[Interest] Write from different thread

Thiago Macieira thiago.macieira at intel.com
Thu Aug 23 14:28:11 CEST 2012

Marc replied about functions, but let's talk about classes and objects. That 
is, let's talk about calling different member functions of a particular class.

On quinta-feira, 23 de agosto de 2012 12.00.43, Giuseppe D'Angelo wrote:
> 1) non-reentrant: safe to be used by only one thread

Non-reentrant means this function or class can only be called from one thread 
at a time, regardless of which object it's operating on. 

An even stricter requirement is that of GUI classes: not only are they non-
reentrant, the only thread where they can be called is the GUI thread.

> 2) reentrant: safe to be used by multiple threads, provided that
> access is serialized ("one at a time")

That's not what it means. It means that it's safe to call these member 
functions from different threads, provided they operate on separate objects.

> 3) thread-safe: safe to be used by multiple threads, even at the same time

Correct: these functions in this particular class can be called on 
simultaneously from multiple threads, even on the same object, provided that's 
all you're doing to this object.

By definition, all thread-safe functions are reentrant.

There are certain classes in Qt that are hybrid between reentrant and thread-
safe: the implicitly-shared container classes. By construction, the const 
functions are thread-safe and the non-const ones are only reentrant. 

But note the "provided that's all you're doing to this object" part: if you're 
calling a non-thread-safe reentrant functions on that object, you can't use 
the thread-safe functions at the same time.

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
     Intel Sweden AB - Registration Number: 556189-6027
     Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
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