question is that sizeof(coords), a 4-element int array, results in 8.
This would only be the case, of course, if on this implementation sizeof(int) is 2. I suppose 16-bit platforms are not entirely unheard of these days, especially in embedded space. But then afterwards, a claim is made that printing an int also results in 4196592, which cannot be the case with a two-byte int, of course.
The only explanation is that the example given here leaves out key information. The code that calls sendfrom() does not access the coords array directly. Most likely .the coords array gets passed as a parameter to this function. Which, of course, turns it into an int *, with its sizeof being 8, resulting in just the first two four-byte int values being send across the socket. This is also an explanation of the compiler warning.
Who knows what happens on the receive side.