How to build to debug with gdb?

Hello, to debug some issues I built and installed git via

$ make prefix=/usr/local DEVELOPER=1 CFLAGS="-O0 -g" $ sudo make install $ git --version # git version 2.23.0.40.g4d8aada92f

But it seems there's still some optimization going on that prevents gdb from working correctly.

For example

(gdb) b builtin/config.c:752 Breakpoint 1 at 0x43942: file builtin/config.c, line 752. (gdb) r config --global --edit Starting program: /usr/local/bin/git config --global --edit [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libth read_db.so.1".

Breakpoint 1, cmd_config (argc=0, argv=<optimized out>,
    prefix=<optimized out>) at builtin/config.c:753
    753                             if (fd >= 0) {
    (gdb) p fd
    $1 = <optimized out>

 What am I missing?

 -Giuseppe

On Tue, Aug 27, 2019 at 1:27 PM Giuseppe Crino' <giuscri@gmail.com> wrote:

Hmm, could it be perhaps that CFLAGS is being overwritten? To debug Git with GDB I always add this to my config.mak file:

CFLAGS += -g3 -O0 LDFLAGS += -g3 -O0

And then just compile with `make`. Maybe give it a try?

On August 27, 2019 1:56 PM, Matheus Tavares Bernardino wrote:

Given that you are 40 commits in from 2.23.0, is it possible that you have compiled a few objects, like main, using the old CFLAGS? Gdb is notorious for not showing symbols if main does not have it. Have you done a git clean -dxf before building with the new CFLAGS?

Just a thought.

Randall

Am 27.08.19 um 18:27 schrieb Giuseppe Crino':

That is because the command sequence above does not do what you think it does. Didn't you notice that everything was recompiled during `sudo make install`?

You must run

  sudo make prefix=/usr/local DEVELOPER=1 CFLAGS="-O0 -g" install

-- Hannes

On Tue, Aug 27, 2019 at 08:36:40PM +0200, SZEDER Gábor wrote:

Yes, now it works thanks!

On Tue, Aug 27, 2019 at 08:34:23PM +0200, Johannes Sixt wrote:

Shouldn't be this documented somewhere?

To my knowledge `make install` is expected to copy artifacts of the build under prefix. It's unusual to me that `install` has the `all` target as prerequisite.

On Wed, Aug 28, 2019 at 08:15:57AM +0000, Giuseppe Crino' wrote:

Well, perhaps. But would users actually read it? Dunno.

Case in point: the first few lines of 'INSTALL' already tell you to build and install Git with the following commands:

  $ make prefix=/usr all doc info ;# as yourself
  # make prefix=/usr install install-doc install-html install-info ;# as root

Note the same 'prefix=...' in both commands, yet you omitted it when you ran 'make install' ;)

I don't think it's unusual at all that the 'install' target depends on 'all'. A quick and incomprehensive survey of Makefiles from a few open source projects that I happen to have lying around seems to confirm this.

What might be unusual (but is rather convenient for developers) is that Git's build process checks the build flags, and rebuilds the whole thing after the flags changed.

On Wed, Aug 28, 2019 at 12:32:17PM +0200, SZEDER Gábor wrote:

Maybe I'm biased by my incompetence with Makefile's but since rebuilding the project with `install` but different flags from the first make is unusual this is worth mentioning to developers.

On Wed, Aug 28, 2019 at 11:00:46AM +0000, Giuseppe Crino' wrote:

Definitely *not* sarcastic

On Tue, Aug 27, 2019 at 04:27:25PM +0000, Giuseppe Crino' wrote:

What do the output of these two command look like? After the first command build Git with the custom prefix and CFLAGS, the second command is supposed to start with printing "* new build flags" and then to build Git again with the default prefix and CFLAGS.

Just a couple hundred lines worth of build output :)

Try using the same build flags for the install, i.e.:

  make prefix=/usr/local DEVELOPER=1 CFLAGS="-O0 -g" install