Thread: checking programm break with sbrk

I've written this code :

Code:

    #define MAX_ALLOC 1000000
    int number = 1;
    void *ptr = NULL;

    if (argc != 2){
        controlled_exit( "you have to give 1 argument" );
    }


    if ( ( number = strtol( argv[1], NULL, 10 ) ) == 0 ){
        controlled_exit("the argument must be over 0 and not a character");
    }
    else if ( number > MAX_ALLOC || number < 0 ){
        controlled_exit("the argument must be over 0 and 1000000 or less");
    }

    printf("Program break: %p\n", sbrk(0));

    if ( ( ptr = malloc(number) ) == NULL)
                errxit("malloc");

    printf("Program break (after malloc): %p\n", sbrk(0));

    free(ptr);

    printf("Program break (after free): %p\n", sbrk(0));

    return EXIT_SUCCESS;

In the output, the program break before and after free(ptr) is the same. Did I do something wrong (In my book it says, after releasing a block on the top of the heap, the Program break will lower, when the free block at the top is large enough) or is one million bytes too small?

Malloc is fairly intelligent. Your program just allocated a large chunk of memory, and then freed it. As far as malloc is concerned, there's every reason to believe that you will be mallocing memory again. The last thing it wants is to constantly return the memory to the system (with a system call) and then ask for it back again (with another system call).

And remember that the memory is virtual anyway, so it's not like the actual ram is unavailable to other programs.

And considering that the memory is totally new to your process and you don't actually write anything to it, it's probably not even backed up by any real storage of any kind and it costs absolutely nothing to keep it. You should try writing something to all that memory to see if it makes a difference.

Also, sbrk is not necessarily the best way to see what's happening with malloc. GNU malloc uses multiple "arenas", each potentially containing mulitple "heaps", and can mmap memory, too, which will not necessarily affect the program break (except perhaps for bookkeeping).

You should save the initial and final program breaks and print the difference to see if it's at least as big as the memory you asked for.

Also, there are extra malloc control functions in malloc.h, such as malloc_trim and mallopt, malloc_info.

gnu malloc

Malloc Internals