Reading an uint32_t from a byte stream

Question

I'm reading bytes from a PNG image via std::basic_ifstream<std::uint8_t> . I'm having problems reading a sequence of 4 bytes that should be interpreted as 32 bit int.

std::uint32_t read_chunk_length(std::basic_ifstream<std::uint8_t> &ifs) {
    std::uint32_t length;
    ifs.read(reinterpret_cast<std::uint8_t*>(&length), 4);
    return length;
}

When reading a sequence that is 00 00 00 0d and should thus be 0xd (or 13), the above function gives me 0xd000000 (or 218103808 instead). Apologies if the question is trivial.

Answer 1

This is a byte ordering issue - the stream on disk contains the bytes in the opposite order ( big endian as specified in the PNG spec ) than your architecture mandates for integers (likely little-endian). You have to manually reverse the order of bytes to solve this.