Async file IO doesn't return ERROR_IO_PENDING

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I'm trying to make an asynchronous IO operation work. However, both the WriteFile and ReadFile functions do not work; WriteFile doesn't print out prompt1 and ReadFile doesn't return chBuffer[200] so consequently the switch statement returns the default output "option not valid!". Moreover the functions fail to return ERROR_IO_PENDING.

Here is my code for more information:

C++

#include <Windows.h>
#include <iostream>
#include <string>

class Timer {
public:
    Timer() = default;
    ~Timer() {};

    BOOL writeOperation(HANDLE& hFile, LPCSTR lpszPrompt1) {
        if (!WriteFile(
            hFile,
            lpszPrompt1,
            lstrlenA(lpszPrompt1),
            NULL,
            &ov1)) {
            if (GetLastError() != ERROR_IO_PENDING) {
                std::cout << "****WriteFile: Error " << GetLastError() << "****" << std::endl;
                ExitProcess(0);
            }
            else {
                // completes asynchronously
                return TRUE;
            }
        }
        else {
            // completes synchronously
            return FALSE;
        }
    }

    CHAR readOperation(HANDLE& hFile) {
        if (!ReadFile(
            hFile,
            chBuffer,
            dwRead,
            NULL,
            &ov2)) {
            if (GetLastError() != ERROR_IO_PENDING) {
                std::cout << "****ReadFile: Error " << GetLastError() << "****" << std::endl;
                ExitProcess(0);
            }
            else {
                return chBuffer[200];
            }
        }
    }

    BOOL waitOperation(HANDLE& hFile) {
        if (WaitForSingleObject(hFile, dwTimeOut) == WAIT_FAILED) {
            std::cout << "****WaitForSingleObject: Error " << GetLastError() << "****" << std::endl;
        }
        else {
            return WaitForSingleObject(hFile, dwTimeOut) == WAIT_OBJECT_0;
        }
    }

public:
    CHAR chBuffer[201]{};

private:
    DWORD dwRead{ 200 };
    DWORD dwTimeOut{ 5'000 };

    OVERLAPPED ov1 = { 0, 0, 0, 0, CreateEvent(NULL, FALSE, FALSE, TEXT("OverlappedOne")) };
    OVERLAPPED ov2 = { 0, 0, 0, 0, CreateEvent(NULL, FALSE, FALSE, TEXT("OverlappedTwo")) };
};

int main() {
    Timer tm;

    LPCSTR lpszPrompt1{ "Option: " };

    HANDLE hFile = CreateFileA(
        "C:\\Temp\\MainFile.txt",
        GENERIC_READ | GENERIC_WRITE,
        FILE_SHARE_READ | FILE_SHARE_WRITE,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
        NULL
    );

    if (hFile == INVALID_HANDLE_VALUE) {
        std::cout << "****CreateFileA: Error " << GetLastError() << "****" << std::endl;
        return 1;
    }

    tm.writeOperation(hFile, lpszPrompt1);

    //do {
    //    
    //    // tm.readOperation(hFile);
    //
    //    switch (tm.chBuffer[0]) {
    //    case '1': std::cout << "option 1" << std::endl;
    //        break;
    //    case '2': std::cout << "option 2" << std::endl;
    //        break;
    //    case '3': std::cout << "exit" << std::endl;
    //        break;
    //    default: std::cout << "option not valid!" << std::endl;
    //        break;
    //    }
    //} while (tm.waitOperation(hFile));

    CloseHandle(hFile);
    return 0;
}

What I have tried:

I tried changing multiple things such as the CreateFile's file name, the OVERLAPPED structures' values, as well as the parameters of the WriteFile and ReadFile functions, however nothing works except WaitForSingleObject.

Can someone please tell me how to make these async functions work together?

Posted 12-Jul-23 23:13pm

RexyCode

Updated 9-Aug-23 0:17am

v2

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Comments

Richard MacCutchan 9-Aug-23 5:46am

I just ran that code and it worked. So what is the new problem?

RexyCode 9-Aug-23 5:52am

Do you actually see "Option: " being printed it out to the terminal? Because like I previously stated WriteFile does have ERROR_IO_PENDING but it doesn't print out the actual string given to it.

Richard MacCutchan 9-Aug-23 6:19am

When I ran the code it printed the result of the read operation ("Option: "). You can also check to see what the text file contains. See also my latest Solution below.

Richard MacCutchan 9-Aug-23 7:19am

I have reworked Solution to so it is much easier to see how the operations should be managed.

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Richard MacCutchan · Answer 1 · 2023-07-13T00:14:00

Solution 1

The main problem that I can see is that you call the ioOperations which will return as soon as the WriteFile has started, since WriteFile returns false for overlapped IO. Returning the value of GetLastError from that call is pointless as your main code does not check it. The main loop is only checking the value of the read buffer which will never contain anything because the read never gets actioned. And even if it did the first byte of the buffer will not contain 1, 2, or 3 since those characters are never written to the file. There may be other logic issues, but i will leave that for you to check.

One other item worth mentioning is in the following code:

C++

LPCSTR prompt1{ "Option: " };

DWORD dwRead{ 200 };
DWORD bytesPrompt1{ sizeof(prompt1) };

The variable prompt1 is a pointer (to a charactger array) so the sizeof(prompt1) will always be 4, as it is the size of the pointer, not the size of the array.

Posted 13-Jul-23 0:14am

Richard MacCutchan

Comments

RexyCode 13-Jul-23 8:53am

Thank you for your input. I changed sizeof(prompt1) to lstrlenA(prompt1) and I also changed the ioOperations function by giving the ReadFile function its own function. However the problem still remains for both the WriteFile and ReadFile functions.

BOOL writeOperation() {
if (WriteFile(hFile, prompt1, lstrlenA(prompt1), NULL, &ov1) != ERROR_INVALID_HANDLE) {
std::cout << "TEXT PRINTED" << std::endl; // Successfully prints out the aforementioned string.
}
else {
std::cout << "****WriteFile: Handle Error****" << std::endl;
return GetLastError();
}
}

BOOL readOperation() {
if (ReadFile(hFile, chBuffer, dwRead, NULL, &ov2) != ERROR_INVALID_HANDLE) {
return chBuffer[200];
}
else {
std::cout << "****ReadFile: Handle Error****" << std::endl;
return GetLastError();
}
}

Richard MacCutchan 13-Jul-23 9:04am

The WriteFile function does not return an error code, it returns TRUE or FALSE as explained in the documentation at WriteFile function (fileapi.h) - Win32 apps | Microsoft Learn[^]. You should remove most of the code from your program and just test the write function, and examine exactly what gets written to the file. Until you get that working correctly you will go round in circles.

RexyCode 2-Aug-23 8:38am

After tinkering a bit longer with the WriteFile function, I was able to get ERROR_IO_PENDING. However, I still can't manage to print out "Option: ", perhaps you have an idea of what the issue could be. By the way, if you would like I can provide you with the new code. Thank you in advance!

Richard MacCutchan 2-Aug-23 9:58am

Yes, because you are testing for a value which does not exist, as I already suggested. The statement:

        switch (tm.chBuffer[0]) {

is meaningless, until all Write and Read operations have completed successfully. So you need to rewrite your main method to check the results of all the I/O before examining any data area that you are (trying to) reading into.

Also, the definitions of your methods are incorrect. You declare them all as returning a BOOL value, but then you return any one of BOOL. Error code, character or void. So strictly speaking this code should not even compile successfully.

RexyCode 8-Aug-23 7:46am

Well like I previously stated I got WriteFile to return ERROR_IO_PENDING and it does write to the file when I check it in my file explorer. However, it can't print the string out to the console terminal (by the way, the new version of my program returns proper BOOL values and not error messages and tm.chBuffer[0] is commented out for now until I get the rest working).

Richard MacCutchan 8-Aug-23 11:20am

If you still have a problem then you need to edit your question and post the latest version of your code.

Richard MacCutchan · Answer 2 · 2023-08-09T00:17:00

I have reworked this to simplify the code so you can see how to manage overlapped operations, and capture the results correctly.

C++

#include <Windows.h>
#include <iostream>
#include <string>

int main()
{
    HANDLE hFile = CreateFileA(
        "MainFile.txt",
        GENERIC_READ | GENERIC_WRITE,
        FILE_SHARE_READ | FILE_SHARE_WRITE,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
        NULL
    );

    if (hFile == INVALID_HANDLE_VALUE) {
        std::cout << "****CreateFileA: Error " << GetLastError() << "****" << std::endl;
        return 1;
    }

    LPCSTR lpszPrompt1{ "Option: " };
    DWORD dwWritten;
    DWORD dwError;

    OVERLAPPED ovWrite = { 0, 0, 0, 0, CreateEvent(NULL, FALSE, FALSE, "ovWrite") };

    if (!WriteFile(
        hFile,
        lpszPrompt1,
        lstrlen(lpszPrompt1),
        &dwWritten,
        &ovWrite))
    {
        dwError = GetLastError();
        if (dwError != ERROR_IO_PENDING)
        {
            std::cout << "****WriteFile: Error " << GetLastError() << "****" << std::endl;
            ExitProcess(0);
        }
        else
        {
            // wait for it to complete
            std::cout << "WriteFile returned ERROR_IO_PENDING" << std::endl;
            BOOL bw = GetOverlappedResult(hFile, &ovWrite, &dwWritten, TRUE);
            if (!bw)
            {
                // something went wrong
                std::cout << "GetOverlappedResult failed" << std::endl;
            }
        }
    }
    std::cout << "WriteFile wrote " << dwWritten << " bytes" << std::endl;

    DWORD dwRead;
    CHAR chBuffer[200]{};
    OVERLAPPED ovRead = { 0, 0, 0, 0, CreateEvent(NULL, FALSE, FALSE, "ovRead") };
    if (!ReadFile(
        hFile,
        chBuffer,
        200,
        &dwRead,
        &ovRead))
    {
        dwError = GetLastError();
        if (dwError != ERROR_IO_PENDING)
        {
            std::cout << "****ReadFile: Error " << GetLastError() << "****" << std::endl;
            ExitProcess(0);
        }
        else
        {
            // wait for it to complete
            std::cout << "ReadFile returned ERROR_IO_PENDING" << std::endl;
            BOOL bw = GetOverlappedResult(hFile, &ovRead, &dwRead, TRUE);
            if (!bw)
            {
                // something went wrong
                std::cout << "GetOverlappedResult failed" << std::endl;
            }
        }
    }
    std::cout << "ReadFile read " << dwRead << " bytes: ";
    std::cout << "\"" << chBuffer << "\"" << std::endl;


    CloseHandle(hFile);
    return 0;
}

I have run tests on the above code and the output is:

WriteFile returned ERROR_IO_PENDING
WriteFile wrote 8 bytes
ReadFile returned ERROR_IO_PENDING
ReadFile read 8 bytes: "Option: "