Programming Forums > Application Development > C++

Polyphemus_ · Jan 26th, 2006, 10:21 AM

Hi,

I'm currently working on a program with multiple threads. All of the threads print to the screen, so the problem is that the messages are often mixed. Creating the function to print into this way:

(Toggle Plain Text)

int lock = 0;
void blah() {
 while(lock != 0);
 lock++;

 // code here

 lock--;
}

wouldn't work: when two threads are pending they will 50% of the time print at the same time.

My question is: what is the best way to make sure the messages won't be mixed? I was thinking of locking the thread in a safe way - but what is a safe way?

TIA,
Poly.

MBirchmeier · Jan 26th, 2006, 10:35 AM

I'm a big fan of mutexes

Simple, easy to use, OS level implementation.

(note: this might not be the best library, it's just what came in first in google for C++ mutex)

-MBirchmeier

Symptom · Jan 26th, 2006, 11:14 AM

Take a look at semaphores as well. It's a way to handle data control in multithreaded programs.

I don't know if you can use it in C++ but C has a data type that, access to it is atomic. It is defined by :

(Toggle Plain Text)

sig_atomic_t varname;

Atomic access means that reading/writing to the variable takes one single instruction, and therefore, your process cannot be interupted while in the middle of such a task. It can be used to access shared data or to implement manual locks for them.

If you are interested in the whole concept of multithreading and synchronizing processes, I know that the book of Tannenbaum about Operating Systems has some notes and algorithms which may help...

More info on sig_atomic_t can be found here.

DaWei · Jan 26th, 2006, 11:35 AM

Note that many things (such as signals) are not portable between OSes. Polyphemus may or not may be interested, but his user info indicates that he uses Win and Linux.

Polyphemus_ · Jan 26th, 2006, 11:54 AM

Quote:

Originally Posted by DaWei

Note that many things (such as signals) are not portable between OSes. Polyphemus may or not may be interested, but his user info indicates that he uses Win and Linux.

Yeah, I'm interested in creating it cross-platform, but some wrappers and #ifdefs will do

.

Thanks guys, I'll check them out.

Quote:

Originally Posted by Symptom

If you are interested in the whole concept of multithreading and synchronizing processes, I know that the book of Tannenbaum about Operating Systems has some notes and algorithms which may help...

Thanks - I forgot about that, and that book is 3 meters away from my computer

.

clsk · Jan 26th, 2006, 1:06 PM

Maybe what you're looking for are conditional variables. POSIX threads include this so it's postable as far as systems that are POSIX complaints.

You might want too look at:
http://users.actcom.co.il/~choo/lupg...thread_condvar

Maybe even read the whole article

. This is what I used when I first started learning threads.

Kaja Fumei · Jan 26th, 2006, 2:33 PM

Quote:

Originally Posted by Polyphemus_

Yeah, I'm interested in creating it cross-platform, but some wrappers and #ifdefs will do

.

I had to deal with the #ifdef and all that fun stuff a bunbch of time when I made multithreaded things. But I made one that supports a bunch of systems if you're interested. The file is here. It has threads, mutexes, semaphores, events (aka pthread condition variables), read-write locks, and a couple other useful things. Read the README to see how to build (it's easy). There's no real documentation yet, but if you want to use it and have questions about the API, feel free to let me know.

Quote:

Originally Posted by clsk

Maybe what you're looking for are conditional variables. POSIX threads include this so it's postable as far as systems that are POSIX complaints.

You might want too look at:
http://users.actcom.co.il/~choo/lup...#thread_condvar

Maybe even read the whole article . This is what I used when I first started learning threads.

Conditional variables (I call them events) would work if you signalled them one at a time. However, it would have to be pre-signalled once so the first one can get in, or else it'll be stuck waiting. I would just use a semaphore with an initial value of 1, but mutexes would work too.

Jan 26th, 2006, 10:21 AM	#1
Polyphemus_ Expert Programmer Join Date: Aug 2005 Location: Rotterdam, the Netherlands Posts: 942 Rep Power: 4	Locking a thread Hi, I'm currently working on a program with multiple threads. All of the threads print to the screen, so the problem is that the messages are often mixed. Creating the function to print into this way: (Toggle Plain Text) int lock = 0; void blah() { while(lock != 0); lock++; // code here lock--; } int lock = 0; void blah() { while(lock != 0); lock++; // code here lock--; } wouldn't work: when two threads are pending they will 50% of the time print at the same time. My question is: what is the best way to make sure the messages won't be mixed? I was thinking of locking the thread in a safe way - but what is a safe way? TIA, Poly.

Jan 26th, 2006, 11:14 AM	#3
Symptom Newbie Join Date: Sep 2005 Posts: 28 Rep Power: 0	Take a look at semaphores as well. It's a way to handle data control in multithreaded programs. I don't know if you can use it in C++ but C has a data type that, access to it is atomic. It is defined by : (Toggle Plain Text) sig_atomic_t varname; sig_atomic_t varname; Atomic access means that reading/writing to the variable takes one single instruction, and therefore, your process cannot be interupted while in the middle of such a task. It can be used to access shared data or to implement manual locks for them. If you are interested in the whole concept of multithreading and synchronizing processes, I know that the book of Tannenbaum about Operating Systems has some notes and algorithms which may help... More info on sig_atomic_t can be found here. __________________ The geeks shall inherit the earth.

Jan 26th, 2006, 11:35 AM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Note that many things (such as signals) are not portable between OSes. Polyphemus may or not may be interested, but his user info indicates that he uses Win and Linux. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Jan 26th, 2006, 10:35 AM	#2
MBirchmeier Hobbyist Programmer Join Date: Oct 2005 Posts: 211 Rep Power: 3	I'm a big fan of mutexes Simple, easy to use, OS level implementation. (note: this might not be the best library, it's just what came in first in google for C++ mutex) -MBirchmeier

Jan 26th, 2006, 1:06 PM	#6
clsk Newbie Join Date: Jan 2006 Posts: 13 Rep Power: 0	Maybe what you're looking for are conditional variables. POSIX threads include this so it's postable as far as systems that are POSIX complaints. You might want too look at: http://users.actcom.co.il/~choo/lupg...thread_condvar Maybe even read the whole article . This is what I used when I first started learning threads.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Programming Forums > Application Development > C++

Locking a thread