Segmentation fault: 11 Server/Client - c

Question

enum recstate {
initial
};
int num_clients = 0;
static void addclient(int fd, struct in_addr addr){
struct client *p = (struct client *) malloc (sizeof(struct client));
if (!p) {
fprintf(stderr, "out of memory!\n");
exit(1);
}
printf("Adding client %s\n", inet_ntoa(addr));
p->fd = fd;
p->next = top;
p->state = initial;
top = p;
num_clients++;
}
struct client {
int fd; // socket descriptor for this client
enum recstate state; // current state of data transfer for this client
struct client *next; // a pointer to the next client in the list
struct in_addr ipaddr;
} *top = NULL;
int main(int argc, char* argv[]){
int listenfd, clientfd, maxfd, nready;
struct client *p;
struct sockaddr_in self, client;
listenfd = socket(AF_INET, SOCK_STREAM, 0);
memset(&self, '\0', sizeof(self));
self.sin_family = AF_INET;
self.sin_addr.s_addr = INADDR_ANY;
self.sin_port = htons(PORT);
if (bind(listenfd, (struct sockaddr *) &self, sizeof(self))){
perror("bind");
exit(1);
}
if (listen(listenfd, 5) < 0){
perror("listen");
exit(1);
}
while (1){
fd_set allset, rset;
FD_ZERO(&allset);
FD_SET(listenfd, &allset);
maxfd = listenfd;
rset = allset;
for (p = top; p; p = p->next){
FD_SET(p->fd, &allset);
if (p->fd > maxfd){
maxfd = p->fd;
}
}
if(FD_ISSET(listenfd, &rset)){
printf("Listenfd is ready\n");
len = sizeof(client);
if ((clientfd = accept(listenfd, (struct sockaddr *) &client, &len)) < 0){
perror("accept");
return(1);
}
else {
printf("Connection from %s\n", inet_ntoa(client.sin_addr));
addclient(clientfd, client.sin_addr);
FD_SET(clientfd, &rset);
printf("clientfd has been added to fdset\n");
if(p->state == initial){ //cannot get into this if statement
printf("now in initial state\n");
nready = select(maxfd + 1, &rset, NULL, NULL, NULL);
if(nready == 0){
printf("timeout happened\n");
continue;
}
else if(nready == -1){
perror("select");
continue;
}
else if (nready > 0){
printf("Data is now available.\n");
continue;
}
if (FD_ISSET(listenfd, &rset)){ //returns a value for fd in rset
//read data into file
}
}
}
}
}
}
This is part of my server code.
I'm trying to send files from the client to the server, but when I send the files from the client, the server will execute until just above the if(p->state == initial) statement, and hangs there. When I terminate the server, it'll give me a segmentation fault: 11 error.
Also, I'm using select() inside the initial state to allow multiple clients to connect simultaneously.
Don't know where I went wrong, any help would be greatly appreciated.
Thanks.

Answer 1

for (p = top; p; p = p->next){
Unless I'm mistaken, you loop until p is NULL
if(p->state == initial){
and then you dereference p.
KABOOM.