c - multiple variables over udp socket

Question

I have to send multiple variables types over an udp socket: an int array and a char. I would like to send it on the same udp packet. What is the standard solution? Convert everything to bytes or something similar?

I'm using: sendto(int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen);

My code is something like:

int buffer[100];
char flag = '0';
int i = 0;

for (i = 0; i < 50; i++) {
    buffer[i] = i * 2;
}

if (sendto(s, buffer, sizeof(buffer), 0, (struct sockaddr *) &si_client, slen) == -1 ){
    //error
}

//rest of the program

user3553031 user3553031 · Accepted Answer · 2014-08-03T00:30:17

Yes, you need to serialize your message into a byte array. There is no version of sendto that accepts an int array. Try something like this:

int arr[] = {1, 2, 3};
char str[] = "hello";
size_t buflen = sizeof arr + sizeof str;

char* buf = malloc(buflen);
if (NULL == buf)
    abort();
unsigned i = 0;
for (unsigned j=0; j<3; ++j)
{
    buf[i++] = (arr[j] >> 24) & 0x000000ff;
    buf[i++] = (arr[j] >> 16) & 0x000000ff;
    buf[i++] = (arr[j] >>  8) & 0x000000ff;
    buf[i++] = (arr[j] >>  0) & 0x000000ff;
}
strcpy(&(buf[i]), str);

if (sendto(s, buffer, sizeof(buffer), 0, (struct sockaddr *) &si_client, slen) == -1 ){
    //error
}

Note that your message may still be sent as more than one packet, for instance if its size exceeds the path MTU.

c - multiple variables over udp socket

2 Answers