Re: What is the point of IPC_PRIVATE of shmget?

[Peng Yu <pengyu.ut@gmail.com>]

On Mon, Apr 12, 2021 at 1:59 PM Manfred via Libc-help
<libc-help@sourceware.org> wrote:
>
>
>
> On 4/12/2021 6:14 PM, Florian Weimer via Libc-help wrote:
> > * Peng Yu:
> >
> >> On Mon, Apr 12, 2021 at 11:07 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>>
> >>> * Peng Yu via Libc-help:
> >>>
> >>>> I don't get the point of IPC_PRIVATE of shmget(). Since it is just
> >>>> used by the current process, why not just use malloc?
> >>>>
> >>>> Can anybody give a real example in which IPC_PRIVATE must be used, but
> >>>>   malloc or other variant of *alloc functions are not appropriate to
> >>>> use? Thanks.
> >>>
> >>> shmget(2) says this:
> >>>
> >>> | BUGS
> >>> |        The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
> >>> |        would more clearly show its function.
> >>>
> >>> Does this answer your question?
> >>
> >> No. That is just a naming issue. It is still privately used by the
> >> current process instead of other processes.
> >
> > Oh, it can be shared if the process forks.  With malloc, you'd lose the
> > sharing.
>
> Out of curiosity,
> Does it need to fork? Can the identifier be shared in some other way?

I had the same question. I made a test case for it. It seems that it
doesn't matter whether fork is used or not. My test shows that as long
as another process obtains the id somehow, it can access it. Can this
conclusion be safely made?

==> main.c <==
// vim: set noexpandtab tabstop=2:

#include <stdlib.h>
#include <stdio.h>
#include <sys/shm.h>

int main(int argc, char *argv[]) {
    int shmid = atoi(argv[1]);
    char *str = shmat(shmid, NULL, 0);
    printf("%s\n",str);
    return 0;
}

==> main_create.c <==
// vim: set noexpandtab tabstop=2:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/shm.h>

int main(int argc, char *argv[]) {
    key_t key = atoi(argv[1]);
    int shmid;
    if((shmid = shmget(key, 1024, 0666|IPC_CREAT)) == -1) {
        perror("shmget()");
        return 1;
    }
    printf("%d\n", shmid);
    char *str = shmat(shmid, NULL, 0);
    strcpy(str, argv[2]);
    return 0;
}

$ shmid=$(./main_create 0 xyz); ./main.exe "$shmid"
xyz

-- 
Regards,
Peng