How to get magic number of a binary file

Question

There is a magic number associated with each binary file , does anyone know how to retrieve this information from the file?

Answer 1

Expounding on @nos's answer:

Example below uses the default magic database to query the file passed on the command line. (Essentially an implementation of the file command. See man libmagic for more details/functions.

#include 
#include 
#include 
int main(int argc, char **argv) {
    if (argc == 1) {
            std::cerr << "Usage "  << argv[0] << " [filename]" << std::endl;
            return -1;
    }
    const char * fname = argv[1];
    magic_t cookie = magic_open(0);
    assert (cookie !=nullptr);
    int rc = magic_load(cookie, nullptr);
    assert(rc == 0);
    auto f=  magic_file(cookie, fname);
    if (f ==nullptr) {
        std::cerr << magic_error(cookie) << std::endl;
    } else {
        std::cout << fname << ' ' << f << std::endl;
    }

}