pthread_join fails in forking TCP server when handling multiple clients - c

Question

I have to create a multithread TCP/IP server which contains a variable to count the number of clients connected (and those which disconnect) and print the number of clients connected when a client connects to the server.
This is my client.c file:
#define PORT 4444
int main ()
{
int clientSocket;
struct sockaddr_in serverAddress;
char buffer[1024];
ssize_t nread;
clientSocket = socket(AF_INET, SOCK_STREAM, 0);
if (clientSocket == -1) {
perror("[-]Errore durante la creazione della socket\n");
exit(-1);
}
printf("[+]Client socket has been created\n");
memset(&serverAddress, '\0', sizeof(serverAddress));
serverAddress.sin_family = AF_INET;
serverAddress.sin_port = htons(PORT);
serverAddress.sin_addr.s_addr = INADDR_ANY;
if (connect(clientSocket, (struct sockaddr*)&serverAddress, sizeof(serverAddress)) == -1) {
perror("Errore con la connessione\n");
exit(-1);
}
while (1) {
printf("> ");
fflush(stdin);
scanf("%s", buffer);
if (nread != -1)
buffer[nread] = '\0';*/
if (send(clientSocket, buffer, strlen(buffer), 0) == -1) {
perror("Errore con l'invio");
exit(1);
}
if(strcmp(buffer, ":exit") == 0) {
close(clientSocket);
printf("[-]Disconnected from Server\n");
exit(0);
}
if ( (nread=recv(clientSocket, buffer, sizeof buffer - 1, 0)) <= 0) {
perror("[-]Error in receiving data from server\n");
}
else {
buffer[nread] = '\0';
printf("Server received: %s\n", buffer);
}
}
close(clientSocket);
return 0;
}
and this is my server.c file:
#define PORT 4444
#define MAX_CONNESSIONI 100
typedef struct myStruct {
int clientCollegati;
int clientCheSiSonoScollegati;
pthread_mutex_t mutex; // Creazione del mutex per sincronizzare la struttura
} myStruct;
myStruct *test;
myStruct *initStruct();
void *incrementa(void*);
int main ()
{
int serverSocket, bindStatus;
struct sockaddr_in serverAddress;
int clientSocket;
struct sockaddr_in newAddress;
char buffer[1024];
pid_t child;
socklen_t addrSize;
ssize_t nread;
pthread_t tid;
test = initStruct();
if (pthread_create(&tid, NULL, incrementa, NULL) != 0) {
perror("Errore nella creazione del thread t1\n");
exit(1);
}
serverSocket = socket(AF_INET, SOCK_STREAM, 0);
if (serverSocket == -1) {
perror("[-]Errore durante la creazione della socket\n");
exit(-1);
}
memset(&serverAddress, '\0', sizeof(serverAddress));
serverAddress.sin_family = AF_INET;
serverAddress.sin_port = htons(PORT);
serverAddress.sin_addr.s_addr = INADDR_ANY;
bindStatus = bind(serverSocket, (struct sockaddr*)&serverAddress, sizeof(serverAddress));
if (bindStatus == -1) {
perror("[-]Errore durante il binding\n");
exit(1);
}
printf("[+]Bind to port %d\n", PORT);
if (listen(serverSocket, MAX_CONNESSIONI) != -1) {
printf("Listening . . .\n\n");
}
else {
perror("[-]Error during listening\n");
exit(1);
}
while (1) {
clientSocket = accept(serverSocket, (struct sockaddr*)&newAddress, &addrSize);
if (clientSocket == -1) {
exit(-1);
}
printf("%s:%d joined\n", inet_ntoa(newAddress.sin_addr), ntohs(newAddress.sin_port));
if (pthread_join(tid, NULL)) { // returns 3
perror("pthread_join error\n");
exit(1);
}
printf("There is/are %d client(s) connected\n", test->clientCollegati);
child = fork();
if (child == 0) {
close(serverSocket);
while (1) {
if ( (nread=recv(clientSocket, buffer, sizeof buffer - 1, 0)) <= 0) {
perror("[-]Error in receiving data from server\n");
}
else {
buffer[nread] = '\0';
}
if (strcmp(buffer, ":exit") == 0) {
printf("%s:%d left\n", inet_ntoa(newAddress.sin_addr), ntohs(newAddress.sin_port));
break;
}
else {
printf("%s:%d wrote: %s\n", inet_ntoa(newAddress.sin_addr), ntohs(newAddress.sin_port), buffer);
send(clientSocket, buffer, strlen(buffer), 0);
bzero(buffer, sizeof(buffer));
}
}
}
}
close(clientSocket);
return 0;
}
myStruct *initStruct()
{
struct myStruct *ptr = malloc(sizeof(myStruct));
ptr->clientCollegati = 0;
ptr->clientCheSiSonoScollegati = 0;
pthread_mutex_init(&ptr->mutex, NULL); // inizializzazione dinamica del mutex
return ptr;
}
void *incrementa(void *ptr)
{
pthread_mutex_lock(&test->mutex);
test->clientCollegati++;
pthread_mutex_unlock(&test->mutex);
pthread_exit(0);
}
Unfortunately this seems not to work properly, ans you can see server output and here client1 and here client2 output
Where's the mistake? I suppose that the problem is when tid finish and a second call to that pthread_join is invoked to a non-existent thread (because there is a single thread)

Answer 1

Your code logic is wrong. You are doing a single pthread_create() and then multiple pthread_join(). If you want to increment the values for every connected client you have to do one pthread_create() after every single successful accept(), followed by one pthread_join(). Note: all this must happen before the call to fork().
Where's the mistake? I suppose that the problem is when tid finish and a second call to that pthread_join is invoked to a non-existent thread (because there is a single thread)
That's exactly what happens. In your current program, the first pthread_join() is successful, and any successive call fails with errno 3 (no such process) because the thread doesn't exist anymore. So you already figured this out. Why are you creating one thread but joining it multiple times if you know that's wrong?
The way your program is written (fork after the increment) also means that there really is no reason at all to use threads to do the increment, and you don't even need the mutex, since only one single thread will access the value at any given time. In other words, your server isn't multithreaded at all. If you only want to do this as an experiment that's ok, but it doesn't make much sense.
What you probably want to do (to have a multithreaded server) is to have one thread per client instead of forking each time, with the client code in the thread function (in that case having a mutex makes sense). Beware though that this does not scale well for large numbers of clients (in general, the classical fork() approach is better, in which case you really don't need any thread).