[QT] for multithreaded programs, do not use the printf() function to print out

I happened to see in the book , I feel this is a hidden knowledge point , It's easy to get into it by mistake , But no reason , It is recorded here , Help beginners to put into the pit .

according to 《Qt5 Development and examples 》（ Electronic industry press ,ISBN：978-7-121-36157-9） Book No 299 page , speak of ：

The thread will call printf()  Function to hold a control  I/O  Lock of (lock), Multiple threads call printf()  Function will block console output in some cases , While using  Qt  Provided  qDebug()  Function as console output will not cause the above problem .

It means If each thread uses printf() Normalization of functions , In fact, each thread of the whole program is Use the same I/O process , So it will cause resource preemption and process mutual exclusion . While using qDebug() Will not be .

take printf(yourVar) function Replace with qDebug()<< yourVar , Also remember to add the header file reference ：include <QtDebug>

QString hello = "hello world";
printf("%s", hello);

Change it to ： 

#include <QtDebug>

QString hello = "hello world";

qDebug()<<hello;

in addition ：

《Qt5 Development and examples 》（ Electronic industry press ,ISBN：978-7-121-36157-9） The book is very good , Suitable for manual translation , If you want to use it Qt, This book is very suitable for reference .