WiringPi: question about multiThread program

[roycejp]

Hello,

I have made a C program under gcc using wiringPi, which is working well.

I want tp try multi threading to improve performance of certain part of my code.

I'm not clear about the piLock / piUnLock functions.

To quote from the wiringPi reference page,

Code: Select all

piLock (int keyNum) ;
piUnlock (int keyNum) ;

These allow you to synchronise variable updates from your main program to any threads running in your program. keyNum is a number from 0 to 3 and represents a “key”. When another process tries to lock the same key, it will be stalled until the first process has unlocked the same key.


So does this mean I can have only 4 (0 to 3) shared variables between my 'main' and all another threads ?
How to handle more shared variables ?

Can anyone shed some light ?

Thanks !

[gordon@drogon.net]

Hello,

I have made a C program under gcc using wiringPi, which is working well.

I want tp try multi threading to improve performance of certain part of my code.

I'm not clear about the piLock / piUnLock functions.

To quote from the wiringPi reference page,

Code: Select all

piLock (int keyNum) ;
piUnlock (int keyNum) ;

These allow you to synchronise variable updates from your main program to any threads running in your program. keyNum is a number from 0 to 3 and represents a “key”. When another process tries to lock the same key, it will be stalled until the first process has unlocked the same key.


So does this mean I can have only 4 (0 to 3) shared variables between my 'main' and all another threads ?
How to handle more shared variables ?

Can anyone shed some light ?

Thanks !

You can have millions - they're just there, like a lot of wiringPi as "helpers". To make life easy for you.

If you want more, just look at the source code, (piThread.c) or type: man pthread_mutex_lock

-Gordon

[roycejp]

Thank You.

So to increase the number of shared variables I edit this line:

Code: Select all

static pthread_mutex_t piMutexes [4] ;

and change the '4' to any value I want ?

BTW,

Code: Select all

man pthread_mutex_lock 

gives me:

Code: Select all

No manual entry for pthread_mutex_lock

[gordon@drogon.net]

Thank You.

So to increase the number of shared variables I edit this line:

Code: Select all

static pthread_mutex_t piMutexes [4] ;

and change the '4' to any value I want ?

Yes - however I'd suggest that if you're at that level, then you might want to look at using the functions directly.

BTW,

Code: Select all

man pthread_mutex_lock 

gives me:

Code: Select all

No manual entry for pthread_mutex_lock

I think the manual pages might not be installed by default. (as not everyone does C programming on the Pi) Try:

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sudo apt-get install glibc-doc

The whole package is called pthreads:

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man pthreads

Happy reading!

-Gordon

[roycejp]

Things are not going all that well ...

I'm getting this error (where 'webradio' is the name of my executable) :

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*** glibc detected *** ./webradio: double free or corruption (top): 0x00662228 ***

and it aborts.

I'm using 2 threads besides main(), and 2 mutexes in the application.

What does this mean ?

Thank You.

[gordon@drogon.net]

Things are not going all that well ...

I'm getting this error (where 'webradio' is the name of my executable) :

Code: Select all

*** glibc detected *** ./webradio: double free or corruption (top): 0x00662228 ***

and it aborts.

I'm using 2 threads besides main(), and 2 mutexes in the application.

What does this mean ?

Thank You.

It means you have a bug in your program - probably calling free() more than once on the same pointer. (or something is, if it's not you)

although wiringPi does use dynamic memory allocation (calloc), it never frees it up, so I'm fairly sure it's not wiringPi producing that error.

Compile it with the -g flag, enable core dumps, run it - get a core file, then run gdb on it and issue the 'where' command.

-Gordon

[roycejp]

I managed to track down the bug.
I was calling a shell command inside one thread using a file pointer which was also being used by the main thread.

I just created separate file pointer for the thread, and the problem went away.

Many thanks for your help !

I'll post further updates here

I plan to eventually release my project to the public.