From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Andreas Schwab Newsgroups: gmane.emacs.devel Subject: Re: how to reading 0 byte files properly Date: Thu, 17 Nov 2022 11:15:50 +0100 Message-ID: <87o7t5ubs9.fsf@igel.home> References: Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="14896"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) Cc: emacs-devel@gnu.org To: Binbin YE Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Thu Nov 17 11:17:26 2022 Return-path: Envelope-to: ged-emacs-devel@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1ovbxW-0003dV-0f for ged-emacs-devel@m.gmane-mx.org; Thu, 17 Nov 2022 11:17:26 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ovbwD-0008LE-2u; Thu, 17 Nov 2022 05:16:05 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ovbw8-0008KK-Ua for emacs-devel@gnu.org; Thu, 17 Nov 2022 05:16:03 -0500 Original-Received: from mail-out.m-online.net ([2001:a60:0:28:0:1:25:1]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ovbw5-0008SD-1x for emacs-devel@gnu.org; Thu, 17 Nov 2022 05:16:00 -0500 Original-Received: from frontend03.mail.m-online.net (unknown [192.168.6.182]) by mail-out.m-online.net (Postfix) with ESMTP id 4NCbQv08sYz1s953; Thu, 17 Nov 2022 11:15:51 +0100 (CET) Original-Received: from localhost (dynscan3.mnet-online.de [192.168.6.84]) by mail.m-online.net (Postfix) with ESMTP id 4NCbQt6qfgz1qqlS; Thu, 17 Nov 2022 11:15:50 +0100 (CET) X-Virus-Scanned: amavisd-new at mnet-online.de Original-Received: from mail.mnet-online.de ([192.168.8.182]) by localhost (dynscan3.mail.m-online.net [192.168.6.84]) (amavisd-new, port 10024) with ESMTP id OyopIgAcIRcK; Thu, 17 Nov 2022 11:15:50 +0100 (CET) X-Auth-Info: Ezh1ZyCVHJAjBYHeETSOFPKueLHmwQEq+cb21slwJAred612zvtJK0LzelQLElQF Original-Received: from igel.home (aftr-82-135-86-21.dynamic.mnet-online.de [82.135.86.21]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mail.mnet-online.de (Postfix) with ESMTPSA; Thu, 17 Nov 2022 11:15:50 +0100 (CET) Original-Received: by igel.home (Postfix, from userid 1000) id 0CB0F2C00B1; Thu, 17 Nov 2022 11:15:50 +0100 (CET) X-Yow: ..Hmmm...This thing is really VIBRATING! ..It's getting WARM, too! In-Reply-To: (Binbin YE's message of "Thu, 17 Nov 2022 17:45:43 +0900") Received-SPF: pass client-ip=2001:a60:0:28:0:1:25:1; envelope-from=whitebox@nefkom.net; helo=mail-out.m-online.net X-Spam_score_int: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.25, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Original-Sender: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.devel:300036 Archived-At: On Nov 17 2022, Binbin YE wrote: > /* emacs/src/fileio.c:4587 */ > > if (seekable || !NILP (end)) > total = end_offset - beg_offset; > else > /* For a special file, all we can do is guess. */ > total = READ_BUF_SIZE; > ``` > Judging from the code, it assume the total size would be READ_BUF_SIZE For a non-seekable file this is just a buffer size, see the read loop later in the function (how_much stays zero then). If the file is seekable, the important part is this: /* The file size returned from fstat may be zero, but data may be readable nonetheless, for example when this is a file in the /proc filesystem. */ if (end_offset == 0) end_offset = READ_BUF_SIZE; -- Andreas Schwab, schwab@linux-m68k.org GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1 "And now for something completely different."