The big ones for me with log/printf debugging are:
- I can get a good idea of the temporal behavior of the program, and I can just scroll up to see earlier state, rather than having to restart the program in the debugger. (I know that "time travel" debuggers exist, but I've found them finicky.) I can scrub back and forth through time just by scrolling.
- I can compare runs by diffing the logs. Sometimes that alone is enough to show where things start going amiss. Or I can keep instrumented logs from baseline runs.
- If there's a personally useful set of printf statements in an area that I'm in a lot, I can save those off to a patch file or a local branch. I don't have to reapply my breakpoints / watchpoints in the debugger each time. Easy persistence.
(That said, I do like to start with a debugger when tackling reproducible crashes.)
- I can get a good idea of the temporal behavior of the program, and I can just scroll up to see earlier state, rather than having to restart the program in the debugger. (I know that "time travel" debuggers exist, but I've found them finicky.) I can scrub back and forth through time just by scrolling.
- I can compare runs by diffing the logs. Sometimes that alone is enough to show where things start going amiss. Or I can keep instrumented logs from baseline runs.
- If there's a personally useful set of printf statements in an area that I'm in a lot, I can save those off to a patch file or a local branch. I don't have to reapply my breakpoints / watchpoints in the debugger each time. Easy persistence.
(That said, I do like to start with a debugger when tackling reproducible crashes.)