dtrace discuss - Aug 2007 - Tracing shared memory calls

Howdy,

I wrote a DTrace script [1] to dump shared memory calls, and for some
reason the value returned in arg1 (the shared memory identifer) of
fbt::shmget:return doesn''t match the value returned by a simple C
program [2] that calls shmget:

$ dtrace -s shm.d
operation  key        shmid      size       flags      CPU        Wall Time
shmget     0           0            1048576  932       585000     613900

$ ./shm
Called shmget(): key: 0 size: 1048576 shmid: 268435534
Called shmat(): data: ff100000 shmid: 268435534
Called shmdt(): data: ff100000

The size and flags look correct, but the shmid is not quite right
(based on the sample program, it should be 268435534). Does anyone
happen to see anything wrong with my DTrace script? Based on the docs,
it looks like the return value should be in arg1, and it also looks
like the shmget() kernel routine returns the shmid back to the caller
(at least this is what I devised by reading through shm.c).

Thanks for any insight,
- Ryan
-- 
UNIX Administrator
http://prefetch.net

[1] shm.d

#pragma D option quiet

dtrace:::BEGIN
{
    /* operation  key  shmid  size  flags cpu wall */
    printf("%-10s %-10s %-10s %-10s %-10s %-10s %-10s\n",
"operation",
                                                    "key",
"shmid",
                                                    "size",
"flags",
                                                    "CPU", "Wall
Time");
}

fbt::shmget:entry
{
   self->key = arg0;
   self->size = arg1;
   self->flags = arg2;
   self->shmgettimestamp = timestamp;
   self->shmgetvtimestamp = vtimestamp;
}

fbt::shmget:return
/ self->shmgettimestamp /
{
   /* operation  key  shmid  size  flags cpu wall */
   printf("%-10s %-10x %-10x %-10d %-10d %-10d %-10d\n", probefunc,
                                          self->key, arg1,
                                          self->size, self->flags,
                                          vtimestamp -
self->shmgetvtimestamp,
                                          timestamp - self->shmgettimestamp);
}

[2] shm.c

include <stdio.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>

int main(int arg, char **argv)
{
    key_t key = 0;
    int shmid = 0;
    int size = 1024 * 1024;
    char *data;

    /* Create a 1MB shared memory segment */
    shmid = shmget(key, size, 0644 | IPC_CREAT);
    printf("Called shmget(): key: %d size: %d shmid: %d\n", key, size,
shmid);

    /* Attach to the shared memory segment */
    data = shmat(shmid, (void *)0, 0);
    printf("Called shmat(): data: %x shmid: %d\n", data, shmid);

    /* Detach from the shared memory segment */
    shmdt(data);
    printf("Called shmdt(): data: %x\n", data);

    return(0);
}

Matty wrote:
> Howdy,
> 
> I wrote a DTrace script [1] to dump shared memory calls, and for some
> reason the value returned in arg1 (the shared memory identifer) of
> fbt::shmget:return doesn''t match the value returned by a simple C
> program [2] that calls shmget:
> 
> $ dtrace -s shm.d
> operation  key        shmid      size       flags      CPU        Wall Time
> shmget     0           0            1048576  932       585000     613900
> 
> $ ./shm
> Called shmget(): key: 0 size: 1048576 shmid: 268435534
> Called shmat(): data: ff100000 shmid: 268435534
> Called shmdt(): data: ff100000
> 
> The size and flags look correct, but the shmid is not quite right
> (based on the sample program, it should be 268435534). Does anyone
> happen to see anything wrong with my DTrace script? Based on the docs,
> it looks like the return value should be in arg1, and it also looks
> like the shmget() kernel routine returns the shmid back to the caller
I read that differently; if you look at the various return statements in 
shmget(), you see:

return (error);
return (EINVAL);
return (ENOMEM);
return (0);

I didn''t dig much further, but IMO this looks like errno values (0 
meaning success), and I strongly suspect the value you''re looking for
is
in rvp:

static int
shmget(key_t key, size_t size, int shmflg, uintptr_t *rvp)
{

....
	*rvp = (uintptr_t)(sp->shm_perm.ipc_id);

	mutex_exit(lock);
	return (0);
}

HTH
Michael
-- 
Michael Schuster	Sun Microsystems, Inc.
recursion, n: see ''recursion''