File IO, malloc allocating incorrect number of bytes

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C++

file

Hi, I rather new in C++ but have a basic understanding of programming in terms of Java. Right now I am having problems trying to read data from disk. After reading the number of bytes from the file, I allocate memory via malloc and create a new string to store the string of data from file. Then i remove the white spaces and try to create the byte array, however it crashes when the character is invalid

this is the string of data in my readingfile.dat

10 02 01 02 03 04 0a 00 03 10

converting method to byte array

void string_to_bytearray(std::string str, unsigned char* &array, int& size)
{
	int length = str.length();
	// make sure the input string has an even digit numbers
	if(length%2 == 1)
	{
		str = "0" + str;
		length++;
	}

	// allocate memory for the output array
	array = new unsigned char[length/2];
	size = length/2;

	std::stringstream sstr(str);
	for(int i=0; i < size; i++)
	{
		char ch1, ch2;
		sstr >> ch1 >> ch2;
		int dig1, dig2;
// crashes here when an invalid char was passed in
		if(isdigit(ch1)) dig1 = ch1 - '0';
		else if(ch1>='A' && ch1<='F') dig1 = ch1 - 'A' + 10;
		else if(ch1>='a' && ch1<='f') dig1 = ch1 - 'a' + 10;
		if(isdigit(ch2)) dig2 = ch2 - '0';
		else if(ch2>='A' && ch2<='F') dig2 = ch2 - 'A' + 10;
		else if(ch2>='a' && ch2<='f') dig2 = ch2 - 'a' + 10;
		array[i] = dig1*16 + dig2;
	}
}

my read file method

unsigned long ReadFromDisk()
{

	FILE	*pFile;
	long	lSize;
	char	*pCharbuff;
	
	byte	abBuffer[128];
	
	pFile = fopen ( "W:\\SW\\readingfile.dat" , "rb" );

	fseek(pFile , 0 , SEEK_END);
	lSize = ftell (pFile);
	rewind(pFile);

	// allocate the buffer
	pCharbuff = (char*) malloc (sizeof(char)*lSize);

// problem is here, the file has only 29 bytes of data and yet it allocates extra 4 bytes, and when checking bytes if it is a digit or not, it gives an error
	fread (pCharbuff,1,lSize,pFile);

// sorry forgot this line
std::string strString(pCharbuff);

	// remove white spaces
	bool spacesLeft = true;
	std::string::size_type pos = 0;
	while(spacesLeft)
	{
		pos = strString.find(" ");
		if(pos != std::string::npos)
			strString.erase( pos, 1 );
		else
			spacesLeft = false;
	}

	unsigned char* array = NULL;
	int size;
	string_to_bytearray(strString, array, size);	

	return 0;
}

furthermore i wish to implement this as a thread, so i can read from the same postion i left off everytime. How do i do that? can i set some offset under fseek?