Segmentation fault using the buffer recived from recv() tcpServer c - c

Question

I'm trying to create an application client/server in c, but after recv() when I try to use the buffer received the program give segmentation fault (core dump created), I can't work out it.
This is my code at server side:
int req_socket_id;
int comunication_socket_id;
struct sockaddr_in server_add;
struct sockaddr_in client_add;
socklen_t client_add_size;
char buffer[255];
//char mess[1024];
int i, n;
//int index;
unsigned int num;
// AF_INET = famiglia di indirizzi iPv4
req_socket_id = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if(req_socket_id<0)
{
printf("Socket initialization failed !!");
return -1;
}
e
memset(&server_add, 0, sizeof(server_add)); // azzeramento struttura
server_add.sin_family = AF_INET; // dominio indirizzi IP
server_add.sin_addr.s_addr = 0; // indirizzo IP
server_add.sin_port = htons(23165); // numero di porta UDP
if(bind(req_socket_id, (struct sockaddr*) &server_add, sizeof(server_add)) < 0)
{
perror("\nErrore associazione porta e socket!\n");
close(req_socket_id);
return -1;
}
if(listen(req_socket_id, 1)<0)
{
perror("\nErrore nell'ascolto!\n");
close(req_socket_id);
return -1;
}
while(1)
{
client_add_size = sizeof(client_add);
comunication_socket_id = accept(req_socket_id, (struct sockaddr*) &client_add, &client_add_size);
if(comunication_socket_id>=0)
{
//index = 0;
char tmp[100];
while(1)
{
printf("\nOk");
//n = recv(comunication_socket_id, (char*) buffer, sizeof(buffer)+1, 0);
//n = recv(comunication_socket_id, (void*) buffer, sizeof(buffer)+1, 0);
n = recv(comunication_socket_id, buffer, sizeof(buffer)+1, 0);
printf("\nReceved! n: %d", n);
printf("\nReceved: %s", buffer);
if(strcmp(buffer, "end")==0)
{
close(comunication_socket_id);
printf("\n...socket closed");
return -1;
}
[...]
and this is client side code :
unsigned long start, now;
unsigned int *num = (unsigned int*)buffer;
int i, n;
TCPclient_send(buffer, strlen(buffer)+1);
printf("\nSent: |%s|\n", buffer);
start = clock(); // tempo iniziale attesa
now = clock(); // tempo attuale
while ((now - start) < TIMEOUT)
{
if ((n = TCPclient_receive(&buffer[i], sizeof(buffer)-i)) > 0)
{
i += n;
if (i >= sizeof(unsigned int))
{
// risposta completa
printf("Receved number %u.\r\n", ntohl(*num));
TCPclient_disconnect();
return 0;
}
}
now = clock();
}
printf("No answer receved!\r\n");
TCPclient_disconnect();
Console server side output:
davide#davide-VirtualBox:~/Documenti/RubricaTCP$ ./TCPserver
Ok
Receved! n: 11
Errore di segmentazione (core dump creato) //-->segmentation fault, end of process
Console client side output:
Insert the command (end to close): SET=A;A;A;
Sent: |SET=A;A;A;|
No answer receved!
PS: this is my first application with sockets, so it's possible that i've done some stupid mistakes. I looked for answers in many topics, but i didn't find anything that can work out it.
Thank you very much

Answer 1

n = recv(comunication_socket_id, buffer, sizeof(buffer)+1, 0);
sizeof(buffer)+1 is inherently incorrect. It should be sizeof(buffer). You are using memory that does not exist.
printf("\nReceved! n: %d", n);
OK.
printf("\nReceved: %s", buffer);
Wrong. This should be
printf("\nReceved: %.*s", n, buffer);
Then:
if(strcmp(buffer, "end")==0)
This is also wrong. There is no guarantee that you've received a null terminated string or a complete command. It's a byte-stream protocol. If you want messages you have to implement them yourself. It is rarely correct to write networking code or any I/O code for that matter that doesn't have read or receive loops.