CString::Format() causes debug assertion - visual-c++

Question

Cstring::Format causes debug assertion in visual studio 2008 at vsprintf.c line 244 with "buffer too small".
//inside the function.
somefile.Open (//open for mode read) //somefile is CFile.
char* buff = new [somefile.GetLength()];
somefile.Read ((void*)buff, somefile.GetLength());
CString cbuff;
cbuff.Format ("%s",buff); //this line causes the debug assertion.
//and so on
Any idea why CString::Format() causes "buffer too small" error ? This doesn't always get debug assertion error.

Answer 1

An alternate solution is:
somefile.Open (//open for mode read) //somefile is CFile.
int buflen = somefile.GetLength();
CString cbuff;
somefile.Read ((void*)cbuff.GetBuffer(buflen), buflen);
cbuff.ReleaseBuffer();
It reads directly into a string buffer instead of the intermediate variable. The CString::GetBuffer() function automatically adds the extra byte to the string which you forgot to do when you allocated the "new char[]".

Answer 2

string end with '\0'
so buffer size will not be enough

Answer 3

The problem is that CFile::Read() does not guarantee that it reads as much data as you ask for. Sometimes it's reading less and leaving your buffer without a null terminator. You have to assume that you might only get one byte on each read call. This will also crash sometimes, when an un-readable memory block immediately follows your buffer.
You need to keep reading the file until you get to the end. Also, the null terminator is generally not written to the file at all, so you shouldn't assume that it will be read in but rather ensure that your buffer is always null-terminated no matter what is read.
In addition, you shouldn't use the file size as the buffer size; there's no reason to think you can read it all in at once, and the file size might be huge, or zero.
You should also avoid manual memory management, and instead of new[]/delete[], use a vector, which will ensure that you don't forget to free the buffer or use delete instead of delete[], and that the memory is released even in case of an exception. (I wouldn't recommend using CString or CFile either, for that matter, but that's another topic...)
// read from the current file position to the end of
// the file, appending whatever is read to the string
CString ReadFile(CFile& somefile, CString& result)
{
std::vector<char> buffer(1024 + 1);
for (;;)
{
int read = somefile.Read(&buffer[0], buffer.size() - 1);
if (read > 0)
{
// force a null right after whatever was read
buffer[read] = '\0';
// add whatever was read to the result
result += &buffer[0];
}
else
{
break;
}
}
}
Note that there's no error handling in this example.