Is qDebug() thread-safe?

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.

The docs say If a function is not marked as thread-safe or reentrant, it should not be used from different threads. In case of qDebug() it says: Note: This function is thread-safe.

(this answer was updated... the docs didn't state that the function was thread-safe before.)

I'm afraid that it is not thread-safe. Also, I tried your code and had mixed output.

aaaaaaaaaaaabbbbbaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbabbbbbbbbbbbbbbbbbb

I had the same luck with qDebug() << "..."

Tested in Qt5.2.1 with mingw48_32 compiler.