Date: Sat, 14 Feb 2015 14:53:48 -0500 From: Pedro Giffuni <pfg@FreeBSD.org> To: Ian Lepore <ian@freebsd.org>, Gleb Smirnoff <glebius@FreeBSD.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: svn commit: r278737 - head/usr.sbin/flowctl Message-ID: <54DFA7CC.20305@FreeBSD.org> In-Reply-To: <1423938828.80968.148.camel@freebsd.org> References: <201502132357.t1DNvKda075915@svn.freebsd.org> <20150214193210.N945@besplex.bde.org> <20150214181508.GL15484@FreeBSD.org> <1423938828.80968.148.camel@freebsd.org>
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On 02/14/15 13:33, Ian Lepore wrote:
> On Sat, 2015-02-14 at 21:15 +0300, Gleb Smirnoff wrote:
>> Bruce,
>>
>> On Sat, Feb 14, 2015 at 08:46:58PM +1100, Bruce Evans wrote:
>> B> Using VLAs and also the C99 feature of declarations anwhere, and extensions
>> B> like __aligned(), we can almost implement a full alloca() using the fixed
>> B> version of this change:
>> B>
>> B> /*
>> B> * XXX need extended statement-expression so that __buf doesn't go out
>> B> * of scope after the right brace.
>> B> */
>> B> #define my_alloca(n) __extension__ ({
>> B> /* XXX need unique name. */ \
>> B> char __buf[__roundup2((n), MUMBLE)] __aligned(MUMBLE); \
>> B> \
>> B> (void *)__buf; \
>> B> })
>>
>> I like this idea. But would this exact code work? The life of
>> __buf is limited by the code block, and we exit the block
>> immediately. Wouldn't the allocation be overwritten if we
>> enter any function or block later?
>>
> Why put any effort into avoiding alloca() in the first place? Is it
> inefficient on some platforms? On arm it's like 5 instructions, it just
> adjusts the size to keep the stack dword-aligned and subtracts the
> result from sp, done.
Because it's non-standard and the alloca(3) man page discourages it:
_____
...
BUGS
The alloca() function is machine and compiler dependent; its use is dis-
couraged.
____
It is not disappearing anytime soon though, some even say the man
page is wrong.
Pedro.
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