libjpeg custom source manager

Question

So I've been reading the libjpeg documentation and it is extremely lackluster.

I have been trying to figure out how to read from a custom memory buffer rather than a file and am not sure how to even test if my solution is working correctly.

At the moment my function for loading a jpeg from memory is like so:

struct error_mgr{
    jpeg_error_mgr pub;
    std::jmp_buf buf;
};

bool load_jpeg(void *mem, size_t size, output_struct &output){
    jpeg_source_mgr src;

    src.next_input_bytes = static_cast<JOCTET*>(mem)-size;
    src.bytes_in_buffer = size;

    src.init_source = [](j_compress_ptr){};

    src.fill_input_buffer = [](j_decompress_ptr cinfo) -> boolean{
        // should never reach end of buffer
        throw "libjpeg tried to read past end of file";
        return true;
    };

    src.skip_input_data = [](j_compress_ptr cinfo, long num_bytes){
        if(num_bytes < 1) return; // negative or 0 us no-op
        cinfo->src.next_input_byte+=num_bytes;
        cinfo->src.bytes_in_buffer-=num_bytes;
    };

    src.resync_to_restart = jpeg_resync_to_restart;

    src.term_source = [](j_decompress_ptr){};

    struct jpeg_decompress_struct cinfo;

    error_mgr err;
    cinfo.err = jpeg_std_error(&err.pub);
    err.pub.error_exit = [](j_common_ptr cinfo){
        error_mgr ptr = reinterpret_cast<error_mgr*>(cinfo->err);
        std::longjmp(ptr->buf, 1);
    };

    if(std::setjmp(err.buf)){
        jpeg_destroy_decompress(&cinfo);
        return false;
    }

    cinfo.src = &src;

    jpeg_create_decompress(&cinfo);

    (void) jpeg_read_header(&cinfo, TRUE);

    // do the actual reading of the image

    return true;
} 

But it never makes it past jpeg_read_header.

I know that this is a jpeg file and I know that my memory is being passed correctly because I have libpng loading images with the same signature and calling function fine, so I'm sure it is how I am setting the source manager in cinfo.

Anybody with more experience in libjpeg know how to do this?

Answer 1

In my code I am setting cinfo.src before calling jpeg_create_decompress, setting it afterwards fixed the issue :)