svn commit: r278737 - head/usr.sbin/flowctl

[Pedro Giffuni]

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.