Trigger an interrupt when the value of a memory location is modified in FreeBSD/Linux

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Is it possible to generate an interrupt when the value of a variable or memory location get modified in FreeBSD or Linux environment using C program ?

In a C application there is an dynamically allocated array which is being used/modified from multiple locations. The application is pretty large and complex, it is difficult to trace all the places the array being used or modified from. The problem is in some condition/flow the array[2] element become 0 which is not expected as per this application. I can't run the application using gdb to debug this issue (because of some constraint). The only way to debug this issue is to modify the source code and run the binary where the issue is happening.

Is it possible to generate an interrupt when the arra[2] element is modified and print the backtrace to know which part of the codebase has modified it ?

Thanks!!!

interrupt-handling backtrace gdb freebsd linux
1 Answers

You want a data breakpoint, also called watchpoint; GDB provides the following commands:

  • watch for writes
  • rwatch for reads
  • awatch for both

You can ask GDB for a specific condition as well, so the following expression (or something similar) should work:

watch array[2] if array[2] == 0

You must run the expression in the scope of the variable, the easiest way is to just set a breakpoint in the line after the allocation, then set the watchpoint after the breakpoint triggers and resume execution.

OTOH, to implement such a debugging facility within the application is rather complex and hardware-specific (in case hardware support isn't available, software watchpoints require implementing an entire debugger), so I would recommend using liblldb (which is Apache-2.0 licensed IIRC), as it provides a lldb::SBWatchpoint class which you can leverage. The Python API is documented: https://lldb.llvm.org/python_api/lldb.SBWatchpoint.html.

The C++ API is similar, but there's a lot of boilerplate to write that I don't see documented anywhere, so the API is private; you'd have to look at LLDB's own source code.

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