We are stepping through usually using step. At each moment, gdb tells us the next line of code it is ready to execute not a line that has already executed. When the next line indicates a call to a standard library function like rand or puts , we use next to do the function call and come back on the next line after it. Now check out the last few steps. This should return back to some point in main. Do you see what went wrong yet? The problem in the original program is a common error but a deadly one: we forgot to load the link register lr back from the stack at the end of the function.
That caused the br lr to return back to the same place of the most recent function call , which is the line after the bl puts. This is an example of debugging in action. This is saying that strlen. S is seg faulting at line It will however, still give some useful information. The output above is telling us:. But we can still use the information that the line of code that we actually wrote that seems to be involved is line Examining the code reveals that is this line:.
So, we called puts , and that caused a segfault. In fact, in this hypothetical example, just before calling puts I replaced this correct line:. Note that the where command did not tell us that the error was this line of code, but it did point us to the call to puts being problematic, which is very useful to know.
Why gdb or gdbgui? Quick start A little more details on commands Getting help Work flow Breakpoints Running your program Looking at registers and stack Looking at the code within gdb Debugging example Postscript — one more very useful command Why gdb or gdbgui? Quick start You will use gdb to run your program. Again, both will become zero if you step over the initialisers. The object has not been constructed yet, but space has been allocated for it. You're seeing the entire backtrace -- you're in main , period.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How does backtrace work? Ask Question. Asked 9 years, 9 months ago. Active 9 years, 9 months ago. Viewed 3k times. Improve this question. Add a comment. Active Oldest Votes. What do frames 1 and 2 show exactly? Why does x have two different values here, assuming that the breakpoint is on main? Improve this answer.
We now have all callstacks for all threads with full local variable information. First, however, some introductions are in order. Compilers and linkers emit DWARF debug information and store it in specific sections in an executable, assuming it is compiled with debug information enabled. Debug information may sometimes live elsewhere — see. Some differences between standards can result in some hairy bugs in debuggers themselves, as we will see in a future post.
These represent elements of the source program — types, variables, functions, compilation units, etc. Each DIE has an identifying tag, which defines the type of the entry e. DIEs are organized into a graph. The portion of the graph representing ownership e.
We can see this graph in the readelf output above. Each increase in depth indicates the aforementioned parent-child ownership relationship.
To save space, DIEs are stored by writing just the values of their attributes; the identifiers for each individual attribute are not written. Instead, the entire DIE is preceded by an abbreviation code; these codes are defined as attribute sets with identifying information stored in a separate debug section of the executable file. Abbreviation codes are shared amongst DIEs. This website uses cookies so that we can provide you with the best user experience possible.
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If you disable this cookie, we will not be able to save your preferences. This means that every time you visit this website you will need to enable or disable cookies again. Previous Next. Any sufficiently advanced technology is indistinguishable from magic. How do they work? What is a debugger? How do we, the debugger, extract this information? Attach and stop Before we can extract any data from the process, we need to pause its execution.
To stop a process, we use the monolithic ptrace syscall. The main process thread is stopped first.
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