std::condit...::notify_all(3)

🎀 NAME 🎀

Okay bestie, welcome to the std::condit...::notify_all(3) page from the C++ Standard Libary manual. The header truncates the name like it’s texting with a character limit, but the full main-character name is std::condition_variable_any::notify_all. Yes, that’s the whole name: std::condition_variable_any::notify_all. Periodt.

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✨ Synopsis ✨

void notify_all() noexcept;  (since C++11)

Unblocks all threads currently waiting for *this.

It’s giving “everybody up, the notification is here” energy, and it’s noexcept since C++11. Slay.

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💅 Parameters 💅

(none)

Zero parameters. Not a single one. She takes no notes and no drama.

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🌸 Return value 🌸

(none)

No return value, bestie. It does the thing and exits the group chat.

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🦋 Notes 🦋

Okay so the notes are lowkey important, so stay with me.

The effects of notify_one()/notify_all() and each of the three atomic parts of wait()/wait_for()/wait_until() — that’s unlock+wait, wakeup, and lock — take place in a single total order that can be viewed as modification order of an atomic variable. The order is specific to this individual condition variable. This makes it impossible for notify_one() to, for example, be delayed and unblock a thread that started waiting just after the call to notify_one() was made. No cap, that ordering detail is the whole vibe.

The notifying thread does not need to hold the lock on the same mutex as the one held by the waiting thread(s). Doing so may be a pessimization, since the notified thread would immediately block again, waiting for the notifying thread to release the lock — though some implementations recognize the pattern and do not attempt to wake up the thread that is notified under lock. So yeah: holding the lock while notifying can be kinda extra, but some implementations are hip to the pattern. 💅

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🔥 Example 🔥

Example time, and yes the comment says what it says:

// Run this code

#include <chrono>
#include <condition_variable>
#include <iostream>
#include <thread>

std::condition_variable_any cv;
std::mutex cv_m; // This mutex is used for three purposes:
                 // 1) to synchronize accesses to i
                 // 2) to synchronize accesses to std::cerr
                 // 3) for the condition variable cv
int i = 0;

void waits()
{
    std::unique_lock<std::mutex> lk(cv_m);
    std::cerr << "Waiting... \n";
    cv.wait(lk, []{ return i == 1; });
    std::cerr << "...finished waiting. i == 1\n";
}

void signals()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    {
        std::lock_guard<std::mutex> lk(cv_m);
        std::cerr << "Notifying...\n";
    }
    cv.notify_all();

    std::this_thread::sleep_for(std::chrono::seconds(1));

    {
        std::lock_guard<std::mutex> lk(cv_m);
        i = 1;
        std::cerr << "Notifying again...\n";
    }
    cv.notify_all();
}

int main()
{
    std::thread t1(waits), t2(waits), t3(waits), t4(signals);
    t1.join();
    t2.join();
    t3.join();
    t4.join();
}

🍓 Possible output 🍓

Waiting...
Waiting...
Waiting...
Notifying...
Notifying again...
...finished waiting. i == 1
...finished waiting. i == 1
...finished waiting. i == 1

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👑 See also 👑

• notify_one — notifies one waiting thread (public member function).
• C documentation for cnd_broadcast.

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💀 Hidden category 💀

• Pages with unreviewed LWG DR marker

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