First buffer from UDP connection contains junk - subsequent connections are fine - c++

Question

I'm really going to try to avoid posting my entire project on here - unless it's needed :)
I have a client/server program I'm writing where the client contacts the server via a UDP port, and expects to receive a packet in the format "#7777~15~3701" - the first time I connect, I get garbage in the buffer. However, once the client exits, and I launch it again, the next buffer "#7777~15~3702" comes through just fine.
int
RoutingManager::SendMessage(struct sockaddr_in toNode, char buffer[1024])
{
#if logging > 1
cout << "Sending: " << buffer << endl;
#endif
int n;
unsigned int length = sizeof(struct sockaddr_in);
//buffer = "#7777~15~3702"
n = sendto(mySocket, buffer, strlen(buffer),0,
(const struct sockaddr *)&toNode,length);
if (n < strlen(buffer))
perror("Sendto");
cout << "Sent: " << n << " bytes of data\n";
}
.. the buffer I pass in here is generated from:
//FIXED: This was the source of my issue it appears - a corrected
//implementation of this method has been included at the bottom.
char*
RoutingManager::GenerateConnectionString(struct Node n)
{
char buffer[512];
bzero(buffer,512);
sprintf(buffer, "#7777~15~%d", n.id);
cout << MYPORT << endl;
return buffer;
}
Server Output:
Sending: #7777~15~3701
Sent: 1 bytes of data
Waiting for nodes...
Client Side:
RoutingNode::GetMyID()
{
int n;
char buffer[256];
bzero(buffer,256);
unsigned int length = sizeof(struct sockaddr_in);
struct sockaddr_in from;
n = sendto(mySocket,"!", strlen("!"),0,(const struct sockaddr *)&server,length);
if (n < 0)
perror("Sendto");
//once we have our information from the manager, let's hog some cpu
//remove this crap when stuff gets more reliable
fcntl(mySocket, F_SETFL, O_NONBLOCK);
while(buffer[0] != '#')
n = recvfrom(mySocket,buffer,256,0,(struct sockaddr *)&from, &length);
if (n < 0)
perror("recvfrom");
parser.ParseMessage(buffer, fromNode, messages);
}
When I parse the message:
bool
RoutingMessage::ParseMessage(char* buffer, int &fromNode, map<int, string> &messages, const int MAX_CHARS_PER_LINE,
const int MAX_TOKENS_PER_LINE, const char* const DELIMITER)
{
#if logging > 1
cout << "Buffer: " << buffer << endl;
#endif
if (buffer[0] != '#')
{
perror("Buffer malformated!");
return false;
}
//remove the '#'
buffer++;
char buf[MAX_CHARS_PER_LINE];
strcpy(buf, buffer);
char* temp = strtok(buf, DELIMITER);
if (temp == NULL)
{
perror("Buffer malformated!");
return false;
}
fromNode = atoi(temp);
temp = strtok(NULL, DELIMITER);
vector<string> tokens;
while(temp != NULL)
{
string val(temp);
tokens.push_back(val);
temp = strtok(NULL, DELIMITER);
}
//store messages in the map: <message-type>, <message>
for (int i = 0; i < tokens.size(); i+=2)
messages.insert(pair<int, string>(atoi(tokens[i].c_str()), tokens[i+1]));
//all good
return true;
}
And output the results, I get junk:
Buffer: <junk-symbol>
Buffer malformated!: Success
Node: 0
But when the client disconnects, and I relaunch the same executable, I get:
Buffer: #7777~15~3702
Node: 7777
Message Type: 15 Message: 3702
As I'd expect. Anyone know of something I could check?
Corrected Method --
void
RoutingManager::GenerateConnectionString(struct Node n, char* buffer)
{
bzero(buffer,512);
sprintf(buffer, "#7777~15~%d", n.id);
}
The above seems to solve my issues.

Answer 1

The problem seems to be in GenerateConnectionString() function:
char*
RoutingManager::GenerateConnectionString(struct Node n)
{
char buffer[512];
....
return buffer;
}
It returns a pointer to a local variable.
So the returned pointer points to the stack. So the data will be corrupted later when the program uses stack from that area.