UDP socket: invalid argument error in sendto - c

Question

I'm trying to write a simple UDP socket client-server program. The client machine is supposed to send a string to the server, that will answer with an ACK message.
Here's the implementation of the client side:
int main() {
message_send('L');
return EXIT_SUCCESS;
}
int message_send(char code) {
int sockfd;
ssize_t n;
char recvline[MAXLINE + 1];
struct sockaddr_in servaddr;
// Create an UDP socket
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
if (sockfd < 0) {
perror("socket");
return -1;
}
// Setup the socket
memset((void *) &servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = (in_port_t) htonl(SERV_PORT);
if (inet_pton(AF_INET, SERVIP, &servaddr.sin_addr) <= 0) {
fprintf(stderr, "Error in inet_pton for %s\n", SERVIP);
exit(1);
}
// Send a test string
char *test = malloc(MAXLINE);
snprintf(test, MAXLINE, "SENDING:%c", code);
if (sendto(sockfd, &test, sizeof(test), 0, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) {
perror("sendto");
return -1;
}
// Get an answer from the server
n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL);
if (n < 0) {
perror("recvfrom");
exit(1);
} else if (n > 0) {
recvline[n] = 0; // Add ending character
if (fputs(recvline, stdout) == EOF) { // Print the received message in stdout
perror("fputs");
return -1;
}
}
return 0;
}
If I run this (whether the server machine is running or not) I get the following error:
sendto: Invalid argument
Why am I getting this error?

Answer 1

The most likely cause of the problem is the line
servaddr.sin_port = (in_port_t) htonl(SERV_PORT);
I'm guessing that gives you a bad port number (i.e. it will give you port 0 on a little endian machine). Port numbers are 16-bit, so you should be using htons.
Also, passing &test and sizeof(test) to sendto will send the pointer value over the network. To send the string, you need to use test and strlen(test)+1.