MrDuk MrDuk - 28 days ago 12
C++ Question

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

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

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.