Reading asynchronous pipe - loosing data [duplicate] - c

Question

Modified code from
Named Pipe Server Using Overlapped I/O
https://msdn.microsoft.com/en-us/library/windows/desktop/aa365603(v=vs.85).aspx
The server code is as follows:
#include <windows.h>
#include <stdio.h>
#include <tchar.h>
#include <strsafe.h>
#include <iostream>
#define CONNECTING_STATE 0
#define READING_STATE 1
#define INSTANCES 4
#define PIPE_TIMEOUT 5000
#define BUFSIZE 4096
typedef struct
{
OVERLAPPED oOverlap;
HANDLE hPipeInst;
TCHAR chRequest[BUFSIZE];
DWORD cbRead;
TCHAR chReply[BUFSIZE];
DWORD cbToWrite;
DWORD dwState;
BOOL fPendingIO;
} PIPEINST, *LPPIPEINST;
BOOL ConnectToNewClient(HANDLE, LPOVERLAPPED);
PIPEINST Pipe[INSTANCES];
HANDLE hEvents[INSTANCES];
int _tmain(VOID)
{
DWORD i, dwWait, cbRet, dwErr;
BOOL fSuccess;
LPTSTR lpszPipename = TEXT("\\\\.\\pipe\\mynamedpipe");
for (i = 0; i < INSTANCES; i++)
{
hEvents[i] = CreateEvent(
NULL, // default security attribute
FALSE, // manual-reset event
TRUE, // initial state = signaled
NULL); // unnamed event object
if (hEvents[i] == NULL)
{
printf("CreateEvent failed with %d.\n", GetLastError());
return 0;
}
Pipe[i].oOverlap.hEvent = hEvents[i];
DWORD dwOpenMode = PIPE_ACCESS_DUPLEX | FILE_FLAG_OVERLAPPED;
Pipe[i].oOverlap.Offset = 0;
Pipe[i].oOverlap.OffsetHigh = 0;
Pipe[i].hPipeInst = CreateNamedPipe(
lpszPipename,
dwOpenMode,
PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
INSTANCES,
BUFSIZE*sizeof(TCHAR),
BUFSIZE*sizeof(TCHAR),
PIPE_TIMEOUT,
NULL);
if (Pipe[i].hPipeInst == INVALID_HANDLE_VALUE)
{
printf("CreateNamedPipe failed with %d.\n", GetLastError());
return 0;
}
BOOL rc = ConnectNamedPipe(Pipe[i].hPipeInst, &Pipe[i].oOverlap); // Overlapped ConnectNamedPipe should return FALSE.
if (!rc && GetLastError() == ERROR_PIPE_CONNECTED) {
std::cout<<"pipe connected setting event " << std::endl;
rc = SetEvent(&Pipe[i].oOverlap.hEvent);
} else if (rc || GetLastError() != ERROR_IO_PENDING) {
std::cout<<"exiting... " << std::endl;
rc = CloseHandle(Pipe[i].hPipeInst);
return 0;
}
}// for INSTANCES
while (1)
{
dwWait = WaitForMultipleObjects(
INSTANCES, // number of event objects
hEvents, // array of event objects
FALSE, // does not wait for all
INFINITE); // waits indefinitely
i = dwWait - WAIT_OBJECT_0; // determines which pipe
if (i < 0 || i > (INSTANCES - 1))
{
printf("Index out of range.\n");
return 0;
}
fSuccess = GetOverlappedResult(
Pipe[i].hPipeInst, // handle to pipe
&Pipe[i].oOverlap, // OVERLAPPED structure
&cbRet, // bytes transferred
FALSE); // do not wait
std::cout<<"GetOverlappedResult " << cbRet;
std::cout<<" success " << fSuccess;
std::cout<<" state " << Pipe[i].dwState;
std::cout<<" GetLastError " << GetLastError() << std::endl;
fSuccess = ReadFile(
Pipe[i].hPipeInst,
Pipe[i].chRequest,
BUFSIZE*sizeof(TCHAR),
&Pipe[i].cbRead,
&Pipe[i].oOverlap);
if(!fSuccess)
std::wcout<<L" Error: "<< GetLastError() <<std::endl;
if (fSuccess && Pipe[i].cbRead != 0)
{
Pipe[i].fPendingIO = FALSE;
Pipe[i].dwState = READING_STATE;
std::wcout<<L"Message " << Pipe[i].chRequest << std::endl;
continue;
}
dwErr = GetLastError();
if (! fSuccess && (dwErr == ERROR_IO_PENDING))
{
std::cout<<"Error IO is still pending" << std::endl;
Pipe[i].fPendingIO = TRUE;
continue;
}
break;
}
return 0;
}
The client code is as follows:
#include <windows.h>
#include <stdio.h>
#include <conio.h>
#include <tchar.h>
#include <string>
#include <sstream>
#include <time.h>
#include <iostream>
#define BUFSIZE 4096
int _tmain(int argc, TCHAR *argv[])
{
HANDLE hPipe;
TCHAR chBuf[BUFSIZE];
BOOL fSuccess = FALSE;
DWORD cbRead, cbToWrite, cbWritten, dwMode;
LPTSTR lpszPipename = TEXT("\\\\.\\pipe\\mynamedpipe");
BOOL rc;
do {
hPipe = CreateFileW(lpszPipename, GENERIC_WRITE, FILE_SHARE_WRITE, 0, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, 0);
if (hPipe == INVALID_HANDLE_VALUE) {
if (GetLastError() == ERROR_PIPE_BUSY) {
// wait for the pipe to become available
rc = WaitNamedPipeW(lpszPipename, NMPWAIT_USE_DEFAULT_WAIT);
if (!rc) return false;
} else {
return false;
}
}
} while (hPipe == INVALID_HANDLE_VALUE);
dwMode = PIPE_READMODE_MESSAGE;
fSuccess = SetNamedPipeHandleState(
hPipe, // pipe handle
&dwMode, // new pipe mode
NULL, // don't set maximum bytes
NULL); // don't set maximum time
if ( ! fSuccess)
{
_tprintf( TEXT("SetNamedPipeHandleState failed. GLE=%d\n"), GetLastError() );
return -1;
}
//===================================================================
while(1)
{
std::cout<<"press a key to send " << std::endl;
_getch();
fSuccess = WriteFile(
hPipe,
"A",
1,
&cbWritten,
NULL);
if ( ! fSuccess)
{
_tprintf( TEXT("WriteFile to pipe failed. GLE=%d\n"), GetLastError() );
return -1;
}
fSuccess = WriteFile(
hPipe,
"B",
1,
&cbWritten,
NULL);
if ( ! fSuccess)
{
_tprintf( TEXT("WriteFile to pipe failed. GLE=%d\n"), GetLastError() );
return -1;
}
//Sleep(1000);
fSuccess = WriteFile(
hPipe,
"C",
1,
&cbWritten,
NULL);
if ( ! fSuccess)
{
_tprintf( TEXT("WriteFile to pipe failed. GLE=%d\n"), GetLastError() );
return -1;
}
fSuccess = WriteFile(
hPipe,
"D",
1,
&cbWritten,
NULL);
if ( ! fSuccess)
{
_tprintf( TEXT("WriteFile to pipe failed. GLE=%d\n"), GetLastError() );
return -1;
}
}// loop
printf("\n<End of message, press ENTER to terminate connection and exit>");
_getch();
CloseHandle(hPipe);
return 0;
}
The server never receives message A but B, C, D are received OK.
If you uncomment //Sleep(1000) in the client, only B and D are received.
Any idea as to why this happens? The server output with out the sleep is as follows:
GetOverlappedResult 0 success 1 state 0 GetLastErr
or 997
Error: 997
Error IO is still pending
GetOverlappedResult 1 success 1 state 0 GetLastErr
or 997
Message B
GetOverlappedResult 1 success 1 state 1 GetLastErr
or 997
Message C
GetOverlappedResult 1 success 1 state 1 GetLastErr
or 997
Message D
GetOverlappedResult 1 success 1 state 1 GetLastErr
or 997
Error: 997
Error IO is still pending

Answer 1

In the server's read loop, you are discarding any data that arrives asynchronously.
After GetOverlappedResult() has reported that the pending I/O operation is complete, the buffer contains the data from that operation. You're ignoring that data and issuing a new read operation into the same buffer.
The only reason you get any of the messages is that (on most runs) all four messages will be written into the pipe's internal buffer at the same time. The first message arrives asynchronously, so you miss it, but the remaining three messages are already in the pipe so those reads can complete immediately.