Issue with Unix signals and QTestStream readline

[ulix]

Hello I'm trying to implement a signal handler mechanism into my Qt Console application to quit it when a unix signal is catched
So I used the standard library c function signal in signal.h registering a handler for SIGTERM signal.
Then I start doing some stuff and in particular I'm reading the stdin using a QTextStream(stdin).readline() method.
The point is that when I'm waiting for the input on the readline the application does not catch the SIGTERM signal
Can someone explain me why?

thank you in adavance

[wasteschirm]

Your application doesn't catch the `SIGTERM` signal while blocked on `QTextStream(stdin).readLine()` because `readLine()` is a blocking call, preventing the Qt event loop or signal handler from running.

Solution: Use `QSocketNotifier` to monitor `stdin` asynchronously instead of blocking on `readLine()`, or configure `sigaction` with `SA_RESTART` unset to allow signals to interrupt the blocking call.
phrazle

[Selena Britt]

The issue you're encountering arises because when you use QTextStream(stdin).readline() in your Qt console application, it internally calls the standard C function fgets() to read input. While this method is blocking (i.e., it waits for user input), it can also cause the signal handler to not be triggered when the signal is sent (in your case, SIGTERM). This behavior is due to how signals are handled in Unix-like systems and how they interact with blocking system calls like fgets() or QTextStream::readline() Drive Mad Game

[foniwe]

Hello

The issue arises because QTextStream(stdin).readLine() is a blocking call, and signals like SIGTERM are not handled immediately during such operations. The signal handler will execute only after the blocking call completes.

Solution:
Use non-blocking I/O with QSocketNotifier to read from stdin asynchronously, allowing signals to be handled promptly:

#include <QCoreApplication>
#include <QSocketNotifier>
#include <QTextStream>
#include <signal.h>
 
QCoreApplication* app = nullptr;
 
void signalHandler(int) {
    if (app) app->quit();
}
 
int main(int argc, char* argv[]) {
    QCoreApplication application(argc, argv);
    app = &application;
 
    signal(SIGTERM, signalHandler); // Register signal handler
 
    QSocketNotifier notifier(STDIN_FILENO, QSocketNotifier::Read);
    QObject::connect(&notifier, &QSocketNotifier::activated, [&]() {
        QTextStream input(stdin);
        QString line = input.readLine();
        if (!line.isEmpty()) qDebug() << "Input:" << line;
    });
 
    return application.exec();
}

#include <QCoreApplication>
#include <QSocketNotifier>
#include <QTextStream>
#include <signal.h>

QCoreApplication* app = nullptr;

void signalHandler(int) {
    if (app) app->quit();
}

int main(int argc, char* argv[]) {
    QCoreApplication application(argc, argv);
    app = &application;

    signal(SIGTERM, signalHandler); // Register signal handler

    QSocketNotifier notifier(STDIN_FILENO, QSocketNotifier::Read);
    QObject::connect(&notifier, &QSocketNotifier::activated, [&]() {
        QTextStream input(stdin);
        QString line = input.readLine();
        if (!line.isEmpty()) qDebug() << "Input:" << line;
    });

    return application.exec();
}

To copy to clipboard, switch view to plain text mode 

This ensures signal handling works even while waiting for input.