python - Identify if a file is a gzip file

Question

I need to check using C++ if a file being opened is a gzip file or not.
In Python I use the following code to identify if a file is gzipped:

test_file = "junk.txt.gz"
with open(test_file, "rb") as f:
   f_read_first_two_bytes = f.read(2)
   if f_read_first_two_bytes==b'x1fx8b':
      print("The file is a gzipped file", end='
')

What is the equivalent in C++?

I am new to C++ and tried the following but that obviously is not the right way.

int main() {
        char p[3] = {0};
        p[2] = '
';
        // open the junk.txt.gz file. We do not want to just go by the '.gz' in the file name.
        // but want to check just like the way we did in the Python code.
        ifstream is("./junk.txt.gz", std::ios::in|std::ios::out|std::ios::binary);
        //read  two characters into p
        is.read(p,2);
        cout << std::hex << p[0] << " " << std::hex << p[1] << endl;
        return 0;
}

question from:https://stackoverflow.com/questions/65839155/identify-if-a-file-is-a-gzip-file

Answer 1

but that obviously is not the right way.

Well obviously not, since you don't compare the bytes with anything. Otherwise, it pretty much is "right" as much as the Python program is.

A simple way to do the comparison is to interpret the bytes as unsigned char:

auto up = reinterpret_cast<unsigned char*>(p);
if (up[0] == 0x1f && up[1] == 0x8b)

---

P.S. This is not necessarily the most accurate test for gzip files. It can have false positives.

I recommend not attempting to implement the test manually. There are open source libraries for this purpose (like there are for most purposes).