Bug 16874 - Setting a breakpoint on function in anonymous namespace is unnecessarily awkward
Summary: Setting a breakpoint on function in anonymous namespace is unnecessarily awkward
Alias: None
Product: gdb
Classification: Unclassified
Component: c++ (show other bugs)
Version: 7.7
: P2 critical
Target Milestone: ---
Assignee: patrick
Depends on:
Reported: 2014-04-25 08:06 UTC by Richard Biener
Modified: 2017-07-19 16:03 UTC (History)
7 users (show)

See Also:
Last reconfirmed:

Candidate patch (1.33 KB, patch)
2014-11-26 04:32 UTC, patrick
Details | Diff
updated patch (1.27 KB, patch)
2015-08-13 06:54 UTC, Markus Trippelsdorf
Details | Diff

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Description Richard Biener 2014-04-25 08:06:33 UTC
namespace { int foo (void) { return 0; } } int main() { return foo (); }

> gdb ./a.out
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warning: /etc/gdbinit.d/gdb-heap.py: No such file or directory
Reading symbols from ./a.out...done.
(gdb) b foo
Function "foo" not defined.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) start
Temporary breakpoint 1 at 0x4005ec: file t.C, line 1.
Starting program: /tmp/a.out 

Temporary breakpoint 1, main () at t.C:1
1       namespace { int foo (void) { return 0; } } int main() { return foo (); }
(gdb) b foo
Function "foo" not defined.
Make breakpoint pending on future shared library load? (y or [n]) n

so it doesn't work, not from toplevel context before starting the program
nor from a context within the TU that contains the anonymous namespace.

Breaks debugging of GCC big times - try to break on cfgexpand.c:pass_expand::execute - we want to be able to do that from a
context that is not necessarily local to that TU.
Comment 1 Richard Biener 2014-04-25 09:06:20 UTC

(gdb) b '(anonymous namespace)::foo'
Breakpoint 1 at 0x4005e1: file t.C, line 1.

this can't be really the desired way to do this ...

There is a DW_TAG_imported_module of the anon namespace in the TU context
so at least there referencing 'foo' needs to work.
Comment 2 Richard Biener 2014-04-25 09:10:48 UTC
And funnily

(gdb) b '(anonymous namespace)::pass_expand::<tab>

auto-"completes" to '(anonymous namespace)::

how useful.  All auto-completion seems to strip everything up to the
first ::.  Bah.
Comment 3 Dave Malcolm 2014-07-29 21:04:37 UTC
FWIW I've written a custom breakpoint-setting command using gdb's Python interface to try to workaround this from the gcc side:
Comment 4 Dave Malcolm 2014-08-06 14:12:32 UTC
Renaming.  It *is* possible to set a breakpoint on a function in an anonymous namespace, it's just way too tricky to do it right now, given that
(A) you have to type the full qualified name, and
(B) tab-completion is broken

and these make it sufficiently difficult that it's effectively not possible for a non-expert user.

So please can (A) and (B) be fixed.
Comment 5 patrick 2014-09-19 14:55:39 UTC
You can do

gdb> break *foo

or if foo is inside an anonymous namespace of another TU, you can do

gdb> break *'foo.c'::foo

Is this functionality sufficient for your use case?
Comment 6 Markus Trippelsdorf 2014-09-19 15:22:47 UTC
(In reply to patrick from comment #5)
> You can do
> gdb> break *foo
No symbol "foo" in current context.

markus@x4 ~ % gdb -v
GNU gdb (GDB)
Comment 7 patrick 2014-09-19 15:26:47 UTC
(In reply to Markus Trippelsdorf from comment #6)
> (In reply to patrick from comment #5)
> > You can do
> > 
> > gdb> break *foo
> No symbol "foo" in current context.
> markus@x4 ~ % gdb -v
> GNU gdb (GDB)

The program has to be running for "break *" to work, it seems.


gdb> break main
gdb> run
gdb> break *foo
Comment 8 Richard Biener 2014-09-22 07:51:11 UTC
The workaround works to some extent, but tab-completion is still broken
and for

namespace { int foo (void) { return 0; } }
int bar(void);
int main() { return foo () + bar (); }

namespace { int foo2 (void) { return 1; } }
int bar () { return foo2 (); }

it doesn't allow me to set a breakpoint on t2.C::foo2 until I enter bar().
That is, using 'b *<name>' seems to do symbol lookup from the current
scope only.

For the GCC case the workaround doesn't work in practice for this reason.

b '(anonymous namespace)::foo2'

works from any context (but is quite awkward due to tab-completion being broken).
Comment 9 patrick 2014-11-26 04:32:52 UTC
Created attachment 7974 [details]
Candidate patch

Here is a candidate patch that fixes this issue by allowing the user to omit the "(anonymous namespace)::" prefix when referencing symbols that are defined inside an anonymous namespace.  Tab completion is fixed accordingly, e.g. by allowing one to tab-complete "pass_exp" into "pass_expand" even though pass_expand is actually (anonymous namespace)::pass_expand.  This should work in breakpoint and expression context.

So the patch enables you to do

(gdb) break pass_exp<TAB>::execu<TAB>

to break on (anonymous namespace)::pass_expand::execute.
Comment 10 Markus Trippelsdorf 2015-08-13 06:54:23 UTC
Created attachment 8515 [details]
updated patch

I've updated Patrick's patch for gdb trunk.

It works fine:
Example from comment0:

 % gdb ./a.out
Reading symbols from ./a.out...done.
(gdb) b fo<tab> 
Breakpoint 1 at 0x40058a: file anon_namesp.cpp, line 1.
(gdb) run
Starting program: /home/markus/a.out 
Breakpoint 1, (anonymous namespace)::foo () at anon_namesp.cpp:1

Example from comment8:

Reading symbols from ./a.out...done.
(gdb) b fo<tab><tab>
foo()   foo2()  
(gdb) b foo2() 
Breakpoint 1 at 0x4005b3: file t2.cpp, line 1.
(gdb) run
Starting program: /home/markus/a.out 
Breakpoint 1, (anonymous namespace)::foo2 () at t2.cpp:1
1       namespace { int foo2 (void) { return 1; } }

Any chance to move this issue forward?
Comment 11 patrick 2015-08-26 01:19:04 UTC
I will try to.
Comment 12 Andrew Pinski 2016-12-13 02:49:51 UTC
I just ran into this today too while trying debug a new pass I was writing for GCC which uses anonymous namespaces :).
Comment 13 Martin Jambor 2017-06-20 16:45:33 UTC
...and so did I.
Comment 14 Pedro Alves 2017-06-21 08:46:42 UTC
This series will help with this:

 [PATCH 00/40] C++ debugging improvements: breakpoints, TAB completion, more

You can find it in the users/palves/cxx-breakpoint-improvements branch meanwhile, if you want to give it a try.