Passing arbitrary data to a child process. Inspired by https://github.com/microsoft/vscode-python/issues/18561#issuecomment-1054306197

CrustyFAS.c

#include <assert.h>

#include <Windows.h>


#pragma pack(push, 1)
typedef struct MAGIC_SAUCE
{
    INT    nHandleCount;
    HANDLE hEvent;
} MAGIC_SAUCE;
#pragma pack(pop)


INT wmain(VOID)
{
    HRESULT                 hrResult                    = E_FAIL;
    DWORD                   eWaitStatus                 = WAIT_FAILED;
    STARTUPINFOW            tOriginalStartupInfo        = {0};
    MAGIC_SAUCE UNALIGNED * ptSauce                     = NULL;
    HANDLE                  hEventToSignal              = NULL;
    WCHAR                   wszExecutable[MAX_PATH + 1] = {L'\0'};
    DWORD                   cchCopied                   = 0;
    SECURITY_ATTRIBUTES     tEventAttributes            = {0};
    HANDLE                  hEvent                      = NULL;
    MAGIC_SAUCE             tSauce                      = {0};
    STARTUPINFOW            tNewStartupInfo             = {0};
    PROCESS_INFORMATION     tProcessInfo                = {0};
    HANDLE                  ahObjectsToWaitOn[2]        = {NULL};

    GetStartupInfoW(&tOriginalStartupInfo);
    if (sizeof(MAGIC_SAUCE) == tOriginalStartupInfo.cbReserved2)
    {
        ptSauce = (MAGIC_SAUCE UNALIGNED *)tOriginalStartupInfo.lpReserved2;
        if (NULL != ptSauce && 0 == ptSauce->nHandleCount)
        {
            // This is the child process.

            hEventToSignal = ptSauce->hEvent;
            if (NULL == hEventToSignal)
            {
                hrResult = E_INVALIDARG;
                goto lblCleanup;
            }

            if (!SetEvent(hEventToSignal))
            {
                hrResult = HRESULT_FROM_WIN32(GetLastError());
                goto lblCleanup;
            }

            hrResult = S_OK;
            goto lblCleanup;
        }
    }

    cchCopied = GetModuleFileNameW(NULL, wszExecutable, ARRAYSIZE(wszExecutable));
    if (0 == cchCopied)
    {
        hrResult = HRESULT_FROM_WIN32(GetLastError());
        goto lblCleanup;
    }
    else if (ARRAYSIZE(wszExecutable) == cchCopied)
    {
        hrResult = HRESULT_FROM_WIN32(ERROR_INSUFFICIENT_BUFFER);
        goto lblCleanup;
    }

    tEventAttributes.nLength        = sizeof(tEventAttributes);
    tEventAttributes.bInheritHandle = TRUE;

    hEvent = CreateEventW(&tEventAttributes, TRUE, FALSE, NULL);
    if (NULL == hEvent)
    {
        hrResult = HRESULT_FROM_WIN32(GetLastError());
        goto lblCleanup;
    }

    tSauce.nHandleCount         = 0;
    tSauce.hEvent               = hEvent;
    tNewStartupInfo.cb          = sizeof(tNewStartupInfo);
    tNewStartupInfo.cbReserved2 = sizeof(tSauce);
    tNewStartupInfo.lpReserved2 = (PBYTE)&tSauce;

    if (!CreateProcessW(wszExecutable,
                        NULL,
                        NULL,
                        NULL,
                        TRUE,
                        CREATE_UNICODE_ENVIRONMENT,
                        NULL,
                        NULL,
                        &tNewStartupInfo,
                        &tProcessInfo))
    {
        hrResult = HRESULT_FROM_WIN32(GetLastError());
        goto lblCleanup;
    }

    ahObjectsToWaitOn[0] = hEvent;
    ahObjectsToWaitOn[1] = tProcessInfo.hProcess;

    eWaitStatus =
        WaitForMultipleObjects(ARRAYSIZE(ahObjectsToWaitOn), ahObjectsToWaitOn, TRUE, INFINITE);
    if (WAIT_OBJECT_0 != eWaitStatus)
    {
        hrResult = HRESULT_FROM_WIN32(GetLastError());
        assert(WAIT_FAILED == eWaitStatus);
        goto lblCleanup;
    }

    hrResult = S_OK;

lblCleanup:
    if (NULL != tProcessInfo.hThread)
    {
        (VOID) CloseHandle(tProcessInfo.hThread);
        tProcessInfo.hThread = NULL;
    }
    if (NULL != tProcessInfo.hProcess)
    {
        (VOID) CloseHandle(tProcessInfo.hProcess);
        tProcessInfo.hProcess = NULL;
    }
    if (NULL != hEvent)
    {
        (VOID) CloseHandle(hEvent);
        hEvent = NULL;
    }
    if (NULL != hEventToSignal)
    {
        (VOID) CloseHandle(hEventToSignal);
        hEventToSignal = NULL;
    }

    return (INT)hrResult;
}