Segmentation Fault in UDP - c

Question

I got a segmentation fault problem when I write a client-server project in UDP. It happens on server side, when I receive a packet from client and going to send an ACK back. I tried to search the solutions and got UDP Server giving Segmentation Fault and C concurrent UDP socket , weird segmentation fault, but seems both of those are not the answer I'm looking for.
Here is my server side code
#include <ctype.h> /* for toupper */
#include <stdio.h> /* for standard I/O functions */
#include <stdlib.h> /* for exit */
#include <string.h> /* for memset */
#include <sys/socket.h> /* for socket, sendto, and recvfrom */
#include <netinet/in.h> /* for sockaddr_in */
#include <unistd.h> /* for close */
#define STRING_SIZE 1024
#define SERV_UDP_PORT 12311
int main(void) {
int sock_server;
struct sockaddr_in server_addr;
unsigned short server_port;
struct sockaddr_in client_addr;
unsigned int client_addr_len;
char sentence[STRING_SIZE];
char modifiedSentence[STRING_SIZE];
unsigned int msg_len;
int bytes_sent, bytes_recd;
unsigned int i;
struct Pkt
{
short length;
short seqnum;
char databytes[80];
};
struct Pkt* pkt;
int j ; //for loop
int seq;
short num_of_bytes;
//char ack_num[2];
struct Ack
{
short ack_num;
};
struct Ack* ack;
/* open a socket */
if ((sock_server = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
perror("Server: can't open datagram socket\n");
exit(1);
}
/* initialize server address information */
memset(&server_addr, 0, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = htonl (INADDR_ANY);
server_port = SERV_UDP_PORT;
server_addr.sin_port = htons(server_port);
/* bind the socket to the local server port */
if (bind(sock_server, (struct sockaddr *) &server_addr,
sizeof (server_addr)) < 0) {
perror("Server: can't bind to local address\n");
close(sock_server);
exit(1);
}
/* wait for incoming messages in an indefinite loop */
printf("Waiting for incoming messages on port %hu\n\n",
server_port);
client_addr_len = sizeof (client_addr);
for (;;) {
bytes_recd = recvfrom(sock_server, pkt, sizeof(*pkt), 0, (struct sockaddr *) &client_addr, &client_addr_len);
ack->ack_num = pkt->seqnum;
printf("%02d\n", ack->ack_num);
num_of_bytes = pkt->length;
printf("The sequence number is: %d\n", ack->ack_num);
printf("Received Sentence is: %s\n with length %d\n\n", pkt->databytes, num_of_bytes);
msg_len = 3;
/* send message */
bytes_sent = sendto(sock_server, (struct Ack*)&ack, msg_len, 0, (struct sockaddr*) &client_addr, client_addr_len); //Here is the segmentation fault comes from
}
}
I'm not really good at C, so forgive me if the code is silly.
Please point out anything wrong or just looks weird.
Thanks in advance for any help.

Answer 1

pkt is pointer to a Pkt, but you haven't initialized it. Likewise with ack. You can either a) malloc a Pkt and assign the result to pkt, or b) change pkt to be a Pkt structure (rather than a pointer). The second option would look something like:
struct Pkt pkt;
struct Ack ack;
bytes_recd = recvfrom(sock_server, &pkt, sizeof(pkt), 0, (struct sockaddr *) &client_addr, &client_addr_len);
ack.ack_num = pkt.seqnum;
printf("%02d\n", ack.ack_num);
num_of_bytes = pkt.length;
printf("The sequence number is: %d\n", ack.ack_num);
printf("Received Sentence is: %s\n with length %d\n\n", pkt.databytes, num_of_bytes);
/* send message */
bytes_sent = sendto(sock_server, &ack, sizeof(ack), 0, (struct sockaddr*) &client_addr, client_addr_len);