From owner-freebsd-hackers@freebsd.org Mon Sep 10 15:28:36 2018 Return-Path: Delivered-To: freebsd-hackers@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id ED34A1093161 for ; Mon, 10 Sep 2018 15:28:35 +0000 (UTC) (envelope-from gljennjohn@gmail.com) Received: from mail-wr1-x444.google.com (mail-wr1-x444.google.com [IPv6:2a00:1450:4864:20::444]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 6B55C868E1 for ; Mon, 10 Sep 2018 15:28:35 +0000 (UTC) (envelope-from gljennjohn@gmail.com) Received: by mail-wr1-x444.google.com with SMTP id s14-v6so13434318wrw.6 for ; Mon, 10 Sep 2018 08:28:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:in-reply-to:references:reply-to :mime-version:content-transfer-encoding; bh=rjUzendxQLWHy27xHr+kw9T29nBdT/Lv6ddfFsLqpOs=; b=bqzL4DSJW4FMuhdx31Xm3nEQXmw4EQevWaiayol8KxGwzLsyXjkgnhONv3UJu4TDfq DcOBRJukJ5/FFUwdS2vwgABDukjyV9Y/QIL3AVz8MooO4I7oCxj5UqJwWlxllDf+hwPQ OLTuhAKeJAnqDrYg/GSdeOzK7N+DTj6loPAm5PmRIWaB3nDfyoBS5ZvCFriiLr3gkttS N5cXF5XPoAqqqFLKvyG5AXL8ExiGwThCh0NJbupgVC6TB6MNVlGWaS+VBX4gYQC93rNL FCssWK34Gw5Z9/54894Syx9UyL5MofUDIFkvhwmQCvrY1o6AkCq3Hky0I8QtYfnMvcP7 SFPA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:in-reply-to :references:reply-to:mime-version:content-transfer-encoding; bh=rjUzendxQLWHy27xHr+kw9T29nBdT/Lv6ddfFsLqpOs=; b=FcqqD0wUUbf5lKo25LXQFwy5eabdgfUEYCLhUZq9F0456S5P6JG2p32sdTVvcHx8Ib 8LPBhixEDxyirTDvt0nbybTd4tvwRYytTlmRGL0R/1vzG6Njef2vTKK18NIvQfjhiMHf iJlW5pgi8yqD6klZrobRkculul/Aas9WiexCgC7W++TUXdyV4JKCl1I0mak/Dir3fJxp U2GAAiCEhQs/x3jzz92IlvRo7p9gdDO+h9mGbFEJl+odYcfGbVkC/CpuoWjbfgfJMxod fxLVTyMO7Ie7lmG9DLmQTSWrdaVicFk9aYpuetrkv0Q4i2CFIU1FL9ezvK+qL74b5Z9h ki3w== X-Gm-Message-State: APzg51DgaeyEXcUeEu7A9qQiNQ2awWh8xNssQr4TiO3JP8R7qvLWAhaD XdibEtdVcpOUnPalZzo2KxZSB15f X-Google-Smtp-Source: ANB0VdYchwKeeLKCUoyw5d709yFdBVWLH5qmypJG2hWRoJ1Uvk90I07xR7UtTtATC466bQLO4qVeIA== X-Received: by 2002:adf:bd10:: with SMTP id j16-v6mr15170243wrh.267.1536593314384; Mon, 10 Sep 2018 08:28:34 -0700 (PDT) Received: from ernst.home (p5B3BE904.dip0.t-ipconnect.de. [91.59.233.4]) by smtp.gmail.com with ESMTPSA id t9-v6sm38920857wra.91.2018.09.10.08.28.32 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 10 Sep 2018 08:28:33 -0700 (PDT) Date: Mon, 10 Sep 2018 17:28:32 +0200 From: Gary Jennejohn To: Tamas Szakaly Cc: freebsd-hackers@freebsd.org Subject: Re: Attempting to receivce zero-length message with recvmsg Message-ID: <20180910172832.2c0a1357@ernst.home> In-Reply-To: <20180910085833.d4py4ladlyqchjvo@pamparam> References: <20180910085833.d4py4ladlyqchjvo@pamparam> Reply-To: gljennjohn@gmail.com X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; amd64-portbld-freebsd12.0) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.27 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 10 Sep 2018 15:28:36 -0000 On Mon, 10 Sep 2018 10:58:33 +0200 Tamas Szakaly wrote: > Hi, > > I have a question about the recvmsg syscall. According to POSIX, unless > O_NONBLOCK is set on the socket fd, recvmsg [1] should block until a message > arrives. However, recvmsg returns immediately with 0, if we are trying to > receive a 0-byte message from a SOCK_SEQPACKET AF_UNIX socket. Consider the > following code: > > #include > #include > > int main(int argc, char** argv) { > int sock[2]; > socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sock); > > struct msghdr msghdr = {0}; > int ret = recvmsg(sock[1], &msghdr, 0); > > printf("ret=%d, msghdr.msg_flags=0x%08x\n\n", ret, msghdr.msg_flags); > } > > Running this yields this output: > > [0x00 socketstuff]$ cc socketpair.c -o socketpair && ./socketpair > ret=0, msghdr.msg_flags=0x00000000 > > You can see that recvmsg returns with 0, even though there were no messages > sent, and neither of the sockets are closed, so it should block indefinitely. > > Is this behavior intentional to match the semantics of read [2] (i.e. > attempting to read zero bytes should be a no-op)? > > > [1] recvmsg: http://pubs.opengroup.org/onlinepubs/9699919799/functions/recvmsg.html > [2] read: http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html > You have to initalize msghdr.msg_iov and msghde.iov_len, otherwise the kernel notices there is no place to put a received message and simply returns 0. I did that and recvmsg() did not return. -- Gary Jennejohn