What is causing a C++ Buffer Overrun error with fprintf? - c++

Question

I am trying to set up a Client, Router, Server in C++ with sockets and UDP using GoBackN windowing. I'm not sure why I'm getting a buffer overrun error in Client::write_log(), at the fprintf() line.
Main run() function
void Client::run()
{
srand ( time(NULL) );
//socket data types
SOCKET client_socket; // Client socket
SOCKADDR_IN sa_out; // fill with server info, IP, port
char buffer[RAWBUF_SIZE]; // Buffer
WSADATA wsadata; // WSA connection
char router[11]; // Host data
char cusername[128], filename[128], direction[3]; // Other header data
DWORD dwusername = sizeof(cusername); // Retains the size of the username
char trace_data[128];
FILE* logfile = fopen("client.log", "w");
try
{
if (WSAStartup(0x0202,&wsadata)!=0)
{
throw "Error in starting WSAStartup";
}
else
{
/* Display the wsadata structure */
cout<< endl
<< "wsadata.wVersion " << wsadata.wVersion << endl
<< "wsadata.wHighVersion " << wsadata.wHighVersion << endl
<< "wsadata.szDescription " << wsadata.szDescription << endl
<< "wsadata.szSystemStatus " << wsadata.szSystemStatus << endl
<< "wsadata.iMaxSockets " << wsadata.iMaxSockets << endl
<< "wsadata.iMaxUdpDg " << wsadata.iMaxUdpDg << endl << endl;
}
client_socket = open_port(PEER_PORT2);
prompt("Enter the router hostname: ",router);
sa_out = prepare_peer_connection(router, ROUTER_PORT2);
prompt("Enter a filename: ",filename); // Retrieve a filename from the client
prompt("Direction of transfer [get|put]: ", direction); // Retrieve a transfer direction
// Make sure the direction is one of get or put
if(!strcmp(direction,GET) || !strcmp(direction,PUT))
{
// Retrieve the local user name
GetUserName(cusername, &dwusername);
int selected = rand() % 256;
int received, verify;
int client_num = 0; // Client packet number
int server_num = 0; // Server packet number
int progress = 0;
int rcv;
cout << "Starting packet ID negotiation..." << endl;
while(1)
{
// Send a random number to the server
if(progress < 1)
{
memset(buffer, 0, sizeof(buffer));
sprintf(buffer,"RAND %d",selected);
cout << "Sending " << buffer << endl;
if((rcv = send_safe(client_socket, sa_out, buffer, RAWBUF_SIZE, 200)) == 201)
{
progress = 1;
}
else if(rcv != 200)
{
continue;
}
// Finally wait for a response from the server with the number
if(recv_safe(client_socket, sa_out, buffer, RAWBUF_SIZE, 100) == 100)
{
cout << "Received " << buffer << endl;
sscanf(buffer,"RAND %d %d",&verify,&received);
}
else
continue;
progress = 1;
}
// Send acknowledgement to the server along with our random number
memset(buffer, 0, sizeof(buffer));
sprintf(buffer, "RAND %d", received);
cout << "Sending " << buffer << endl;
if(send_safe(client_socket, sa_out, buffer, RAWBUF_SIZE, 201) != 201)
{
progress = 0;
continue;
}
break;
}
client_num = selected % WINDOW_SIZE + 1;
server_num = received % WINDOW_SIZE + 1;
cout << "Negotiated server start " << server_num << " and client start " << client_num << endl;
sprintf(trace_data, "Negotiated srv %d and cli %d", server_num, client_num);
write_log(logfile, cusername, trace_data);
// Send client headers
sprintf(buffer, HEADER, cusername, direction, filename);
while((rcv = send_safe(client_socket,sa_out,buffer,RAWBUF_SIZE,777)) != 777)
{
if(rcv == 101)
break;
}
// Perform a get request
if(!strcmp(direction,GET))
{
get(client_socket, sa_out, cusername, filename, client_num, server_num, logfile);
}
else if(!strcmp(direction,PUT))
{
put(client_socket, sa_out, cusername, filename, client_num, server_num, logfile);
}
}
else
{
throw "The method you requested does not exist, use get or put";
}
}
catch (const char *str)
{
cerr << str << WSAGetLastError() << endl;
}
//close the client socket and clean up
fclose(logfile);
closesocket(client_socket);
WSACleanup();
}
PUT() CODE
void Client::put(SOCKET s, SOCKADDR_IN sa, char * username, char* filename, int client_num, int server_num, FILE* logfile)
{
char window[FRAME_SIZE * WINDOW_SIZE]; // data retention window
char buffer[FRAME_SIZE]; // send buffer
int filesize;
int size = 0, sent = 0; // Trace variables
char tracebuf[128];
FILE* send_file;
fopen_s(&send_file, filename, "rb");
if(send_file != NULL)
{
// open the file
// Determines the file size
fseek(send_file, 0L, SEEK_END);
filesize = ftell(send_file);
fseek(send_file, 0L, SEEK_SET);
//write filesize to logfile
sprintf(tracebuf, "Filesize %d", filesize);
write_log(logfile, username, tracebuf);
//add file size to buffer
strncpy_s(buffer, "SIZ", 3);
memcpy(buffer + (3 * sizeof(char)), &filesize, sizeof(int));
//if filesize was sent to server successfully
if(send_safe(s,sa,buffer,FRAME_SIZE,101) == 101)
{
cout << "Sent filesize, starting transfer..." << endl;
//reset buffer
memset(buffer, 0, sizeof(buffer));
int count = 0;
int offset = 0;
int frames_outstanding = 0; //for frames not ACKED
int next = 0;
bool resend = false;
int packet_id;
int pid_max = WINDOW_SIZE + 1;
// Start sending the file
while (1)
{
// If the acks mismatch with the current send offset and there are frames not ACKED, this has to be a resend
if(next != offset && frames_outstanding > 0)
resend = true;
// Send as many frames as available for the given window size
while((!feof(send_file) && frames_outstanding < WINDOW_SIZE) || resend)
{
if(next == offset)
resend = false;
if(!resend)
{
if(feof(send_file))
break;
fread(buffer,1,FRAME_SIZE,send_file); // Read the next block of data
memcpy(window + (offset * FRAME_SIZE), buffer, FRAME_SIZE); // Store the data in the local window
//memcpy(window, buffer, FRAME_SIZE); // Store the data in the local window
send_packet(s, sa, buffer, FRAME_SIZE, offset); // Send the packet to peer
offset = (offset + 1) % pid_max; // Update the offset
frames_outstanding++;
}
else
{
// Resend by copying the data from the window
memcpy(buffer, window + (next * FRAME_SIZE), FRAME_SIZE);
send_packet(s, sa, buffer, FRAME_SIZE, next);
//log
sprintf(tracebuf, "Resending packet %d", next);
write_log(logfile, username, tracebuf);
next = (next + 1) % pid_max;
}
}
// Receive ACKs before continuing sending
//while the # of unacked frames are greater than 0
while(frames_outstanding > 0)
{
//if packet id < 0
if((packet_id = recv_packet(s,sa,buffer,FRAME_SIZE,next)) < 0)
{
if(count < filesize)
resend = true;
break;
}
// Receive acknowledgment from the client
if(!strncmp(buffer,"NAK", 3))
{
if(packet_id >= 0)
next = packet_id; // Set the next packet to the packet id
break;
}
else if(!strncmp(buffer,"ALL", 3))
{
frames_outstanding = 0;
break;
}
//log
sprintf(tracebuf, "Sent %d bytes", count);
write_log(logfile, username, tracebuf);
memset(buffer, 0, sizeof(buffer)); // Zero the buffer
next = (next + 1) % pid_max; // Update the next frame tracker
frames_outstanding--; // Another frame has been acked
}
if(feof(send_file) && frames_outstanding == 0)
break; // Break when done reading the file and all frames are acked
}
cout << "File transfer completed" << endl;
fclose(send_file);
}
else
{
fclose(send_file);
return put(s,sa,username,filename, client_num, server_num, logfile);
}
}
}
WRITE_LOG() CODE
void Client::write_log(FILE* logfile, char* username, char* message)
{
fprintf(logfile, "%s > %s\n", username, message);
memset(message, 0, sizeof(message));
}

Answer 1

Not at all sure what you actually intdended to do here:
memset(message, 0, sizeof(message));
but it's likely not doing what you intended. sizeof(message) is either 4 or, more likely, 8 on a modern system. So you are writing 8 zeros to message. Which may not be 8 bytes long in the first place.
Possibly you want:
memset(message, 0, strlen(message));
or:
message[0] = 0;
(Which will do set the string to "empty", but not fill the entire thing with zeros).