reading binary from a file gives negative number - c++

Question

Hey everyone this may turn out to be a simple stupid question, but one that has been giving me headaches for a while now. I'm reading data from a Named Binary Tag file, and the code is working except when I try to read big-endian numbers. The code that gets an integer looks like this:
long NBTTypes::getInteger(istream &in, int num_bytes, bool isBigEndian)
{
long result = 0;
char buff[8];
//get bytes
readData(in, buff, num_bytes, isBigEndian);
//convert to integer
cout <<"Converting bytes to integer..." << endl;
result = buff[0];
cout <<"Result starts at " << result << endl;
for(int i = 1; i < num_bytes; ++i)
{
result = (result << 8) | buff[i];
cout <<"Result is now " << result << endl;
}
cout <<"Done." << endl;
return result;
}
And the readData function:
void NBTTypes::readData(istream &in, char *buffer, unsigned long num_bytes, bool BE)
{
char hold;
//get data
in.read(buffer, num_bytes);
if(BE)
{
//convert to little-endian
cout <<"Converting to a little-endian number..." << endl;
for(unsigned long i = 0; i < num_bytes / 2; ++i)
{
hold = buffer[i];
buffer[i] = buffer[num_bytes - i - 1];
buffer[num_bytes - i - 1] = hold;
}
cout <<"Done." << endl;
}
}
This code originally worked (gave correct positive values), but now for whatever reason the values I get are either over or underflowing. What am I missing?

Answer 1

Your byte order swapping is fine, however building the integer from the sequences of bytes is not.
First of all, you get the endianness wrong: the first byte you read in becomes the most significant byte, while it should be the other way around.
Then, when OR-ing in the characters from the array, be aware that they are promoted to an int, which, for a signed char, sets a lot of additional bits unless you mask them out.
Finally, when long is wider than num_bytes, you need to sign-extend the bits.
This code works:
union {
long s; // Signed result
unsigned long u; // Use unsigned for safe bit-shifting
} result;
int i = num_bytes-1;
if (buff[i] & 0x80)
result.s = -1; // sign-extend
else
result.s = 0;
for (; i >= 0; --i)
result.u = (result.u << 8) | (0xff & buff[i]);
return result.s;