How to output to the currently visible terminal

Question

I want to output some text as response to the signal SIGUSR1

I.e. a user sends kill -USR1 <pid> to a process started in background or in another terminal.

I would like to get some output in the terminal where kill was invoked.

How can I achieve this?

Answer 1

The comment of @BoBTFish is working. A possible pseudo C++ implementation:

// somewhere in code:
std::memset(&sa, 0, sizeof(struct sigaction));
sa.sa_sigaction = sh_dump;
sa.sa_flags = static_cast<int>(SA_SIGINFO); // <- important, else you'll get an invalid siginfo_t pointer
sigaction(SIGUSR1, &sa, NULL);

void sh_dump(int, siginfo_t *info, void *) {

    if(info) {

        // do some locking of your choice

        char *p = NULL;
        char sp[PATH_MAX] = "";

        std::snprintf(sp, PATH_MAX, "/proc/%d/stat", info->si_pid);

        int tty_nr = 0;
        FILE *spf;

        if((spf = std::fopen(sp, "r"))) {

            int iDummy;
            char cDummy, *sDummy;

            // proc(5)
            if(std::fscanf(spf, "%d %ms %c %d %d %d %d", &iDummy, &sDummy, &cDummy, &iDummy, &iDummy, &iDummy, &tty_nr)) {}

            free(sDummy);
            std::fclose(spf);
        }

        // see http://goo.gl/L0pGK1 for an implementation
        if(!(p = ttynameCheckDir(static_cast<dev_t>(tty_nr), "/dev/pts"))) {
            p = ttynameCheckDir(static_cast<dev_t>(tty_nr), "/dev");
        }

        std::ofstream out(p ? p : "/dev/null");

        free(p);

        if(out.is_open()) out << "HELLO" << std::endl;

        // do some unlocking of your choice
    }
}

prints HELLO on the terminal invoking kill -USR1 <pid>

EDIT: Using /proc/#/stat (Linux only)