Sending signal to thread from annother non-process and logging stack not happening

Question

I am trying to send signal to a POSIX thread from another process (Not from the process that created that thread. What I did to send signal using kill(...)::

int trap_handle(pid_t child_waited )
 69 {
 70     printf("%s, new value: %d, child_waited=<%ld>\n", __func__,g_var_x, child_waited);
 71     int errno_ = -1;
 72     errno_ = kill(child_waited, SIGUSR1);
 73     //syscall(SYS_tgkill, -1, child_waited, SIGUSR1);
 74     //errno_ = pthread_kill(child_waited, SIGUSR1);
 75     if(0==errno_)
 76         printf("Signal sent to thread: %ld\n", child_waited);
 77     else
 78         printf("pthread_kill failed: error:%d", errno_);
 79 }

And in a thread that registered SIGUSR1:

230 void baz() {
231     g_var_x++;
232 }
233
234 void bak() { baz(); }
235 void bar() { bak(); }
236 void foo() { bar(); }
237
238 void stack_dump()
239 {
240     printf("******trap() entry ******\n");
241     void *array[100];
242     size_t size;
243     // get void*'s for all entries on the stack
244     size = backtrace(array, 100);
245
246     // print out all the frames to stderr
247 //    fprintf(stderr, "Error: signal %d:\n", sig);
248     backtrace_symbols_fd(array, size, STDERR_FILENO);
249     printf("*******trap() exit ******\n");
250 }
251
252 void* thread_proc_one(void *lParam)
253 {
254     printf("--Entry: thread_one debugee tid<%ld> \n", syscall(SYS_gettid));
255     g_arg_params.debugee_tid =  syscall(SYS_gettid);
256
257     struct sigaction trap_action;
258     //printf("Childprocess <tid> %d\n", syscall (SYS_gettid));
259     memset(&trap_action, 0, sizeof(trap_action));
260     sigaction(SIGUSR1, NULL, &trap_action);
261     trap_action.sa_sigaction = stack_dump;
262     trap_action.sa_flags = SA_SIGINFO | SA_RESTART | SA_NODEFER;
263     sigaction(SIGUSR1, &trap_action, NULL);
....

Now This is expected that it will backtrace the Thraed stack not the main process that invoked it. But is not happening. stack_dump is called, but instead of logging thread stack, it is logging its parents stack. Backtrace is showing stack of the process that created this thread_proc_one thread.

Anyone here faced this issue? Hope I am clear.

Answer 1

sigaction() installs a signal handler for the whole process.

From man sigaction (italics by me):

The sigaction() system call is used to change the action taken by a process on receipt of a specific signal.

Which of the process' threads handles it is left to the OS.

From man 7 signal:

The signal disposition is a per-process attribute: in a multithreaded application, the disposition of a particular signal is the same for all threads.

To make sure a certain signal is handled by a specific thread use pthread_sigmask() to mask out the signal for all threads but the one to handle it.

Again from man 7 signal:

Each thread in a process has an independent signal mask, which indicates the set of signals that the thread is currently blocking. A thread can manipulate its signal mask using pthread_sigmask(3).

So this, for example, could be done by callîng pthread_sigmask() in the main thread masking out the signal in question prior to creating any thread, and then inside the thread to handle the signal call pthread_sigmask() again to unmask the signal to be handled.