Question about struct in shared memory (C on linux)

Discussion in 'C Programming' started by hugo.arregui@gmail.com, Jul 3, 2008.

  1. Guest

    Hi!

    I have two struts like that:

    struct {
    int num;
    int num2;
    struct b arrayOfB[SIZE];
    } a;

    struct {
    int num3;
    int num4
    } b;

    I put struct a into Shared Memory:
    shmid = shmget(IPC_PRIVATE, sizeof(struct b), 0666)

    and atach it in two process:
    ptr = shmat(shmid, 0, 0);

    This works fine, but now i must delete the constant SIZE:

    I will obtain the SIZE in runtime, before execute the shmget function.

    the new struct a:

    struct {
    int num;
    int num2;
    struct b *arrayOfB;
    } a;

    now:

    I put struct a into Shared Memory:
    shmid = shmget(IPC_PRIVATE, sizeof(struct a) + sizeof(struct b) *
    size, 0666);

    and atach it in two process:
    ptr = shmat(shmid, 0, 0);

    my question is: how can tell arrayOfB to point over the shared memory?

    i thought:

    struct a *ptr,p2*;
    p2 = ptr; //ptr is the shmat return
    p2++;
    ptr->arrayOfB

    but doesn't work. in another way i read about the shmat parametrs, but
    don't understand how (if its possible) use to solve my trouble.

    Thanks.
    , Jul 3, 2008
    #1
    1. Advertising

  2. On Jul 3, 12:29 pm, "" <>
    wrote:
    > Hi!
    >
    > I have two struts like that:
    >
    > struct {
    > int num;
    > int num2;
    > struct b arrayOfB[SIZE];
    >
    > } a;
    >
    > struct {
    > int num3;
    > int num4
    >
    > } b;
    >
    > I put struct a into Shared Memory:
    > shmid = shmget(IPC_PRIVATE, sizeof(struct b), 0666)
    >
    > and atach it in two process:
    > ptr = shmat(shmid, 0, 0);
    >
    > This works fine, but now i must delete the constant SIZE:
    >
    > I will obtain the SIZE in runtime, before execute the shmget function.
    >
    > the new struct a:
    >
    > struct {
    > int num;
    > int num2;
    > struct b *arrayOfB;
    >
    > } a;
    >
    > now:
    >
    > I put struct a into Shared Memory:
    > shmid = shmget(IPC_PRIVATE, sizeof(struct a) + sizeof(struct b) *
    > size, 0666);
    >
    > and atach it in two process:
    > ptr = shmat(shmid, 0, 0);
    >
    > my question is: how can tell arrayOfB to point over the shared memory?
    >
    > i thought:
    >
    > struct a *ptr,p2*;
    > p2 = ptr; //ptr is the shmat return
    > p2++;
    > ptr->arrayOfB
    >
    > but doesn't work. in another way i read about the shmat parametrs, but
    > don't understand how (if its possible) use to solve my trouble.
    >
    > Thanks.


    Forgetting the details of shared memory (which you would get better
    advice in comp.unix.programmer, btw, as it is not standard C), you
    could consider using the struct hack to solve your problem. See, e.g.
    http://c-faq.com/struct/structhack.html

    -David
    David Resnick, Jul 3, 2008
    #2
    1. Advertising

  3. Guest


    > Forgetting the details of shared memory (which you would get better
    > advice in comp.unix.programmer, btw, as it is not standard C), you
    > could consider using the struct hack to solve your problem. See, e.g.http://c-faq.com/struct/structhack.html
    >
    > -David


    Thanks you David, but my problem is how to do that in shared memory, i
    will try in comp.unix.programmer.

    Thanks again.
    , Jul 3, 2008
    #3
  4. On Jul 3, 2:52 pm, "" <>
    wrote:
    > > Forgetting the details of shared memory (which you would get better
    > > advice in comp.unix.programmer, btw, as it is not standard C), you
    > > could consider using the struct hack to solve your problem.  See, e.g..http://c-faq.com/struct/structhack.html

    >
    > > -David

    >
    > Thanks you David, but my problem is how to do that in shared memory, i
    > will try in comp.unix.programmer.
    >
    > Thanks again.


    The first option in the faq site should work as well with shared
    memory as elsewhere as far as I know. i.e. put in your structure
    this:

    struct b arrayOfB[1];

    But allocate enough space for the structure AND the desired number of
    "b" elements, then use it. This avoids the problem of having a
    pointer to another block of memory in the struct...

    -David
    David Resnick, Jul 3, 2008
    #4
  5. On Thu, 3 Jul 2008 09:29:37 -0700 (PDT), ""
    <> wrote:

    > struct {
    > int num;
    > int num2;
    > struct b arrayOfB[SIZE];
    > } a;
    >
    > struct {
    > int num3;
    > int num4
    > } b;

    <snip shared-memory>
    Aside: you used sizeof(struct b) where you needed (struct a).

    The shared-memory part is offtopic as it is not standard in C; but
    issues of laying-out and accessing data in arbitrary memory, whether
    it happened to come from shm* or say malloc (a_zillion), is ontopic.
    At least if the memory is suitably aligned in the first place, and
    both <offtopic> shmat and <ontopic> malloc are (for all types).

    > [now] I will obtain the SIZE in runtime, before ... shmget ...
    > struct {
    > int num;
    > int num2;
    > struct b *arrayOfB;
    > } a;

    <snip>
    > my question is: how can tell arrayOfB to point over the shared memory?
    >


    As already answered, you don't need this change; you can use the
    'struct hack' in C89, or the equivalent but standard 'flexible array
    member' in C99, to get the same 'in-place' layout as above.

    But if you WANT to change to a pointer:

    > struct a *ptr,p2*;


    Syntax error; should be *ptr, *p2;

    > p2 = ptr; //ptr is the shmat return
    > p2++;
    > ptr->arrayOfB
    >

    You're close. With the corrected declaration above, this gives you p2
    pointing just after the struct-a. IF that location is suitably aligned
    for a struct-b, you can just do ptr->arrayOfB = (struct b*) p2;
    In fact you don't need a separate variable, just
    ptr->arrayOfB = (struct b*) (ptr+1);
    /* or equivalently */ = (struct b*) &ptr[1];

    Unfortunately there is no standard way to determine what alignment is
    needed for a struct-b, or provided by a struct-a (the size of any type
    in C must be a multiple of its required alignment, to make arrays
    work), so this can be unsafe. For the particular structs you showed,
    it is extremely likely that any requirement for struct-a will (also)
    satisfy struct-b, but it isn't absolutely 100% portably guaranteed.

    If you want to use a pointer and handle the possible alignment issue,
    simplest to use a union which is guaranteed to align for either:

    struct a * aptr = shmat (...);
    union x { struct a a; struct b b; } * xptr = (union x *) aptr;
    aptr->arrayOfB = (struct b*) (xptr + 1);
    /* this is guaranteed located after the struct-a,
    AND aligned for a struct-b. It may _possibly_ be
    offset more than actually needed and waste some space,
    but on most modern systems this isn't worth worrying about,
    unless your (individual) structures are truly huge,
    in which case post a more specific example
    and we can get into the more arcane methods. */

    - formerly david.thompson1 || achar(64) || worldnet.att.net
    David Thompson, Jul 14, 2008
    #5
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Chris Fogelklou
    Replies:
    36
    Views:
    1,357
    Chris Fogelklou
    Apr 20, 2004
  2. gunasingh
    Replies:
    2
    Views:
    2,346
    Keith Thompson
    Apr 5, 2008
  3. Blubaugh, David A.
    Replies:
    3
    Views:
    332
    Diez B. Roggisch
    Aug 27, 2008
  4. Blubaugh, David A.
    Replies:
    0
    Views:
    279
    Blubaugh, David A.
    Aug 26, 2008
  5. Victor Bazarov

    Re: [Linux] Shared memory and pointers

    Victor Bazarov, May 14, 2012, in forum: C++
    Replies:
    0
    Views:
    443
    Victor Bazarov
    May 14, 2012
Loading...

Share This Page