From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/14951 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: non-standard implementation of fflush() Date: Thu, 21 Nov 2019 17:58:49 -0500 Message-ID: <20191121225849.GF16318@brightrain.aerifal.cx> References: <20191121190444.GE16318@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="29904"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mutt/1.5.21 (2010-09-15) Cc: musl@lists.openwall.com To: y38h5z@protonmail.com Original-X-From: musl-return-14967-gllmg-musl=m.gmane.org@lists.openwall.com Thu Nov 21 23:59:08 2019 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by blaine.gmane.org with smtp (Exim 4.89) (envelope-from ) id 1iXvPn-0007cy-U1 for gllmg-musl@m.gmane.org; Thu, 21 Nov 2019 23:59:07 +0100 Original-Received: (qmail 16210 invoked by uid 550); 21 Nov 2019 22:59:05 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 16189 invoked from network); 21 Nov 2019 22:59:05 -0000 Content-Disposition: inline In-Reply-To: Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:14951 Archived-At: On Thu, Nov 21, 2019 at 09:25:31PM +0000, y38h5z@protonmail.com wrote: > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > Am Donnerstag, November 21, 2019 8:04 PM schrieb Rich Felker : > > > On Thu, Nov 21, 2019 at 06:31:02PM +0000, y38h5z@protonmail.com wrote: > > > > > Hello everyone, > > > the implementation of fflush() in musl doesn't seem to conform to > > > the opengroup standard: > > > https://pubs.opengroup.org/onlinepubs/009695399/functions/fflush.html > > > In addition to flushing unwritten data, which is expected, musl > > > flushes the read buffer when calling fflush(). This leads to data > > > loss in bidirectional communication uses. Other standard libraries > > > don't do this. > > > As a reference compare musl to openbsd's libc: > > > https://git.musl-libc.org/cgit/musl/tree/src/stdio/fflush.c > > > https://github.com/openbsd/src/blob/master/lib/libc/stdio/fflush.c > > > I think this is unexpected behavior and should be changed. > > > > ISO C leaves the behavior of fflush undefined unless "stream points to > > an output stream or an update stream in which the most recent > > operation was not input". POSIX further defines it for read, but only > > if the underlying fd is seekable: > > > > "For a stream open for reading with an underlying file > > description, if the file is not already at EOF, and the file is > > one capable of seeking, the file offset of the underlying open > > file description shall be set to the file position of the stream, > > and any characters pushed back onto the stream by ungetc() or > > ungetwc() that have not subsequently been read from the stream > > shall be discarded (without further changing the file offset). > > > > The case of reading from an unseekable stream is left undefined. > > Correct programs should not be doing this at all. The current behavior > > in musl is simply the default effect from not making any special > > provisions to treat unseekable streams differently, since there is not > > any particular behavior we're trying to achieve. > > In my case FILE * is a socket wrapped via fdopen(). According to > https://linux.die.net/man/7/socket it does not support seek but > clearly it makes sense to read from it and also flush written data. > It does not make sense to flush the currently written data and at > the same time flush data which may just have been received without > the application having a chance to read at all. It sounds like you're trying to use the same FILE for both reading and writing to a socket. That simply does not work at all. The only time it's allowed to switch between from reading to writing on a given FILE is after a successful seek operation, which can never happen if the underlying fd is not seekable. See 7.21.5.3 The fopen function, ¶7: "When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file." Violation of such a "shall not" is undefined behavior. For practical purposes, to use stdio with a socket via fdopen, you must limit yourself to one direction through a given FILE. If you want bidirectional access via stdio, you need to dup() the socket file descriptor and fdopen both. None of this is unique to musl. Theoretically an implementation could split the buffer in half to support both read and write at the same time, but there are lots of slippery corner cases involved in doing this, and it penalizes everyone who's actually following the standard and not trying to do undefined things (by giving them half the buffer for the same amount of memory consumed), so it's probably not a good idea to do this. Rich