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Subject: Re: apr shared memory handling
From: michaelr <mypop@freenet.de>
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	 <20111114154209.GA10893@seid-online.de>
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Date: Mon, 14 Nov 2011 17:07:20 +0100
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Hi Ralf,

thanks for the explaination. As i see - i can't use a simple pointer
and after reading your answer it's clear why! So it's not that easy 
to store dynamic content (like arrays) into a shared memory segment.
Actually i have looked at the ldap-caching functions from mod_ldap. 
They do something similar as i wanted to do and it seems to me that 
a lot of more work has to be done to make it work correctly.

Sorry for this stupid question. Ticket closed. 

Greetings 
  Michael
   

On Mon, 2011-11-14 at 16:42 +0100, rm@tuxteam.de wrote:
> On Mon, Nov 14, 2011 at 02:18:20PM +0100, michaelr wrote:
> > Hi all,
> > 
> > first excuse my bad english. As a beginner in apache module development
> > i have a problem understanding the apr shared memeory handling. In the
> > following example i removed all the locks and error handling to bring my
> > problem in front. Even the malloc and free calls are only for testing
> > purposes (in a real module i will use memory pools).
> > 
> > As an example i have the following struct defined in my header:
> > 
> > typedef struct
> >   {
> >     unsigned int active ;
> >     char *test ;
> >   } shm_seg_t ;
> > 
> 
> That char  pointer smells  funky!
> 
> [...]
> > // malloc just for testing purposes!
> > acl_pool->test = (char *) malloc ( 40 * sizeof(char) );  
> 
> And here is why: acl_pool->test will now point to an address on
> the current process' heap. That's just a number with no relevance
> to other processes.
> 
> > strcpy(acl_pool->test, "POSTCONFIGHANDLER");
> > fprintf(stderr," post: --> %s\n", acl_pool->test);
> > fflush(stderr);
> 
> Yes, because you read from your own memory.
> 
> > 
> > This works. So in the module handler i can access the varaibles 
> > like this: 
> > 
> > apr_shm_attach(&acl_pool_shm, "/tmp/file", r->pool) ;
> > acl_pool = (shm_seg_t *) apr_shm_baseaddr_get(acl_pool_shm) ;
> > 
> > printf(stderr, " HANDLER: --> %d - %s\n", 
> > acl_pool->active, acl_pool->test) ;
> > fflush(stderr) ;
> > 
> > 
> > Everything is fine until here - it prints out:
> > 
> > HANDLER: --> 0 - POSTCONFIGHANDLER
> > 
> > to the error log.
> > 
> > 
> > I want to update the data in the shared memeory segement in an
> > monitor_hook which i set up also. In the monitor_hook i do the
> > following:
> > 
> > apr_shm_attach(&acl_pool_shm, "/tmp/file", p) ;
> > 
> > acl_pool = (shm_seg_t *) apr_shm_baseaddr_get(acl_pool_shm) ;
> > 
> > acl_pool->active++ ;
> > 
> > if ( acl_pool->test != NULL)
> >  {
> >  free(acl_pool->test) ;
> 
> Kawoom! You just handed free a number that has only meaning within
> the context of the process that did _set_ this number.
> There's no magic to get your hands on another processes memory (at least
> not since Mac Os7 and OS/2 left this planet).
> 
> >  acl_pool->test = (char *) malloc(40 * sizeof(char)) ;
> >  strcpy(acl_pool->test, "SCHEDULER") ;
> >  fprintf(stderr, " scheduler: --> %s\n", acl_pool->test) ;
> >  fflush(stderr) ;
> >  }
> > 
> > 
> > Now i run into the problem that the module_handler did'nt recognise
> > the changes i have made so far in the monitor_hook. acl_pool->test
> > did not change in the child process and it always prints out:
> > 
> > HANDLER: --> 1 - POSTCONFIGHANDLER
> > HANDLER: --> 2 - POSTCONFIGHANDLER
> > HANDLER: --> 3 - POSTCONFIGHANDLER
> > 
> > The integer varaibles get incremented as expected. The dynamic
> > variable: char *test did'nt change at all. I set MaxRequestsPerChild 
> > to zero in this first try.
> 
> Yes, acl_pool->active is a storage location, i.e. the value get's stored
> and read from shared memory, but acl_pool->test is only a _pointer_ to a
> storage location local to one process.
> 
> > When i set MaxRequestsPerChild to 4 for something else below and the 
> > childs get restarted by the root-Server the next call to the
> > module-handler prints out:
> > 
> > HANDLER: --> 1 - SCHEDULER
> 
> Here you most likely happen to get the request served from the same
> process where acl_pool->test _happens_ to point to the right location
> ...
>   
> > So my questions is: Why i can not see the changes which the
> > monitor hook has done until the childs gets restarted? The integer
> > variable is readable with the correct update at any time.
> 
> You would have to allocate the character data from shared memory, but
> keep in mind that even then a simple pointer wouldn't work (since one
> location in shared memory can and will have different addresses in
> different processes). A bit of googling will bring you further material
> (this has been discussed in this mailing list abd elsewhere).
> 
>  HTH Ralf Mattes
>  
> > Thank's for your help.
> > 
> > Greetings Michael
> >