std::multimap for files

#include <cerrno>
#include <cstring>
#include <cstdlib>
#include <cstdint>
#include <iostream>
#include <map>
#include <string>

#include <unistd.h>
#include <sys/types.h>
#include <dirent.h>
#include <sys/stat.h>

int main(int argc, char *argv[]) {
    const char *dirname = argc > 1 ? argv[1] : nullptr;

    if (! dirname || chdir(dirname)) {
        std::cerr << dirname << ": chdir(2) failed: "
                  << strerror(errno) << std::endl;
        exit(EXIT_FAILURE);
    }

    DIR *dir = opendir(dirname);
    if (! dir) {
        std::cerr << dirname << ": opendir(3) failed: "
                  << strerror(errno) << std::endl;
        exit(EXIT_FAILURE);
    }

    std::multimap<off_t, std::string> files;
    for (dirent *entry = readdir(dir); entry; entry = readdir(dir)) {
        struct stat stats;
        if (stat(entry->d_name, &stats)) {
            std::cerr << entry->d_name << ": stat(2) failed: "
                      << strerror(errno) << std::endl;
        }

        files.insert(std::make_pair(stats.st_size, std::string(entry->d_name)));
    }
    if (errno) {
        std::cerr << dirname << ": readdir(3) failed: "
                  << strerror(errno) << std::endl;
    }
    closedir(dir);

    for (auto entry : files) {
        std::cout << entry.first << "\t" << entry.second << std::endl;
    }

    return EXIT_SUCCESS;
}