Extract embedded .png files from an uncompressed container

pngextract.cpp

#include <iostream>
#include <fstream>
#include <sstream>
#include <cstdint>
#include <iomanip>
#include <cctype>

const uint8_t magicnum[8] = {0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a};
const std::string magicstr(reinterpret_cast<const char*>(magicnum), 8);

void pngextract(const std::string& fname)
{
    std::stringstream readbuf;
    {
        std::ifstream infile(fname);
        readbuf << infile.rdbuf();
    }
    const std::string& buffer = readbuf.str();

    std::string::size_type pos = 0;

    int outnum = 0;
    while ((pos = buffer.find(magicstr, pos)) < buffer.size()) {
        std::cerr << "Found magic number at " << pos << std::endl;
        std::string::size_type start = pos;

        pos += 8;

        while (pos < buffer.size()) {
            uint32_t size = ((uint8_t)buffer[pos] << 24) | ((uint8_t)buffer[pos + 1] << 16) | ((uint8_t)buffer[pos + 2] << 8) |
                            (uint8_t)buffer[pos + 3];
            pos += 4;

            std::string chunk = buffer.substr(pos, 4);
            pos += 4;

            bool isValidChunk = true;
            for (char c : chunk) {
                if (!isprint(c)) {
                    std::cerr << "Got invalid byte " << std::hex << c << " in chunk type; skipping file" << std::endl;
                    isValidChunk = false;
                    break;
                }
            }
            if (!isValidChunk) {
                break;
            }

            if (pos + size > buffer.size()) {
                std::cerr << "png at " << start << " has invalid " << chunk << " length " << size << "; skipping file" << std::endl;
                break;
            }

            std::cerr << "    Found chunk type=" << chunk << " size=" << size << std::endl;

            pos += size + 4;

            if (chunk == "IEND") {
                std::stringstream outfname;
                outfname << fname << "-" << outnum << ".png";
                std::cout << outfname.str() << ": " << start << " - " << pos << std::endl;
                std::ofstream outfile(outfname.str());
                outfile << buffer.substr(start, pos - start);
                ++outnum;
                break;
            }
        }
    }
}

int main(int argc, char* argv[])
{
    for (int i = 1; i < argc; i++) {
        pngextract(argv[i]);
    }

    return 0;
}