This appendix focuses on dbx usage and commands that change your program or change the behavior of your program when you run it under dbx, as compared to running it without dbx. Understanding which commands might make modifications to your program is important.
The chapter contains the following sections:
You use dbx to observe a process, and the observation should not affect the process. However, on occasion, you might drastically modify the state of the process. Sometimes plain observation can affect execution and cause intermittent bug symptoms.
Your application might behave differently when run under dbx. Although dbx strives to minimize its impact on the program being debugged, you should be aware of the following:
You might have forgotten to take out a –C or disable RTC. Having the RTC support library librtc.so loaded into a program can cause the program to behave differently.
Your dbx initialization scripts might have some environment variables set that you have forgotten. The stack base starts at a different address when running under dbx. The address might also different based on your environment and the contents of argv[], forcing local variables to be allocated differently. If the variables are not initialized, they will produce different random numbers. This problem can be detected using runtime checking.
The program does not initialize memory allocated with malloc() before use. This problem can be detected using runtime checking.
dbx has to catch LWP creation and dlopen events, which might affect timing-sensitive multithreaded applications.
dbx does context switching on signals so if your application makes heavy use of signals, things might work differently.
Your program might be expecting that mmap() always returns the same base address for mapped segments. Running under dbx affects the address space sufficiently that mmap() is unlikely to return the same address as when the program is run without dbx. To determine if this is a problem, look at all uses of mmap() and ensure that the address returned is used by the program, rather than a hard-coded address.
If the program is multithreaded, it might contain data races or be otherwise dependent upon thread scheduling. Running under dbx perturbs thread scheduling and might cause the program to execute threads in a different order than normal. To detect such conditions, use lock_lint.
Otherwise, determine whether running with adb or truss causes the same problems.
To minimize perturbations imposed by dbx, try attaching to the application while it is running in its natural environment.