When you send SIGUSR1 signal (say the signal handler has been set in advance) to a program while it is executing sleep(100), the signal is caught correctly but sleep(100) is terminated just after the catch. This may mean sending a signal can forcedly terminate the internal some function.

For example, in a scientific calculation program, I would like to catch SIGUSR1 and print the progress. But what if I happen to send the signal while statements like has_error_occured = true or should_break_this_roop = true are in execution? I think this may cause unexpected behavior.

How can I use SIGUSR1 (and SIGUSR2) safely? It is known that the shell command dd print the progress when it catches SIGUSR1. Why is this safe?

Sample program (I executed kill -SIGUSR1 xxxxx):

#include <iostream>
#include <csignal>
#include <unistd.h>

void my_handler(int signal) {
    ; //some instructions

void just_sleep() {
    std::cout << "sleep() starts.\n";
    sleep(100); //not wait for 100s if a signal caught
    std::cout << "sleep() ends.\n"; //executed even if a signal caught

int main() {

    signal(SIGUSR1, my_handler);


closed as off-topic by Kusalananda, Thomas Dickey, RalfFriedl, Romeo Ninov, muru Nov 13 '18 at 1:34

  • This question does not appear to be about Unix or Linux within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.


You should set your signal handler using sigaction(2) instead of signal(2), and set SA_RESTART in sa_flags if you don't want it to interrupt a blocking system call.

struct sigaction sa;
sa.sa_handler = your_handler;
sa.sa_flags = SA_RESTART;
sigaction(SIGUSR1, &sa, 0);

Or, even better than that, just handle the interrupt yourself.

If nanosleep(), etc. has returned -1, check if errno == EINTR, and print the progress and then redo the call (sleep() is just a wrapper for nanosleep() on linux). You will have to do this anyway if your program grows to something more complex — there's not much you can do safely from a signal handler — see the signal-safety(7) man page on Linux.

  • Thank you. This is exactly what I wanted. I've read all of the links you gave me and I'll check some additional references. – ynn Nov 12 '18 at 20:30

Not the answer you're looking for? Browse other questions tagged or ask your own question.