Raw file functions for GameMaker

I am trying to append the file and continue writing. But when using:
file_raw_seek(a_file,file_raw_seek_end,0);
I don't get to the end of the file. Tested to seek just 1 byte from start and that works. But I can't seek from relative position. Whenever I seek it start from the start of the file. file_raw_seek do return 1.

Tested to manually seek to the end with:

var seek_pos=0;
while (file_raw_eof(a_file)=false)
{
    file_raw_seek(a_file,file_raw_seek_start,seek_pos);
    seek_pos+=1;
}

But it becomes a infinite loop.