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

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.