[lldb-dev] SBProcess::Detach kills target

Pavel Labath via lldb-dev lldb-dev at lists.llvm.org
Wed Mar 30 02:22:33 PDT 2016

So I have made a small test program (which does nothing but spin in a
loop), and indeed it is the SIGHUP that kills it after detach. If the
test program blocks the signal, then it continues running even after
$ cat a.c
#include <unistd.h>
#include <signal.h>

int main() {
    signal(SIGHUP, SIG_IGN);
    for (;;) sleep(1);
$ cc a.c -g
$ ps -A | grep a.out
$ ~/ll/build/dbg/bin/lldb ./a.out
(lldb) target create "./a.out"
Current executable set to './a.out' (x86_64).
(lldb) b 6
Breakpoint 1: where = a.out`main + 19 at a.c:6, address = 0x0000000000400590
(lldb) r
Process 13416 launched: './a.out' (x86_64)
Process 13416 stopped
* thread #1: tid = 13416, 0x0000000000400590 a.out`main + 19 at a.c:6,
name = 'a.out', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400590 a.out`main + 19 at a.c:6
   4   int main() {
   5      signal(SIGHUP, SIG_IGN);
-> 6      for (;;) sleep(1);
   7   }
(lldb) detach
Process 13416 detached
(lldb) q
$ ps -A | grep a.out
 13416 ?        00:00:00 a.out

On 29 March 2016 at 18:38, Eugene Birukov <eugenebi at hotmail.com> wrote:
> I believe this is not SIGHUP on debugger exit. I am using my own C++ program
> that calls into LLDB API. So, this program is still alive after calling
> SBProcess::Detach() but the target dies. Also, the target intercepts SIGHUP
> to do cleanup before exiting. I put printf there, it was not hit.
The fact whether your program is alive irrelevant. What matters is
that by cleaning up the process structure, we will also close the
master end of the pty used for inferior communication, and this is
what causes the SIGHUP. For that reason you also cannot use a printf
to do diagnostics as the output has nowhere to go. Note that lldb's
behavior here will be different from gdb as gdb does not do stdio
redirection, and has the inferior share the pty with the debugger (in
which case your program will die when you close the terminal window).

> I tried interactive LLDB, the target is not there:
> Process 49145 stopped
> * thread #1: tid = 49145, ..., stop reason = signal SIGSTOP
>     frame #0: 0x00007ffff6a5bbed libc.so.6 at syscall-template.S:81
> (lldb) detach
> Process 49145 detached
> (lldb) q
> eugene at EUGENEBI-L1:~/tmp$ ps
>   PID TTY          TIME CMD
> 30714 pts/17   00:00:00 bash
> 49259 pts/17   00:00:00 ps
Note that the inferior will not show up here even if it exists, as ps
will only list the processes with the same tty, but at this point the
inferior process has no tty.

Good luck with your investigations.


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