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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