Is qDebug() thread-safe?

c++, multithreading, qdebug, qt

Solution

Following are my answer and comments:

If the documentation of qDebug() does not mention whether it is thread-safe or not, we should assume it is not. The answer is likely platform-dependent: how qDebug() is implemented at the system level (Linux, Windows, ...).

Instead of the broader question of thread-safety, I think you were asking a more specific question like this: "Will the use of qDebug() in a multi-threaded application lead to interleaved output lines?" The answer is "Yes, occasionally." as demonstrated by the results produced by @dmcontador above. And the probability increases when the strings to be printed out are getting longer, as explained by @quetzalcoatl above.

The answer does not depend on whether you use qDebug("...") or qDebug() << "...", as both will finally call the system-level implementation code.

It is not easy for me to produce interleaved output lines using your original example code. So I have created a new example as shown below:

#include <QCoreApplication>
#include <QtConcurrent>

#define MAX_ITERS 10
#define MAX_LEN   10000

void print_a()
{
    QString a(MAX_LEN, 'a');

    for(int i = 0; i < MAX_ITERS; ++i) {
        qDebug().noquote() << a;
    }
}

void print_b()
{
    QString b(MAX_LEN, 'b');

    for(int i = 0; i < MAX_ITERS; ++i) {
        qDebug().noquote() << b;
    }
}

int main(int argc, char * argv[])
{
    QCoreApplication a(argc, argv);
    QtConcurrent::run(print_a);
    QtConcurrent::run(print_b);
    return 0;
}

The probability increases when you increase MAX_LEN.

- A follow-up question would be: "How to use qDebug() to produce non-interleaved output lines?" One solution would be to use QMutex on each and every qDebug() line. Note that I have not tried this solution which is not practical.

Problem

Original source

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