Is qDebug() thread-safe?

Question

Is qDebug() thread-safe? By thread-safe I don't just mean not-crashing, but also if I call qDebug() from different threads, is it possible for the output to become mixed-up? I tested it with this code, and it doesn't appear to be so, however, I couldn't find anywhere in the documentation where they talk about this.

This is my test code:

#include <QtConcurrent> #include <QApplication> void print_a() {     for (int ii = 0; ii < 10000; ii++) {         qDebug("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");     } } void print_b() {     for (int ii = 0; ii < 10000; ii++) {         qDebug("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");     } } int main(int argc, char *argv[]) {     QApplication a(argc, argv);     QtConcurrent::run(print_a);     QtConcurrent::run(print_b);     return a.exec(); } 

There were no 'a' and 'b' mixed in the same line anywhere, but I'm still not sure if it's 100% thread safe...

Answer 1

Following are my answer and comments:

1. 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, ...).

2. 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.

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

4. 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.

5. 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.