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

If this were two separate questions, then you'd be able to give each one a useful title. — Steve Jessop, Jun 17 '11 at 13:30
`-1&)~(` is not even legal C (a compiler should complain about a missing token), are you sure its not `-1)&~(`? which means AND NOT, (usually used to turn a specific bit off) — Wiz, Jun 17 '11 at 13:31
@Wiz: In this case, OBJECT_GRAIN is probably a power of 2, so if the code were `-1)&~` then it would round `mem+HEADER_SIZE` up to the next multiple of `OBJECT_GRAIN`. — Steve Jessop, Jun 17 '11 at 13:33
Yeah I am sure, I copied the source code! If it was a mistake, there maybe a warning a compile no? — scoobyclown, Jun 17 '11 at 13:33
@Steve: Yes OBJECT_GRAIN is for data alignment, it's equal to 8 — scoobyclown, Jun 17 '11 at 13:34
@scoobyclown i beg to differ: (line 212) http://read.pudn.com/downloads144/sourcecode/others/628545/jamvm-1.4.2/jamvm-1.4.2/src/alloc.c__.htm (I guess it must be a typo in the source you are viewing) — Wiz, Jun 17 '11 at 13:36
@scoobyclown: please accept an answer that solves your question instead of editing that into the title. Stackoverflow.com is not a forum. — rubenvb, Jun 17 '11 at 13:43

score 4 · Answer 1 · answered Jun 17 '11 at 13:32

4

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.

answered Jun 17 '11 at 13:32

Jeff Foster

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Thanks! That why I said it was a stupid question in the first place, I hadn't read the good manual in the first place (I read the 3p) – scoobyclown Jun 17 '11 at 13:36

paxdiablo · Accepted Answer · 2011-06-17T13:53:14.227

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).

Thank you, That's it! But it's strange because I have the latest version and it works fine... — scoobyclown, Jun 17 '11 at 13:39
After downloading a clean copy of JamVM and apply a diff, the only difference seems to be the line with heapbase... I maybe have made bad edits while reading... Thanks for your answer and sorry ! — scoobyclown, Jun 17 '11 at 13:59
@scooby, no need to apologise, helping people is why we are here. Cheers. — paxdiablo, Jun 17 '11 at 14:01

C/Posix Questions

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