Question

My task is to implement JPEG encoder/decoder in Qt. I have to process jpeg markers (for example 0xFFC4 which is start of huffman table) and data between markers. I open my jpeg file using QFile class. While debugging the result is shown in picture bellow. As you can see the value in marker[0] is -1/255 and marker[1] is -40/216, and the if statement is never true I don’t know why? The code is following:

try{
        char markers[2];
        char TableData[64];
        QFile *file= new QFile( "NovaSlika.jpeg");
        if(file->open(QFile::ReadOnly )){
            while(file->read((char*)markers,sizeof(markers))){

                if(markers[0]=='255' && markers[1]=='216'){//FF i C4
                    char TableLength[2];
                    char tableMetaData;
                    file->read((char*)TableLength,sizeof(TableLength));//Read table length
                    file->read((char*)tableMetaData,sizeof(tableMetaData));//Read metadata
                    file->read((char*)TableData,sizeof(TableData));//Read data
                    break;
                }

            }
        }

Picture of debugging process is here

Answer 1

Be careful in your while condition: read() returns

• 0 if there is no more data.
• -1 if there is an error, such as reading past the end.
• # bytes read otherwise.

You should probably do:

while( file->read( (char*)markers, sizeof(markers) ) == 2 ) 

And this line isn’t right either:

markers[0]=='255' && markers[1]=='216'

Get rid of those single quotes. (-Wall on gcc would have warned you on this). It should be:

markers[0]==255 && markers[1]==216