How to Create Executable Files and Produce Debug Information

In this short article I will show you how to create an executable file as well as produce debug information but in a completely separate file.

There has been a number of times when I wanted to understand what went wrong with a program but debug information was not available. Without debug information it is nearly impossible to make sense of what happened. This holds true especially for larger programs.

Today we will work with something super simple:

int factorial(int n) {
  if(n==1) {
    return 1;
  }
  else {
    return n * factorial(n-1);
  }
}

int main(int argc, char **argv) {
  int x = factorial(5);
  switch(x) {
    case 120:
      return 0;
    default:
      return -1;
  }

  return -1;
}

Save this into a file called

main.c

We can now use c compiler to produce an executable:

gcc -o fact main.c

This gives us a binary file, but unfortunatelly there's no debug information in it. If we run a program like this in production and for whatever reason need to debug this program to understand what's wrong we're out of luck. I understand that some of you may say - "hey, you should never debug programs in production" but there will be situations where that's the only way.

Ideally, programs should be so good by the time they go to production that they don't fail. In a future post, I'll talk more about how to do understand failures in production. For now, you will need to trust me that to truly understand failure of a system you will need to inspect that system.

Now let's compile this program with debug info:

gcc -g -Og -o fact-debug main

Now we can use

eu-readelf 

readelf

sed

readelf

debug_aranges

debug_info

debug_abbrev

debug_line

debug_str

If you try using

gdb

fact

Now let's extract debug information from fact-debug binary and put it in another file. That can be done using

obj-copy

eu-strip

objcopy --only-keep-debug fact-debug fact-debug-info
eu-strip -g fact-debug

Now you have the best of both worlds - an executable file that does not include debug information as well as access to debug information if the need arises.

As a teaser to my next article, I encourage you to check out

build-id

eu-readelf -n fact-debug

I'll show you how to put it to good use using the latest

gdb