dtrace discuss - Jul 2008 - simple translator

I''m new to dtrace and trying to figure out how I can use it as a tool
for debugging.

I have a few lines for testing :

====================================#include<stdio.h>
#include<string.h>

typedef struct {
        int age;
        char name[24];
        char class;
} student_t;

int setdata(int count,student_t *first) {
   first->age = 24;
   strcpy(first->name,"Nigel S");
   first->class = ''I'';

return 100;
}

int main() {

  student_t first;
  int count=12;

  setdata(count,&first);
  printf("name is %s\n",first.name);

return 1;
}

====================================
Compiled with "-g", I would like to use dtrace to examine the
structure. Using compiled in probes is not an option.

I''ve tried something like this :
====================================#!/usr/sbin/dtrace -s

typedef struct {
        int age;
        char name[24];
        char class;
} student_t;


translator student_t <student_t *P> {
    age = *(int *)copyin((uintptr_t)&P->age, sizeof (int));
    name = (char *)copyin((uintptr_t)*P->name, sizeof (char *));
    class = *(char *)copyin((uintptr_t)&P->class, sizeof (char)); 
};

pid$target::setdata:return
{
        xlate<struct student_t *>(arg2);
        printf("\nit is %i ", arg2->age); 
} 

====================================
When run it returns :
dtrace: failed to compile script ./structs.d: line 18: cannot translate from
"int64_t" to "struct student_t *"

There''s probably something obvious that I''m missing here. Any
ideas ?

If anyone had any example of examining a structure in thier own code using
dtrace that would be great to see as well.

TIA


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This message posted from opensolaris.org

Actually, my "xlate" line has been partially eaten in posting; it
should be :

xlate < struct student_t *  >(arg2);


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This message posted from opensolaris.org

Hi,

Hi, I''m typing this without any testing, so I might be slightly wrong
somewhere. You don''t need translators.

=======================================================================#!/usr/sbin/dtrace
-s

#pragma D option quiet
/* this will stop dtrace to show whatever it shows by default, so that
 * your printf functions output won''t get distorted */

typedef struct {
        int age;
        char name[24];
        char class;
} student_t;

pid$target::setdata:entry{
	/* You have to remember the address in entry point */
	self->addr = arg1;
}

pid$target::setdata:return
/ self-> addr /
{
	/* Use this probe only when we got address at entry point first */
	/* return probe uses only arg0 - address of the returning instruction in
fuction */
	/*                    and arg1 - return value */
	student = (student_t*)copyin(self->addr, sizeof(student_t));
        printf("it is %i\n", student->age); 
	self->addr = 0; /* this will release self->addr variable, otherwise it
will occupy dtrace''s memory */
} 
=======================================================================
Oh, and for 32bit binaries you should to run it using ''dtrace -32 -s
.....''

Hope this helps

-- 
	Vlad
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> 
> translator student_t <student_t *P> {
>     age = *(int *)copyin((uintptr_t)&P->age, sizeof (int));
>     name = (char *)copyin((uintptr_t)*P->name, sizeof (char *));
>     class = *(char *)copyin((uintptr_t)&P->class, sizeof (char));
> };
> 
> pid$target::setdata:return
> {
>         xlate<struct student_t *>(arg2);
>         printf("\nit is %i ", arg2->age);
> }
> 
> ====================================> 
> When run it returns :
> dtrace: failed to compile script ./structs.d: line 18: cannot translate
from
> "int64_t" to "struct student_t *"
> 
> There''s probably something obvious that I''m missing here.
Any ideas ?
Two problems:

1. Your translator''s input type is student_t *.  The arg2 (and all
argN)
   variables for PID provier have no types associated (they are just integers)
   So you have to cast arg2 to struct student *, or alternatively declare
   the translator input type as integer and cast inside of the translator.

2. Your clause consists of an xlate statement and a printf statement.
   xlate is not a function, it''s an operator -- think of it like a type
cast.
   By making it a separate statement that statement has no effect.
   You need to printf("it is %i", (xlate<struct student_t
*>(arg2))->age);
   or alternatively you can assign to a local variable etc.

-Mike

-- 
Mike Shapiro, Sun Microsystems Fishworks. blogs.sun.com/mws/

Thanks Mike and  neuron, both of your suggestions worked out.

Following Mike''s suggestion, this is what worked :

pid$target::setdata:return
{
        printf("\nit is %i ",(xlate < student_t * > (((student_t
*)arg2)))->age);
}

(added just in case anyone else is curious to see the solution )


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