UDP C Server sendto() error: Can't assign requested address - c

Question

I'm writing a basic Client/Server program in C, using UDP. The idea of the program is that the client sends a message to the server, the server receives it, then echoes it back to the client (the goal being to measure RTT for UDP). Unfortunately, on the server side, when the program attempts to call sendto() to echo the message, I receive the error "Can't assign requested address".
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdlib.h>
#include <strings.h>
#include <unistd.h>
#include <fcntl.h>
#define SERVER_PORT 7000
#define MAX_PENDING 10
#define MAX_LINE 1024000
int main()
{
struct sockaddr_in sin, sout;
socklen_t soutLen;
char buf[MAX_LINE];
int len;
int msgLen;
int s;
char *msg;
if( (s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0){
perror("could not establish UDP socket");
exit(1);
}
/* build address data structure */
bzero((char *)& sin, sizeof( sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(SERVER_PORT);
if( (bind(s, (struct sockaddr *)&sin, sizeof(sin))) < 0){
perror("udpServer: bind");
exit( 1);
}
while(1){
if((msgLen = recvfrom(s, buf, sizeof(buf), 0,(struct sockaddr *)&sout, &soutLen))<0){
perror("udpServer: recvfrom()");
exit( 1);
}
if( (sendto(s, buf, msgLen, 0, (struct sockaddr *)&sout, sizeof(sout)))<0 ){
perror("udpServer: sendto()");
exit( 1);
}
free(msg);
}
}
Thanks in advance: I'm pretty new to C, so any advice is much appreciated!

Answer 1

The problem is that your sout you pass to sendto is not correct, because you are not correctly setting it's size when passing it to recvfrom:
man recvfrom:
ssize_t
recvfrom(int socket, void *restrict buffer, size_t length,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
If address is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The address_len argument is a
value-result argument, initialized to the size of the buffer associated with address, and modified on return to indicate the actual size of the address
stored there.
When you pass &sout to recvfrom, you also have to tell recvfrom the size of the structure you're passing in so it knows how much data it can write there -- soutLen is both an in parameter and an out parameter. Since you are not initializing soutLen, it probably has some value smaller than the actual size of the structure, which means that what you end up with in sout is not valid.
So you need to initialize soutLen:
struct sockaddr_in sin, sout;
socklen_t soutLen = sizeof(sout);
You should then pass this value as the size to sendto instead of sizeouf(sout) (this may not be required but it's good practice):
if( (sendto(s, buf, msgLen, 0, (struct sockaddr *)&sout, soutLen))<0 ){
Also just as a note, you are freeing msg which you never allocated. This is unrelated but might cause problems later.
Hope this helps.