Unusual characters while printing after using ReadFile - c

Question

I have a simple text file that has "SakyLabs. Hello, MDE", and I am trying to reading it using ReadFile function and then printing out, but I am getting:
?SakyLabs. Hello, MDE??²
What am I doing wrong?
Here's the code:
// For CreateFileW
HANDLE createFile;
DWORD fileDesiredAccess = GENERIC_READ | GENERIC_WRITE;
DWORD fileShareMode = FILE_SHARE_READ | FILE_SHARE_WRITE;
DWORD creationDisposition = CREATE_ALWAYS;
DWORD fileFlags = FILE_ATTRIBUTE_NORMAL;
// ReadFile function
BOOL readFile;
DWORD lastError;
DWORD fileSize;
LPWSTR fileBuffer;
DWORD bytesRead;
if ((wcscmp(argv[1], L"-ef") == 0))
{
createFile = CreateFileW(argv[2], fileDesiredAccess, fileShareMode, NULL, OPEN_EXISTING, fileFlags, NULL);
// To get extended error message
lastError = GetLastError();
if (createFile == INVALID_HANDLE_VALUE)
{
wprintf(L"Error %d opening the file: ", lastError);
ShowErrorMessage(lastError);
return;
}
// File was opened! Let's read it's content
fileSize = GetFileSize(createFile, NULL);
fileBuffer = (LPWSTR)malloc((fileSize) + 1);
if (fileBuffer == NULL)
{
wprintf(L"Could not allocated memory.\n");
CloseHandle(createFile);
return;
}
readFile = ReadFile(createFile, fileBuffer, fileSize, &bytesRead, NULL);
lastError = GetLastError();
if (!readFile)
{
wprintf(L"Error code %d reading the file: ", lastError);
ShowErrorMessage(lastError);
CloseHandle(createFile);
return;
}
fileBuffer[fileSize] = '\0';
// Show the content
wprintf(L"%s \n", fileBuffer);
free(fileBuffer);
CloseHandle(createFile);
}
return 0;
}

Answer 1

This line is wrong:
fileBuffer[fileSize] = '\0';
fileSize is expressed in raw bytes, but fileBuffer is an array of WCHAR elements, where WCHAR is 2 bytes in size. As such, you end up writing way past the end of the array into surrounding memory, which is undefined behavior.
Put another way, your file is 40 bytes in size, and you are allocating 40+1 bytes for your array. That array can hold a max of 20 WCHARs, which means its valid indexes are 0..19, but since you are using fileSize as an index, you are writing the '\0' to the WCHAR located at index 40.
To fix that, you need to change this:
fileBuffer = (LPWSTR)malloc((fileSize) + 1);
...
fileBuffer[fileSize] = '\0';
To this instead:
fileBuffer = (LPWSTR)malloc(fileSize + sizeof(WCHAR));
...
fileBuffer[fileSize / sizeof(WCHAR)] = L'\0';
Now it will write the L'\0' into index 20, which is the array element that +sizeof(WCHAR) is allocating space for.
Alternatively, you don't have to null-terminate the array at all, if you instead tell wprintf() how many WCHARs to print, eg:
wprintf(L"%.*s \n", (int)(fileSize / sizeof(WCHAR)), fileBuffer);
On a side note -
Your code is not checking if GetFileSize() failed before using fileSize.
Also, when calling ReadFile(), fileSize is the requested number of bytes to read, but bytesRead receives the number of bytes actually read. ReadFile() can return fewer bytes than requested. So, you should use bytesRead instead of fileSize when terminating/printing fileBuffer. Otherwise, call ReadFile() in a loop to ensure all of the required bytes are actually read.
And, you are leaking fileBuffer if ReadFile() fails.

Answer 2

1.You should initialize the buffer fileBuffer.
2.fileBuffer size should be fileSize+2.
3.FF FE is the leading BOM.
If the 16-bit units use little-endian order ("UTF-16LE"), the BOM is
the (hexadecimal) byte sequence FF FE
Update the revised code:
#include<windows.h>
#include<wchar.h>
int main()
{
// For CreateFileW
HANDLE createFile;
DWORD fileDesiredAccess = GENERIC_READ | GENERIC_WRITE;
DWORD fileShareMode = FILE_SHARE_READ | FILE_SHARE_WRITE;
DWORD creationDisposition = CREATE_ALWAYS;
DWORD fileFlags = FILE_ATTRIBUTE_NORMAL;
// ReadFile function
BOOL readFile;
DWORD lastError;
DWORD fileSize;
LPWSTR fileBuffer;
DWORD bytesRead;
{
createFile = CreateFileW(L"Test.txt", fileDesiredAccess, fileShareMode, NULL, OPEN_EXISTING, fileFlags, NULL);
// To get extended error message
lastError = GetLastError();
if (createFile == INVALID_HANDLE_VALUE)
{
wprintf(L"Error %d opening the file: ", lastError);
return 0;
}
// File was opened! Let's read it's content
fileSize = GetFileSize(createFile, NULL);
fileBuffer = (LPWSTR)malloc((fileSize)+2);
if (fileBuffer == NULL)
{
wprintf(L"Could not allocated memory.\n");
CloseHandle(createFile);
return 0;
}
else
{
memset(fileBuffer,0, (fileSize)+2);
}
readFile = ReadFile(createFile, fileBuffer, fileSize, &bytesRead, NULL);
lastError = GetLastError();
if (!readFile)
{
wprintf(L"Error code %d reading the file: ", lastError);
CloseHandle(createFile);
return 0;
}
//fileBuffer[fileSize] = '\0';
// Show the content
wprintf(L"%s \n", fileBuffer+1);
free(fileBuffer);
CloseHandle(createFile);
}
return 0;
}