Very Computer

System: Sun SPARC

We need to transfer data from one program (a simulation) to another
(a graphic program). Up to now, we used a simple file to do that.
Of course we wanted to speed up things a little bit, so we decided
to use shared memory.
To test this, we wrote 2 little programs, one for writing, one for reading.
The procedure to read the contents could be (writing looks similar):

void
read_shm(shmname)

int     shmname;

{
  double        *start,*tmp,m1,m2;
  char          *shmat();
  int           n;

  tmp = start = (double *)shmat(shmname,0,1);   /* make shm accessable */

  n = (int)(*tmp++);   /* # of value pairs */

  for ( ; n > 0; n--)
  {
    m1 = *tmp++;   /* just reading dummies */
    m2 = *tmp++;
  }

  shmdt ((char*)start);

Quote:}

This works fine (and fast).

BUT:
In a Sun document named "Porting Software to SPARC Systems"
(from the "Programmer's Language Guides"), chapter 2, you
can read the following :

---
   On SPARC machines, all quantities must be aligned on
   boundaries corresponding to their sizes: bytes on byte
   boundaries, (16-bit) halfwords on halfword boundaries,

   ...[deleted]...

   There are several C language constructs, however, that
   may lead to a bus error during execution:

   - Casting a pointer to a char or unsigned char into
     a pointer to a larger quantity, such as ...[deleted]...
     double,

   ...[deleted]...
---

shmat() returns a char *. Is there any way to make sure,
that it points to a suitable boundary for a double?

Another question:
Is it a good idea to control the read/write access via
IPC-Queues (the 'writer' waits until the 'reader' says ok)?
Are there any examples for such a communication?

Thanks in advance,
Bernd

Universitaet Stuttgart
Institut fuer Chemische Verfahrenstechnik
Boeblinger Str. 72
D-7000 Stuttgart 1

Quote:> [program using shared memory]
> tmp = start = (double *)shmat(shmname,0,1);   /* make shm accessable */
> ...
> shmat() returns a char *. Is there any way to make sure,
> that it points to a suitable boundary for a double?

A suitable boundary for a double for a double is any multiple of
sizeof(double) == 8. You can test that after calling shmat(). If the
returned value should not be suitable, detach the shared memory (shmdt())
set 'start' to the next double boundary, and invoce shmat() passing
'start' as second argument. That asks shmat() to allocate at the address
you have passed.

--
Michael Dahmen                                  Tel. +49 89 926 99113

Arabellastr 17, 8000 Munich 81. Germany         UUCP: ...unido!ecrc!dahmen