Substituting gdb debug paths

Gdb is a debugger with many, many, many⁽¹⁾ options. Some of them are rather obscure, others... less so.

While it's always had the ability to read source files and do useful stuff with them, recent versions of gdb have gained another interface (which you get by calling "gdb -tui") that makes it a bit easier to watch the source as you step through the code. Additionally (and I don't know how long that feature's been there), you can do some interesting things with source paths. Such as substituting a source path prefix by another prefix -- useful if you've moved sources around. This is accomplished by way of the 'set substitute-path' command:

(gdb) set substitute-path /home/buildd /home/wouter

Now whenever gdb is told by the debugging info to search for a file /home/buildd/foo/bar/baz.c, it will instead search for /home/wouter/foo/bar/baz.c. Very useful.

One problem with the above approach, however, is that you need to know what the original path is—the /home/buildd in our example. If you've built the binary yourself, you just need to remember where you built it, and you're all set. But what if you haven't built the binary yourself? I used to use strings for that, and look for any likely candidates, but that isn't very reliable and requires a bit of hunting through many many lines that just happen to look like strings to what really is a fairly naive program. Additionally, it doesn't work when the debugging symbols are in a separate file (as implemented by dh_strip), amongst others.

But no worries! There's a simple solution. Rather than using strings, use readelf -w instead, and grep for DW_AT_comp_dir, which will give you the correct source directory to substitute.

I'm happily single-stepping through a library now. If that isn't nice.

¹ no really, many