From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by sourceware.org (Postfix) with ESMTP id 315833958C27 for ; Mon, 12 Apr 2021 16:18:49 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 315833958C27 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-482-r0LNixXZPNGm6xzKijHv8w-1; Mon, 12 Apr 2021 12:18:47 -0400 X-MC-Unique: r0LNixXZPNGm6xzKijHv8w-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4B74780197D; Mon, 12 Apr 2021 16:18:46 +0000 (UTC) Received: from oldenburg.str.redhat.com (ovpn-112-148.ams2.redhat.com [10.36.112.148]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 73F5E60854; Mon, 12 Apr 2021 16:18:45 +0000 (UTC) From: Florian Weimer To: Peng Yu Cc: Peng Yu via Libc-help Subject: Re: What is the point of IPC_PRIVATE of shmget? References: <87wnt7iizx.fsf@oldenburg.str.redhat.com> <87sg3viio7.fsf@oldenburg.str.redhat.com> Date: Mon, 12 Apr 2021 18:18:55 +0200 In-Reply-To: <87sg3viio7.fsf@oldenburg.str.redhat.com> (Florian Weimer's message of "Mon, 12 Apr 2021 18:14:48 +0200") Message-ID: <87o8ejiihc.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-Spam-Status: No, score=-6.4 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-help@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-help mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 12 Apr 2021 16:18:51 -0000 * Florian Weimer: > * Peng Yu: > >> On Mon, Apr 12, 2021 at 11:07 AM Florian Weimer 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. There is also this part: | RETURN VALUE | On success, a valid shared memory identifier is returned. On | error, -1 is returned, and errno is set to indicate the error. That is, IPC_PRIVATE provides the identifier for future invocations. Florian