topic Re: second call to shmat in same process in Operating System - HP-UX

second call to shmat in same process

pramodsharma — Thu, 17 Jan 2008 09:45:26 GMT

If the calling process is already attached to the shared memory segment, shmat() fails and returns SHM_FAILED regardless of what value is passed in shmaddr.

This is not done in other *NIX platforms.

Is there any mechanism to know this is already attached by someone in the process and if yes how to know the address to which it is attached , so that I can use it.

I really don't want to make shared memory point global now.

#include
#include
#include
#include
extern int errno ;

int main()
{
int id = -1 , id2;
key_t key = ftok("/PRAMODTEST" , 700);
id = shmget(key , 16 , 0600 | IPC_CREAT);

if( -1 != id)
{
void *sharedMem = shmat( id ,NULL ,SHM_RDONLY) ;
if((int)sharedMem == -1 )
{
printf("Attaching failed %d , %s \n" , errno , strerror(errno) );
}
else
{
void *p = shmat(id ,NULL ,0) ;
if((int)p == -1 )
{
printf("Second attach is failed %d , %s \n" , errno , strerror(errno));
}
else
{
printf("Second attach paased\n");
shm_unlink("/PRAMODTEST");
}

shmdt(sharedMem);
}
}
else
printf("shmget failde\n");
}

Thanks in advance.

Re: second call to shmat in same process

Don Morris_1 — Thu, 17 Jan 2008 12:02:26 GMT

Yes, that's in the man page and all:

"If the calling process is already attached to the shared memory segment, shmat() fails and returns SHM_FAILED regardless of what value is passed in shmaddr. See exceptions for MPAS processes below."

Quickly skimming over the gory details, this differs from other OS's because HP-UX is a Mostly Global Address Space, they're Mostly (or Completely) Private. And that's because PA doesn't allow (easily) multiple virtual addresses at the same time for the same physical memory (aliasing).

Keep track of things within your own process (as you've considered and which I really think is the best option) or restrict yourself to 11iv2 and higher, IPF only and use MPAS (keeping in mind that using MPAS too much and generating a _lot_ of aliases slows down the system as a whole [because of the collision chains in the translation tables]).

Re: second call to shmat in same process

pramodsharma — Fri, 18 Jan 2008 04:04:01 GMT

Closing thread.