Date: Mon, 23 May 2016 14:31:18 +0200 From: Cedric Blancher <cedric.blancher@gmail.com> To: Matthew Macy <mmacy@nextbsd.org>, Joerg Sonnenberger <joerg@bec.de>, "<freebsd-hackers@freebsd.org>" <freebsd-hackers@freebsd.org> Subject: Re: read(2) and thus bsdiff is limited to 2^31 bytes Message-ID: <CALXu0Ud53O5Rg7cKDp8iQFqdcACLk2o0y8Jfn2CpkjqXoZ%2BUCQ@mail.gmail.com> In-Reply-To: <20160523122131.GC8747@britannica.bec.de> References: <b2515cae-b75d-66e9-4207-3cf100ab3ab0@erdgeist.org> <20160522225414.GB24398@britannica.bec.de> <154dab43060.11208cdfd132112.2616144627831899155@nextbsd.org> <20160522231203.GB25503@britannica.bec.de> <154db353935.dd5e87c1133922.4370692881788049491@nextbsd.org> <20160523122131.GC8747@britannica.bec.de>
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Nothing in POSIX mandates that read()/write() are atomic. Old UNIX, SystemV, AIX, Solaris and HP-UX don't do that nor do they guarantee that. Ced On 23 May 2016 at 14:21, Joerg Sonnenberger <joerg@bec.de> wrote: > On Sun, May 22, 2016 at 06:22:58PM -0700, Matthew Macy wrote: >> >> >> >> ---- On Sun, 22 May 2016 16:12:03 -0700 Joerg Sonnenberger <joerg@bec.de> wrote ---- >> > On Sun, May 22, 2016 at 04:02:02PM -0700, Matthew Macy wrote: >> > > >> > > >> > > >> > > ---- On Sun, 22 May 2016 15:54:14 -0700 Joerg Sonnenberger <joerg@bec.de> wrote ---- >> > > > On Sun, May 22, 2016 at 10:54:30PM +0200, Dirk Engling wrote: >> > > > > When trying to bsdiff two DVD images, I noticed it failing due to >> > > > > read(2) returning EINVAL to the tool. man 2 read says, this would only >> > > > > happen for a negative value for fildes, which clearly was not true. >> > > > >> > > > I would classify that as implementation bug. It seems perfectly sensible >> > > > to turn overly large requests into a short read/write, even for blocking >> > > > files. But erroring out seems to be quite wrong to me. >> > > > >> > > >> > > read(2) takes a size_t so this is clearly an internal bug where it's an int and treating it as a negative value. >> > >> > Not exactly. The reason for cutting it off are many fold. Using int in >> > the kernel is one argument. The requirement for locking the IO range for >> > concurrent read/write operations from other threads is a bigger >> > argument. >> > >> That still doesn't justify EINVAL as a return. Does read(2) need to >> make atomicity guarantees? > > See my first sentence. I consider returning EINVAL for too large buffer > size a bug. Yes, read/write operations should be atomic with regard to > other processes on the system. Atomic meaning in this context that the > read can be observed either completely or not at all. This still doesn't > mean that read must execute the full size. Other cases for short > read/writes are socket, pipes etc. > > Joerg > _______________________________________________ > freebsd-hackers@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org" -- Cedric Blancher <cedric.blancher@gmail.com> Institute Pasteur
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