On 6/2/07, Eduardo Morras <[EMAIL PROTECTED]> wrote:
At 17:18 31/05/2007, you wrote: >On 5/31/07, Eduardo Morras <[EMAIL PROTECTED]> wrote: >>At 23:25 30/05/2007, you wrote: >>>Setting and reading individual bytes (u8 in sqlite-speak) are not >>>threadsafe either. Only reading/setting entire entire words >>>are threadsafe on most architectures. >> >>Using a uint32 for store the flags is threadsafe. There are less than 32 true/false values and read/set is simple. I see no difference doing >> >>if (uint8==0){ // read/test bit >>uint8=1; // set bit >>whatever more >>} > >Not atomic, so not thread-safe. >You have a race condition waiting to happen. > >>and >> >>if (uint32&&MASK){ // read/test bit >>uint32&&=MASK; // set bit >>whatever >>} > >Also not atomic, so not thread-safe.I think i have not explained too well. I have wanted to point that if you can modify/set 8 or 9 values atomically/thread-safe you can do it with one 32 bit flag value.
Ok, but you can't do it in a portable way without some help (e.g. some threads library, like pthreads, or the use of the Interlocked* functions on Windows). You have instructions on the x86 that you can use for this, but some CPUs don't (many RISC ones), so you need to have some kind of memory barrier until you complete the operation. There are other things you may want to look at. For example, on the x86, reading a 32 bit value from memory is only atomic if the address is aligned, else you are actually doing 2 memory reads (meaning another thread can change the value halfway). Other CPUs fix this by simply not allowing un-aligned memory reads (causing a lot of problems when porting x86 code to other platforms). The subject is a bit more complex than this, because to really talk about it we would also need to talk about what the C standard says about the volatile keyword and how different compilers treat it. But it's becoming off-topic. Regards, ~Nuno Lucas ----------------------------------------------------------------------------- To unsubscribe, send email to [EMAIL PROTECTED] -----------------------------------------------------------------------------
