I would like to print on two-sided (duplex), for this I use :

#include <windows.h>
#include <winspool.h>

void SetDuplexPrinting(HANDLE hPrinter) {
    PRINTER_INFO_2 *pPrinterInfo = NULL;
    DWORD dwNeeded = 0;
    GetPrinter(hPrinter, 2, NULL, 0, &dwNeeded);
    pPrinterInfo = (PRINTER_INFO_2 *)malloc(dwNeeded);
    GetPrinter(hPrinter, 2, (LPBYTE)pPrinterInfo, dwNeeded, &dwNeeded);

    DEVMODE *pDevMode = pPrinterInfo->pDevMode;
    if (pDevMode) {
        pDevMode->dmFields |= DM_DUPLEX;
        pDevMode->dmDuplex = DMDUP_VERTICAL; // or DMDUP_HORIZONTAL
        LONG result = DocumentProperties(NULL, hPrinter, pPrinterInfo->pPrinterName, pDevMode, pDevMode, DM_IN_BUFFER | DM_OUT_BUFFER);
        if (result != IDOK) {
            // Handle the error
        }
    }

    free(pPrinterInfo);
}

int main() {
    HANDLE hPrinter;
    DOC_INFO_1 DocInfo;
    DWORD dwJob;

    // Open the printer
    if (OpenPrinter("Printer name", &hPrinter, NULL)) {
        SetDuplexPrinting(hPrinter);

        // Set document information
        DocInfo.pDocName = "My Document";
        DocInfo.pOutputFile = NULL;
        DocInfo.pDatatype = "RAW";

        // Start the print job
        dwJob = StartDocPrinter(hPrinter, 1, (LPBYTE)&DocInfo);
        if (dwJob > 0) {
            StartPagePrinter(hPrinter);
            // Writing data to the printer with WritePrinter
            EndPagePrinter(hPrinter);
            EndDocPrinter(hPrinter);
        } else {
            // Handle the error
        }
        ClosePrinter(hPrinter);
    } else {
        // Handle Printer Opening Error
    }

    return 0;
}

but my HP printer prints one sided.
what is strange, StartDocPrinter() does not even use the default settings that I changed in Windows (duplex long edge).

Thanks