C++ fstream read & write

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C++

I have a brief question as to how you would read a file. Specifically how could you read about 100 characters of a file at a time then write the very same characters to the same file.Like if you wanted to run an xor op on a file without using any file other than the source.

What I have tried:

This
void main ()
{
int theSize = 32;
char *data = new char[36];
fstream fileStreamIn("in",ios::in | ios::out | ios::binary);
while(fileStreamIn)
{
fileStreamIn.read(data,theSize);
size_t count = fileStreamIn.gcount();

cout << data;
fileStreamIn << data;
fileStreamIn.write(data,count);
}
fileStreamIn.close();
system("pause");
}