gdb catch throw, this thread

Question

My binary (generated from C++) has two threads. When I care about exceptions, I care about exceptions thrown in one of them (the worker) but not the other.

Is there a way to tell gdb only to pay attention to one of the threads when using catch throw ? The gdb manual (texinfo document) and googling suggest to me that this isn't possible, although I think I could request catching a specific type of exception, which hopefully only one of the threads would throw, using catch throw REGEXP .

Answer 1

Is there a way to tell gdb only to pay attention to one of the threads

The catch throw is really just a fancy way to set a breakpoint on __cxxabiv1::__cxa_throw (or similar), and you can make a breakpoint conditional on thread number, achieving the equivalent result.

Example:

#include <pthread.h>
#include <unistd.h>

void *fn(void *)
{
  while(true) {
    try {
      throw 1;
    } catch (...) {}
    sleep(1);
  }
  return nullptr;
}

int main() {
  pthread_t tid;
  pthread_create(&tid, nullptr, fn, nullptr);
  fn(nullptr);
  return 0;
}

g++ -g -pthread t.cc

Using catch throw , you would get a breakpoint firing on the main and the second thread. But using break ... thread 2 you would only get the one breakpoint you care about:

gdb -q a.out
Reading symbols from a.out...

(gdb) start
Temporary breakpoint 1 at 0x11d6: file t.cc, line 17.
Starting program: /tmp/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Temporary breakpoint 1, main () at t.cc:17
17        pthread_create(&tid, nullptr, fn, nullptr);
(gdb) n
[New Thread 0x7ffff7a4c640 (LWP 1225199)]

  ## Note: GDB refuses to set thread-specific breakpoint until the thread actually exists.

18        fn(nullptr);
(gdb) break __cxxabiv1::__cxa_throw thread 2
Breakpoint 2 at 0x7ffff7e40370: file ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc, line 77.
(gdb) c
Continuing.
[Switching to Thread 0x7ffff7a4c640 (LWP 1225199)]

Thread 2 "a.out" hit Breakpoint 2, __cxxabiv1::__cxa_throw (obj=0x7ffff0000be0, tinfo=0x555555557dc8 <typeinfo for int@CXXABI_1.3>, dest=0x0) at ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc:77
77      ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc: No such file or directory.
(gdb) bt
#0  __cxxabiv1::__cxa_throw (obj=0x7ffff0000be0, tinfo=0x555555557dc8 <typeinfo for int@CXXABI_1.3>, dest=0x0) at ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc:77
#1  0x00005555555551b5 in fn () at t.cc:8
#2  0x00007ffff7d7eeae in start_thread (arg=0x7ffff7a4c640) at pthread_create.c:463
#3  0x00007ffff7caea5f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(gdb) c
Continuing.

Thread 2 "a.out" hit Breakpoint 2, __cxxabiv1::__cxa_throw (obj=0x7ffff0000be0, tinfo=0x555555557dc8 <typeinfo for int@CXXABI_1.3>, dest=0x0) at ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc:77
77      in ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc
(gdb) bt
#0  __cxxabiv1::__cxa_throw (obj=0x7ffff0000be0, tinfo=0x555555557dc8 <typeinfo for int@CXXABI_1.3>, dest=0x0) at ../../../../src/libstdc++-v3/libsupc++/eh_throw.cc:77
#1  0x00005555555551b5 in fn () at t.cc:8
#2  0x00007ffff7d7eeae in start_thread (arg=0x7ffff7a4c640) at pthread_create.c:463
#3  0x00007ffff7caea5f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Voilà -- thread-specific catch throw equivalent.