compilation_debugging.md

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Compilation flags and debugging

Today:

• What other compilation flags exist
• What compilation flags to use
• How to debug code

📺 Watch the related YouTube video!

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Special symbols used in slides

• 🎨 - Style recommendation
• 🎓 - Software design recommendation
• 😱 - Not a good practice! Avoid in real life!
• ✅ - Good practice!
• ❌ - Whatever is marked with this is wrong
• 🚨 - Alert! Important information!
• 💡 - Hint or a useful exercise
• 🔼1️⃣7️⃣ - Holds for this version of C++(here, 17) and above
• 🔽1️⃣1️⃣ - Holds for versions until this one C++(here, 11)

Style (🎨) and software design (🎓) recommendations mostly come from Google Style Sheet and the CppCoreGuidelines

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Compilation flags

• Lots of flags can be passed while compiling the code
• We have seen some already: -std=c++17, -o, etc.

  c++ -std=c++17 -o test test.cpp

• ✅ Enable most warnings, treat them as errors: -Wall, -Wextra, -Wpedantic, -Werror
• Other warnings possible too: -Wimplicit-fallthrough, -Wsign-conversion, etc.
• Optimization options:
  • -O0 - no optimization [default]
  • -O3 - full optimization [preferred]
• Keep debugging symbols: -g (usually used with -O0)

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Debugging your code

• Debugging will take more time than writing your code
• 💡 Read about how to think about debugging (really, do it!)
• When it comes to tools used for debugging, there are largely two different philosophies:
  • Using print statements [my preference]
  • Using a debugger
• Each has advantages and disadvantages
• No debugging methods is complete without you thinking about the probable cause of the problem
• You can read a discussion on Hacker News about it

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Using print statements

• Just add any printout statements to your code
• I usually use a form of a print statement shown below:

  cerr << __FILE__ << ":" << __LINE__ << ": " << value << endl;

• This will print the filename and a line where its called
• We can also print a value of interest at any point
• Usually requires to recompile only part of the program
• Typical workflow:
  • Observe the behavior
  • Form a hypothesis of what is wrong
  • Try a fix and repeat until problem is solved
• This workflow forces us to understand the problem before rushing into trying a solution

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Using a debugger

• Allows stopping a program at a point and looking around
• Requires debug symbols (-g flag), needs full recompilation
• Might be confusing with optimizations enabled
• Best program to use is probably lldb or gdb
• Insanely popular and powerful (and already installed 😉)
• Allows to print the backtrace
• No built-in gui 😢
• There are some tools built on top of GDB to fix this:
  • gdbgui is a Python tool that provides a web GUI for GDB
  • VSCode C++ extension provides a GUI for the debugger
• I rarely use a debugger and when I do I use lldb/gdb

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Exercise: debug a simple program!

❌ Beware it has an error!

#include <iostream>
#include <vector>
int main() {
  std::vector<int> numbers{1, 2, 3};
  for (auto i = numbers.size() - 1UL; i >= 0; --i) {
    std::cout << numbers[i] << std::endl;
  }
  return 0;
}

Once we run the program it crashes at some point:

[1]    74786 segmentation fault  ./program

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Using print statements

#include <iostream>
#include <vector>
int main() {
  std::vector<int> numbers{1, 2, 3};
  int count{};  // <- Count used for debugging
  for (auto i = numbers.size() - 1UL; i >= 0; --i) {
    std::cerr << "i = " << i << std::endl;  // <- Debug print
    std::cout << numbers[i] << std::endl;
    if (count++ > 10) {break;}  // <- Early exit
  }
  return 0;
}

• We can see smth like this on stderr:

  i = 2
  i = 1
  i = 0
  i = 18446744073709551615
  

• Now we can guess what happens, here is a hint 😉

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Using lldb (or gdb)

• Recompile the code with -g -O0 flags
• Start the code in the debugger:

  λ › lldb ./program
  (lldb) target create "./program"
  Current executable set to '/private/tmp/program' (arm64).
  (lldb) r

• When it fails, lldb shows us where it fails
• To find out why we will add a breakpoint

  (lldb) breakpoint set --file program.cpp --line 6
  

• Restart by typing r, the execution will stop at breakpoint
• We now add a watch on the variable i

  (lldb) watch set var i
  

• Repeatedly enter c and press ⏎ until the issue is found
• Full tutorial directly from lldb

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