On Fri, Oct 17, 2008 at 08:06:55PM +0200, Joern Nettingsmeier wrote: > excuse my chiming in here, i'm not really much of a c programmer... but > how can this > - rb->read_ptr += n1; > - rb->read_ptr &= rb->size_mask; > + rb->read_ptr = (rb->read_ptr + n1) & rb->size_mask; > fix anything? > > iiuc, both versions are equivalent. a context switch could happen just > as well after the parenthesis has been computed..!? > putting stuff on one line doesn't make it atomic. maybe you are now > getting another compiler optimization that helps to hide the bug? The idea is that it is very unlikely that the compiler would store the intermediate result in rb->read_ptr, and so this value is updated only once. I agree it is fragile, there is nothing stopping the compiler from doing it wrong. Unless the the pointers are made volatile again. One real solution is to *not* use the size_mask at this point, but apply it only when read_ptr and write_ptr are read. So only a single addition remains. The pointers are allowed to grow, they will wrap around at the word size, but since the buffer size is a power of two they are still correct when that happens. This is what is done in the C++ class I posted earlier. Ciao, -- FA Laboratorio di Acustica ed Elettroacustica Parma, Italia Lascia la spina, cogli la rosa. _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
