Date: Tue, 29 Dec 2015 15:22:32 -0800 From: John Baldwin <jhb@freebsd.org> To: Ian Lepore <ian@freebsd.org> Cc: Warner Losh <imp@bsdimp.com>, src-committers <src-committers@freebsd.org>, svn-src-head@freebsd.org, svn-src-all@freebsd.org, Warner Losh <imp@freebsd.org> Subject: Re: svn commit: r292809 - head/lib/libc/stdio Message-ID: <4400685.QN9T0cbN9C@ralph.baldwin.cx> In-Reply-To: <1451429489.1369.35.camel@freebsd.org> References: <201512272304.tBRN4C5D034464@repo.freebsd.org> <2345870.SHMMVrpc1D@ralph.baldwin.cx> <1451429489.1369.35.camel@freebsd.org>
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On Tuesday, December 29, 2015 03:51:29 PM Ian Lepore wrote: > On Tue, 2015-12-29 at 11:37 -0800, John Baldwin wrote: > > On Monday, December 28, 2015 01:01:26 PM Warner Losh wrote: > > > I'll look at that, but I don't think posix_memalign is the right > > > way to go. > > > The alignment of FILE is more strict than posix_memalign will > > > return. Ian's > > > idea of __alignof__ is the way to go. We allocate them in one block > > > on > > > purpose for performance, and posix_memalign would be a one at a > > > time affair. > > > > posix_memalign gives you whatever alignment you ask for. Using > > __alignof__ > > to determine the alignment instead of hardcoding sizeof(int64_t) > > would > > certainly be an improvement. If you move the glue after the FILE > > objects > > then you can use posix_memalign() directly as so: > > > > void *mem; > > int error; > > > > error = posix_memalign(&mem, MAX(ALIGNBYTES, > > __alignof__(mbstate_t)), > > n * sizeof(FILE) + sizeof(*g)); > > if (error) > > return (NULL); > > p = (FILE *)mem; > > g = (struct glue *)(p + n); > > g->next = NULL; > > g->niobs = n; > > g->iobs = p; > > ... > > > > (This presumes that the requested alignment of 'struct glue' is less > > than > > the alignment needed by FILE which should be true.) > > > > If there's going to be an assumption that __alignof__(glue) <= > __alignof__(FILE), it might be nice to have a static_assert() of that > to prevent a future time bomb similar to the one that exploded on arm > when it turned out the opposite assumption was wrong. A static assert seems sensible, yes. -- John Baldwin
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