C/Posix Questions

Question

I have been working for two weeks on JamVM, a small but powerful Java Virtual Machine.

Now I am trying to figure how the memory is implemented and I am stuck on two C stupid problems:

char *mem = (char*)mmap(0, args->max_heap, PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANON, -1, 0);

--> The -1 parameter stands for a file descriptor, what does that mean? (I have aleady read the mmap man, but haven't found it, maybe I misunderstood...).

heapbase = (char*)(((uintptr_t)mem+HEADER_SIZE+OBJECT_GRAIN-1&)~(OBJECT_GRAIN-1)) HEADER_SIZE;

--> What is 1& ? I don't find it in the C specification...

Thanks,

Yann

Answer 1

You use the file descriptor when you have an open file that you want to map into memory. In this case, you're creating an anonymous map (one not backed by a file) so the file descriptor isn't needed. Some implementations ignore fd for anonymous maps, some require it to be -1.

The second question is a syntax error (probably a typo). It probably should be something like:

heapbase = (char*)(((uintptr_t)mem+HEADER_SIZE+OBJECT_GRAIN-1)
    &~(OBJECT_GRAIN-1)) - HEADER_SIZE;

In that case, OBJECT_GRAIN will be a power of two and it's a way to get alignment to that power. For example, if it were 8, then ~(OBJECT_GRAIN-1) would be ~7 (~00...001112, which is ~11...110002) which, when ANDed with a value, could be used to force that value to the multiple-of-8 less than or equal to it.

In fact, it's definitely a transcription error somewhere (not necessarily you) because, when I download the JamVM from here and look in src/alloc.c, I get:

void initialiseAlloc(InitArgs *args) {
    char *mem = (char*)mmap(0, args->max_heap, PROT_READ|PROT_WRITE,
                                               MAP_PRIVATE|MAP_ANON, -1, 0);
    :
    << a couple of irrelevant lines >>
    :    
    /* Align heapbase so that start of heap + HEADER_SIZE is object aligned */
    heapbase = (char*)(((uintptr_t)mem+HEADER_SIZE+OBJECT_GRAIN-1)&
               ~(OBJECT_GRAIN-1))-HEADER_SIZE;

(note that your version is also missing the - immediately before HEADER_SIZE, something else that points to transcription problems).

Answer 2

In answer to your first question. From the man page.

fd should be a valid file descriptor, unless MAP_ANONYMOUS is set. If MAP_ANONYMOUS is set, then fd is ignored on Linux. However, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this.

So it's -1 because MAP_ANONYMOUS is being used.