Question

I am calling sysctl() to retrieve mem stats and for the void* oldVal argument, I am passing in a pointer to a double. However instead of setting the double to the correct value, it just sets it to 0.00000

However, when I try doing the exact same thing with a long, it sets it to the correct stat. Why is the double being set to 0.00000 while long is being set to the correct stat?

    int systemInfoNeeded[2] = {CTL_HW, HW_PHYSMEM};
    size_t sizeOfBuffer = sizeof(totalAmount);

    if (sysctl(systemInfoNeeded, 2, &totalAmount, &sizeOfBuffer, NULL, 0)) 
    {
        NSLog(@"Total memory stat retrieval failed.\n");
        exit (EXIT_FAILURE);
    }

totalAmount is a double. The second I change the type of totalAmount to long, it works perfectly. Is there anyway I can get the double to work? I want to directly send in totalAmount rather than sending a long and then assigning the value to totalAmount.

I am using Objective-C/C, on Mac OS X Snowleopard with Xcode 3.2.6

Answer 1

sysctl() accepts a pointer to the type specified, in the manpage, for the property you are querying. The parameter is declared as a void* so that the same generic interface can work with the different types expected by the various properties. That does not mean that you can use any type you want. In the case of HW_PHYSMEM, it is an integer, i.e. an int, not a long or anything else.

The only reason it works if you pass a long is because macs are little endian, thus the first four bytes of a value as a long are the same as the value as an int, but you should of course not depend on this.

If you want to read a double, convert the integer.

You should take a good look at sysctl(3). Look in particular at the example with KERN_MAXPROC.