converting a string read from binary file to integer - c++

Question

I have a binary file. i am reading 16 bytes at a time it using fstream.
I want to convert it to an integer. I tried atoi. but it didnt work.
In python we can do that by converting to byte stream using stringobtained.encode('utf-8') and then converting it to int using int(bytestring.hex(),16). Should we follow such an elloborate steps as done in python or is there a way to convert it directly?
ifstream file(binfile, ios::in | ios::binary | ios::ate);
if (file.is_open())
{
size = file.tellg();
memblock = new char[size];
file.seekg(0, ios::beg);
while (!file.eof())
{
file.read(memblock, 16);
int a = atoi(memblock); // doesnt work 0 always
cout << a << "\n";
memset(memblock, 0, sizeof(memblock));
}
file.close();
Edit:
This is the sample contents of the file.
53 51 4C 69 74 65 20 66 6F 72 6D 61 74 20 33 00
04 00 01 01 00 40 20 20 00 00 05 A3 00 00 00 47
00 00 00 2E 00 00 00 3B 00 00 00 04 00 00 00 01
I need to read it as 16 byte i.e. 32 hex digits at a time.(i.e. one row in the sample file content) and convert it to integer.
so when reading 53 51 4C 69 74 65 20 66 6F 72 6D 61 74 20 33 00, i should get, 110748049513798795666017677735771517696
But i couldnt do it. I always get 0 even after trying strtoull. Am i reading the file wrong, or what am i missing.

Answer 1

You have a number of problems here. First is that C++ doesn't have a standard 128-bit integer type. You may be able to find a compiler extension, see for example Is there a 128 bit integer in gcc? or Is there a 128 bit integer in C++?.
Second is that you're trying to decode raw bytes instead of a character string. atoi will stop at the first non-digit character it runs into, which 246 times out of 256 will be the very first byte, thus it returns zero. If you're very unlucky you will read 16 valid digits and atoi will start reading uninitialized memory, leading to undefined behavior.
You don't need atoi anyway, your problem is much simpler than that. You just need to assemble 16 bytes into an integer, which can be done with shifting and or operators. The only complication is that read wants a char type which will probably be signed, and you need unsigned bytes.
ifstream file(binfile, ios::in | ios::binary);
char memblock[16];
while (file.read(memblock, 16))
{
uint128_t a = 0;
for (int i = 0; i < 16; ++i)
{
a = (a << 8) | (static_cast<unsigned int>(memblock[i]) & 0xff);
}
cout << a << "\n";
}
file.close();

Answer 2

It the number is binary what you want is:
short value ;
file.read(&value, sizeof (value));
Depending upon how the file was written and your processor, you may have to reverse the bytes in value using bit operations.