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The test-case gdb.threads/signal-sigtrap.exp:
- installs a signal handler called sigtrap_handler for SIGTRAP,
- sets a breakpoint on sigtrap_handler, and
- expects the breakpoint to trigger after issuing "signal SIGTRAP".
Usually, that happens indeed:
...
(gdb) signal SIGTRAP^M
Continuing with signal SIGTRAP.^M
^M
Thread 1 "signal-sigtrap" hit Breakpoint 2, sigtrap_handler (sig=5)^M
28 }^M
(gdb) PASS: $exp: sigtrap thread 1: signal SIGTRAP reaches handler
...
Occasionally, I run into this failure on openSUSE Tumbleweed:
...
(gdb) signal SIGTRAP^M
Continuing with signal SIGTRAP.^M
^M
Thread 1 "signal-sigtrap" received signal SIGTRAP, Trace/breakpoint trap.^M
__pthread_create_2_1 () at pthread_create.c:843^M
(gdb) FAIL: $exp: sigtrap thread 1: signal SIGTRAP reaches handler
...
AFAIU, the problem is in the situation that is setup before issuing that
command, by running to a breakpoint in thread_function:
...
void *thread_function (void *arg) {
return NULL;
}
int main (void) {
pthread_t child_thread;
signal (SIGTRAP, sigtrap_handler);
pthread_create (&child_thread, NULL, thread_function, NULL);
pthread_join (child_thread, NULL);
return 0;
}
...
In the passing case, thread 2 is stopped in thread_function, and thread 1 is
stopped somewhere in pthread_join:
...
(gdb) info threads^M
Id Target Id Frame ^M
1 Thread ... (LWP ...) "signal-sigtrap" __futex_abstimed_wait_common64 ()
* 2 Thread ... (LWP ...) "signal-sigtrap" thread_function ()
...
In the failing case, thread 2 is stopped in thread_function, but thread 1 is
stopped somewhere in pthread_create:
...
(gdb) info threads^M
Id Target Id Frame ^M
1 Thread ... (LWP ...) "signal-sigtrap" __GI___clone3 ()
* 2 Thread ... (LWP ...) "signal-sigtrap" thread_function ()
...
What I think happens is that pthread_create blocks SIGTRAP at some point, and
if the "signal SIGTRAP" command is issued while that is the case, the signal
becomes pending and consequently there's no longer a guarantee that the signal
will be delivered to the inferior.
Instead the signal will be handled by gdb like this:
...
(gdb) info signals SIGTRAP
Signal Stop Print Pass to program Description
SIGTRAP Yes Yes No Trace/breakpoint trap
...
Fix this by adding a barrier that ensures that pthread_create is done before
we issue the "signal SIGTRAP" command.
Likewise in test-case gdb.threads/signal-command-handle-nopass.exp.
Using the fixed test-case, I tested my theory by explicitly blocking SIGTRAP:
...
+ sigset_t old_ss, new_ss;
+ sigemptyset (&new_ss);
+ sigaddset (&new_ss, SIGTRAP);
+ sigprocmask (SIG_BLOCK, &new_ss, &old_ss);
+
/* Make sure that pthread_create is done once the breakpoint on
thread_function triggers. */
pthread_barrier_wait (&barrier);
pthread_join (child_thread, NULL);
+ sigprocmask (SIG_SETMASK, &old_ss, NULL);
...
and managed to reproduce the same failure:
...
(gdb) signal SIGTRAP^M
Continuing with signal SIGTRAP.^M
[Thread 0x7ffff7c00700 (LWP 13254) exited]^M
^M
Thread 1 "signal-sigtrap" received signal SIGTRAP, Trace/breakpoint trap.^M
0x00007ffff7c80056 in __GI___sigprocmask () sigprocmask.c:39^M
(gdb) FAIL: $exp: sigtrap thread 1: signal SIGTRAP reaches handler
...
Tested on x86_64-linux.
PR testsuite/26867
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26867
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.