Question

I have a bit of a weird problem here!
I am trying to write the color table for a 8 bit windows 3.x bitmap file. I just want the file to be greyscale, so i am trying to write bbb0, ggg0, rrr0 256 times where r=g=b=1..256

//write greyscale color table
for (int i = 255; i >= 0; i--) {
    writeS = (unsigned short)i;
    outfile.write ((char*)&writeS,sizeof(char)); // b
    outfile.write ((char*)&writeS,sizeof(char)); // g
    outfile.write ((char*)&writeS,sizeof(char)); // r
    writeS = 0;
    outfile.write ((char*)&writeS,sizeof(char)); // 0
}

When I look at the output i am getting with a hex editor, everything looks fine until I get to the number 10, which is written like so:

…0C 0C 0C 00 0B 0B 0B 00 0D 0A 0D 0A 0D 0A 00 09 09 09 00 08 08 08 00…

isntead of:

…0C 0C 0C 00 0B 0B 0B 00 0A 0A 0A 00 09 09 09 00 08 08 08 00…

So that is wierd that it is only doing it on this one number, but what is even wierder is that when I change the code to skip the number 10 and write 9 instead, it works.

//write greyscale color table
for (int i = 255; i >= 0; i--) {
    writeS = (unsigned short)i;
    if (writeS == 10) writeS = 9;
    outfile.write ((char*)&writeS,sizeof(char)); // b
    outfile.write ((char*)&writeS,sizeof(char)); // g
    outfile.write ((char*)&writeS,sizeof(char)); // r
    writeS = 0;
    outfile.write ((char*)&writeS,sizeof(char)); // 0
}

gives:

…0C 0C 0C 00 0B 0B 0B 00 09 09 09 00 09 09 09 00 08 08 08 00…

Is there something weird going on there with notation? Any obvious errors that I have missed? Has anyone ran into something like this before?
Thanks!

Answer 1

The “number 10” in ASCII is the line feed character, \n. In C++, this is the newline character.

You have apparently opened the file as a text stream. Because newlines are represented differently on different platforms, text streams perform newline translation: when reading they translate the platform-specific newline representation into \n and when writing they translate the \n character into the platform-specific newline representation.

On Windows, line breaks are represented by \r\n. When you write a \n to the text stream, it gets written as \r\n.

To write raw binary data, you need to open the stream as a binary stream. This is done by passing the ios_base::binary flag to the constructor of the stream.