Invalid access to memory location with ReadFile - c++

Question

I have a file, C:\demo\Demo.txt, that has a simple "Hello, world" on it. I want to pass the path as argument to my app, open it with CreateFile, read it with ReadFile and show that line out on console. However, I am receiving an error code 998:
Invalid access to memory location.
This is my code:
int wmain(int argc, WCHAR **argv)
{
if (argc != 2)
{
fwprintf(stderr, L"\nWrong arguments. \n");
return 1;
}
// CreateFile function variables
HANDLE hSourceFile;
LPCWSTR fileName = (LPCWSTR)argv[1];
DWORD desiredAccess = FILE_GENERIC_READ;
DWORD shareMode = FILE_SHARE_READ;
DWORD creationDisposition = OPEN_EXISTING;
DWORD flagsAndAttributes = FILE_ATTRIBUTE_NORMAL;
//---------------------------------------------------------------
// Opening file for reading data
hSourceFile = CreateFileW(
fileName,
desiredAccess,
shareMode,
NULL,
creationDisposition,
flagsAndAttributes,
NULL);
if (hSourceFile != INVALID_HANDLE_VALUE)
{
wprintf(L"\nThe source file, %s, is open. \n", fileName);
}
else
{
wprintf(L"Error code: %u\n", GetLastError());
}
// ReadFile function variables
LPVOID dataRead=NULL;
DWORD bytesToRead = 100;
DWORD bytesWritten = 0;
//-----------------------------------------------------------------
// Reading data from file
if (!ReadFile(
hSourceFile,
dataRead,
bytesToRead,
&bytesWritten,
NULL))
{
wprintf(L"Error code: %u\n", GetLastError());
return 1;
}
wprintf(L"%s. \n", (LPWSTR)dataRead);
CloseHandle(hSourceFile);
return 0;
}
First time I use ReadFile, so no idea what I am doing wrong.
Can you help me?

Answer 1

ReadFile wants a pointer to a buffer into which it can write the data. You are passing NULL, so you get the error you see.
I would change the code to
// ReadFile function variables
static const DWORD bytesToRead = 100;
unsigned char dataRead[bytesToRead];
DWORD bytesWritten = 0;
//-----------------------------------------------------------------
// Reading data from file
if (!ReadFile(
hSourceFile,
dataRead,
bytesToRead,
&bytesWritten,
NULL))
{
wprintf(L"Error code: %u\n", GetLastError());
return 1;
}
The next problem you have, is that you are casting your pointer to LPWSTR, that is a pointer to a null-terminated wide string. Does your file contain that null termination? or do you need to add it yourself? Assuming the file doesn't contain the termination, you probably want:
// ReadFile function variables
static const DWORD bufferSize = 50;
WCHAR buffer[bufferSize+1]; // Leave room for null.
DWORD bytesWritten = 0;
//-----------------------------------------------------------------
// Reading data from file
if (!ReadFile(
hSourceFile,
buffer,
bufferSize*sizeof(WCHAR),
&bytesWritten,
NULL))
{
wprintf(L"Error code: %u\n", GetLastError());
return 1;
}
buffer[bytesWritten/sizeof(WCHAR)] = 0; // Null terminate.
wprintf(L"%s. \n", buffer); // Look ma! No cast needed.

Answer 2

You must allocate a memory buffer where you want to place read bytes to. Now you pointer dataRead points to nullptr, in other words nowhere, but you pass the size 100, that states your pointer refers to 100 byte allocated buffer, it is not truth.