What happened in gdb between handle_sigint and async_request_quit?

Amker.Cheng amker.cheng@gmail.com
Thu Feb 5 12:31:00 GMT 2009

HI All:
It's my first message in this list. Please be generous if I break any
rules unintentionally.

I am studying gdb internals by debugging with native gdb-6.8 under
and trying to find out how gdb handles signals, for example, hitting
Ctrol+c during debugging.

In my view, gdb registers "handle_sigint" to SIGINT in function
"async_init_signals" at first,
then creates async_signal_handler with "proc=async_request_quit" for
"sigint_token", here comes the codes:

async_init_signals (void)
  signal (SIGINT, handle_sigint);
  sigint_token =
    create_async_signal_handler (async_request_quit, NULL);
  /*other codes*/

handle_sigint (int sig)
  signal (sig, handle_sigint);

  /* We could be running in a loop reading in symfiles or something so
     it may be quite a while before we get back to the event loop.  So
     set quit_flag to 1 here. Then if QUIT is called before we get to
     the event loop, we will unwind as expected.  */

  quit_flag = 1;

  /* If immediate_quit is set, we go ahead and process the SIGINT right
     away, even if we usually would defer this to the event loop. The
     assumption here is that it is safe to process ^C immediately if
     immediate_quit is set. If we didn't, SIGINT would be really
     processed only the next time through the event loop.  To get to
     that point, though, the command that we want to interrupt needs to
     finish first, which is unacceptable. */
  if (immediate_quit)
    async_request_quit (0);
    /* If immediate quit is not set, we process SIGINT the next time
       through the loop, which is fine. */
    mark_async_signal_handler_wrapper (sigint_token);
---------------cut here---------------

It's clear that in "handle_sigint" it just marks the corresponding
async_signal_handler of SIGINT,
result in the true SIGNAL HANDLER "async_request_quit" will be called
during next event loop.

I think gdb must stops the debuggee between calling to "handle_sigint"
and "async_request_quit",
the question is I cannot locate the codes doing this work.

Also, "handle_sigint" set "quit_flag" to 1, but I traced gdb and found
that it was set back to 0
before "async_request_quit" invoked. Who did this and when?

It seems to me that the asynchronous event loop is hard to trace, does
anybody have any tips?

Thanks in advance.
Best Regards.

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