Alas, the same problem has been known to happen with debugger-provided breakpoints and tracepoints. That's why GCC advertises -O0 as «make debugging produce th

Alas, the same problem has been known to happen with debugger-provided breakpoints and tracepoints. That's why GCC advertises -O0 as «make debugging produce the expected results» (which in practice will avoid the problem in your example too). There are lots of other ways to cause yourself trouble using printf to debug, of course (eg clobbering errno).