How to create and edit a global PCHAR without any memory leak

Question

This might seem like a basic question, but I can't find a solution anywhere.

I have to get Information out of a kernel mode. Therefore i work with an IOCTL. My User-Mode-Applications sends this IOCTL using DeviceIoControl with METHOD_OUT_DIRECT because i need to transfer big amounts of data. This sends a char-Pointer to my Driver.

The Driver gets the IOCTL and can respond to it within the interrupt request.

Code looks like this:

case IOCTL_FILTERIO_REQUEST_DATA:
    {
        PVOID pInputBuffer;
        PVOID pOutputBuffer;
        ULONG outputBufferLength;

        PCHAR pReturnData = g_pDataPack;
        ULONG dwDataSize = 8000;
        ULONG dwDataRead = 0, dwDataWritten = 0;

        status = STATUS_UNSUCCESSFUL;
        pInputBuffer = Irp - > AssociatedIrp.SystemBuffer;
        pOutputBuffer = NULL;

        if (Irp - > MdlAddress) {
            pOutputBuffer = MmGetSystemAddressForMdlSafe(Irp - > MdlAddress, NormalPagePriority);
        }

        if (pOutputBuffer) {
            RtlCopyMemory(pOutputBuffer, pReturnData, dwDataSize);
            status = STATUS_SUCCESS; * g_pDataPack = 0;
            db_counter = 0;
        }
    }
    break;

But i need to copy information from another part of the kernel-mode.

I tried to create a global PCHAR like this:

Header-File (Declaration) : extern PCHAR g_pDataPack;

Source-File (Definition) : PCHAR g_pDataPack = "IOCTL DEFINITION";

Editing (Other Source-File): *g_pDataPack++ = 'some stuff';

My Problem is, that i have to edit and enlarge the global PCHAR many times before the interrupt copies it to the local PCHAR and sends it back to my USER-Mode-Application. I tried to reset the global Pointer every 5000 editions, but that did not realy work.

if (db_counter > 5000)
{
    *g_pDataPack = 0;
    db_counter = 0;
}

The Program crashes after a special amount of time. I guess, because the global pointer allocates to much space?

I tried to create a CHAR Array, but that did not work at all.

Is there another better or easier way to allocate a special size of memory and then edit this one and copy it to the local PCHAR pReturnData when i need it or do i have to learn how to work with DMA for this project?

alk · Accepted Answer · 2013-11-18T13:55:42.333

Declare it this way to have it global to a module:

module.c:

#include <windows.h>

#define GLOBALBUFFERMAX (1024)

CHAR dataPack[GLOBALBUFFERMAX] = "";
PCHAR g_pDataPack = dataPack;

... and the following way to have it global to the program (#include module.h to every module that shall be able to access pglobal_buffer):

module.h:

#include <windows.h>

#define GLOBALBUFFERMAX (1024)

extern PCHAR g_pDataPack;

module.c

#include <windows.h>

#include "module.h"

CHAR dataPack[GLOBALBUFFERMAX] = "";
PCHAR g_pDataPack = dataPack;

How to create and edit a global PCHAR without any memory leak

1 Answers1