MailSlot write sending same thing three times C/C++ - c++

Question

I have a problem with MailSlots in windows (C/C++).
I am trying to make two simple programs right now, but the communication is not good at the last step.
This is my int main from the server.cpp
int main()
{
HANDLE ss, sc, sc2r;
LPCTSTR errMsg;
ss = CreateMailslot("\\\\.\\mailslot\\ss", 0, MAILSLOT_WAIT_FOREVER, NULL);
if (ss == INVALID_HANDLE_VALUE)
{
printf("Invalid ss value");
return -1;
}
for (;;)
{
DWORD msgSize;
DWORD nr;
BOOL err;
/* Get the size of the next record */
err = GetMailslotInfo(ss, 0, &msgSize, 0, 0);
char x[100];
char nrr[10];
if (msgSize != (DWORD)MAILSLOT_NO_MESSAGE)
{
DWORD numRead;
/* Read the record */
err = ReadFile(ss, x, msgSize, &numRead, 0);
int wrds=count(x)+1;
sc = CreateFile("\\\\*\\mailslot\\sc", GENERIC_WRITE, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
itoa(wrds,nrr,10);
err = WriteFile(sc, nrr, sizeof(nrr), &nr, 0);
//cout<<err<<endl;
//cout<<x;
//cout<<err;
strrev(x);
err=WriteFile(sc, x, sizeof(x), &nr, 0);
}
}
return(0);
}
Here is the client source:
int main()
{
HANDLE ss, sc, sc2;
LPCTSTR errMsg;
BOOL err;
DWORD numWritten;
sc = CreateMailslot("\\\\.\\mailslot\\sc", 0, MAILSLOT_WAIT_FOREVER, NULL);
ss = CreateFile("\\\\*\\mailslot\\ss", GENERIC_WRITE, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
if (ss == INVALID_HANDLE_VALUE)
{
printf("CreateFile failed. ");
// Close any mailslot we opened
if (ss != INVALID_HANDLE_VALUE) CloseHandle(ss);
return -1;
}
char x[100];
char z[100];
printf("Write the damn sentence:");
cin.getline(x,100);
err = WriteFile(ss, x, sizeof(x), &numWritten, 0);
if (!err) printf("WriteFile failed. ");
DWORD rd;
ReadFile(sc,x,sizeof(x),&rd,NULL);
cout<<x<<endl;
ReadFile(sc,z,sizeof(z),&rd,NULL);
cout<<z;
return 0;
}
It seems like the server is sending the same thing three times. I tested the client in debugger and he gets it right, but can't figure it out why the server is sending three times the same thing.
Do you have any suggestions ?

Answer 1

Mailslots are an undependable transport -- messages are free to be dropped. To try to ensure that the message gets through, the sender automatically sends the message once using each distinct protocol available (that connects that sender to the intended receiver).
Your networking stack is apparently set up so there are three protocols connecting your sender to your receiver. Since they're (probably) communicating locally, over relatively dependable hardware with no routers that handle congestion by dropping packets, or anything like that, you'll probably get three copies of every packet.
Bottom line: if you want to use mailslots, you pretty much have to do assign a serial number to each packet, so you'll be able to track when you've already received something, so you'll be able to recognize and ignore duplicates on the receiving side.
Alternatively, just don't use mailslots. If (for whatever reason) you want something specific to Windows, a named pipe is generally easier. Unless you're actually bothered by your code being portable and interoperable, sockets are probably simpler still.

Answer 2

You are confusing sizeof with strlen. Calling sizeof(nrr) will always return 10. The server program will do a single write of 10 bytes, even if the buffer only contains 2 valid bytes.
Replace sizeof with 1+strlen to fix the problem.
For example, in server.cpp if wrds is 1, then nrr will be { 0x31, 0x00 } in memory. What looks like a repeated write is really a single write of uninitialzed memory. strlen will give you the count of valid characters, +1 for the terminating null.
It might be a good idea to initialize nrr, with *nrr = 0 first. You can test that itoa succeeeded with if(*nrr) and handle the failure as you see fit.
Oh, and one more thing : you are leaking handles. It might not matter much in the client, but server leaks a handle to a mailslot at every iteration. You should reuse the mailslot handle or close it at every iteration.