[ffmpeg-devel] question about SDL_LockMutex in ffplay

[Andy Parkins]

On Friday 2005 July 08 11:06, zuo bf wrote:

> it too, but shouldn't a mutex can only be locked one time at the same time
> ? With some printf output, i found that every mutex can be locked twice, it
> that the
> correct behaviour ?

I'm going to guess that you tested by doing something like:

  printf( "lock first\n" );
  lock_mutex();
  printf( "unlock first\n" );
  unlock_mutex();

then in another thread

  printf( "lock second\n" );
  lock_mutex();
  printf( "unlock second\n" );
  unlock_mutex();

And have seen that both "lock first" and "lock second" appear one after the 
other.  Try again with the printfs inside the locks, like this:

  lock_mutex();
  printf( "locked first\n" );
  // perform some locked work
  printf( "unlocking first\n" );
  unlock_mutex();

You see it's perfectly acceptable to call lock_mutex() twice from two 
different threads, and the second lock will simply wait until the first 
thread unlocks again.  This is mutex's working exactly as they should.  

Hope that helps.  I'd be very surprised if you really were getting a mutex 
locked twice - it would be a fairly fundamental problem with libpthread.


Andy

-- 
Dr Andrew Parkins, M Eng (hons), AMIEE
andyparkins at gmail.com
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