From owner-freebsd-doc@freebsd.org Tue Dec 27 13:58:57 2016 Return-Path: Delivered-To: freebsd-doc@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 6CA00C8A226 for ; Tue, 27 Dec 2016 13:58:57 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from kenobi.freebsd.org (kenobi.freebsd.org [IPv6:2001:1900:2254:206a::16:76]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 3C9BD14E0 for ; Tue, 27 Dec 2016 13:58:57 +0000 (UTC) (envelope-from bugzilla-noreply@freebsd.org) Received: from bugs.freebsd.org ([127.0.1.118]) by kenobi.freebsd.org (8.15.2/8.15.2) with ESMTP id uBRDwvpa057288 for ; Tue, 27 Dec 2016 13:58:57 GMT (envelope-from bugzilla-noreply@freebsd.org) From: bugzilla-noreply@freebsd.org To: freebsd-doc@FreeBSD.org Subject: [Bug 215612] shm_open.2 man page does not accurately describe support for read(2)/write(2) on POSIX shared-memory objects Date: Tue, 27 Dec 2016 13:58:57 +0000 X-Bugzilla-Reason: AssignedTo X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: Documentation X-Bugzilla-Component: Documentation X-Bugzilla-Version: Latest X-Bugzilla-Keywords: X-Bugzilla-Severity: Affects Only Me X-Bugzilla-Who: rwatson@FreeBSD.org X-Bugzilla-Status: New X-Bugzilla-Resolution: X-Bugzilla-Priority: --- X-Bugzilla-Assigned-To: freebsd-doc@FreeBSD.org X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version rep_platform op_sys bug_status bug_severity priority component assigned_to reporter Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: https://bugs.freebsd.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: freebsd-doc@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Documentation project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 27 Dec 2016 13:58:57 -0000 https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D215612 Bug ID: 215612 Summary: shm_open.2 man page does not accurately describe support for read(2)/write(2) on POSIX shared-memory objects Product: Documentation Version: Latest Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: Documentation Assignee: freebsd-doc@FreeBSD.org Reporter: rwatson@FreeBSD.org The shm_open(2) man page states the following about POSIX shared-memory objects: The result of using open(2), read(2), or write(2) on a shared memory object, or on the descriptor returned by shm_open(), is undefined. It= is also undefined whether the shared memory object itself, or its content= s, persist across reboots. In FreeBSD, read(2) and write(2) on a shared memory object will fail w= ith EOPNOTSUPP and neither shared memory objects nor their contents persist across reboots. However, this is no longer up-to-date: the kernel implementation explicitly includes support for read(2)/write(2). This documentation also does not mention the confusing property that ftruncate(2) must be called to extend a shared-memory object before it can be written to. I.e., the following does require a call to ftruncate(2): uint8_t buffer[getpagesize()]; ssize_t len; int fd; fd =3D shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600); if (fd < 0) err(EX_OSERR, "%s: shm_open", __func__); if (ftruncate(fd, getpagesize()) < 0) err(EX_IOERR, "%s: ftruncate", __func__); len =3D pwrite(fd, buffer, getpagesize(), 0); if (len < 0) err(EX_IOERR, "%s: pwrite", __func__); if (len !=3D getpagesize()) errx(EX_IOERR, "%s: pwrite length mismatch", __func__); If the truncate is missing, then pwrite(2) will return 0 bytes written (EOF= ). It is not clear that this is a bug, but it probably deserves mention. --=20 You are receiving this mail because: You are the assignee for the bug.=