Okay, the question title may be a bit cryptic. But it aptly describes what I'm looking for:
I think every experienced coder went through this numerous times: You get a binary file format specification, you implement the reader for it, and… nothing works like expected. So you run your code in the debugger, go execute through the code line by line, every header field is read in seemingly correct, but when it comes to the bulk data, offset and indices no longer match up.
What would really help in this situation was a binary file viewer, that shows you the progress of your file pointer, as you step through the code, and ideally would also highlight all memory maps. Then you could see the context of the current I/O operations, most notably those darn "off-by-one" mistakes, which are even more annoying when reading a file.
Implementing such a debugger should not be too hard. traces on the process' file descriptors/handles and triggers on the I/O functions, to update the display.
Only: I don't know of such a kind of debugger to exist. Do I just lack knowledge about the existance of such a tool, or is there really no such thing?