From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Warren Date: Wed, 13 Mar 2013 12:36:51 -0600 Subject: [U-Boot] C99 and dynamic arrays In-Reply-To: References: Message-ID: <5140C743.3030508@wwwdotorg.org> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de On 03/13/2013 12:03 PM, M?ns Rullg?rd wrote: > Simon Glass writes: > >> Hi Mans, >> >> On Wed, Mar 13, 2013 at 3:29 AM, M?ns Rullg?rd wrote: >>> Tom Rini writes: >>> >>>> On Tue, Mar 12, 2013 at 7:22 PM, Simon Glass wrote: >>>>> Hi, >>>>> >>>>> Given that we seem to allow C99 features in U-Boot I wonder if it >>>>> would be OK to use dynamic arrays in SPL? >>>>> >>>>> I am trying to replace: >>>>> >>>>> ptr = malloc(size); >>>>> >>>>> with: >>>>> >>>>> char ptr[size]; >>>>> >>>>> to avoid use of malloc in SPL. Can I assume that is permitted? >>>> >>>> Without knowing the underlying mechanics of how that works, "maybe". >>> >>> How it works depends on the compiler. Some compilers implement it by >>> calling malloc(). GCC uses the stack. >>> >>> Regardless of how they are implemented, variable-length arrays should, >>> in my opinion, never be used. There is simply no way they can be used >>> safely since no mechanism for detecting failure is provided. If the >>> requested size is too large, you will silently overflow the stack or end >>> up with an invalid/null pointer. In an environment without full memory >>> protection, errors resulting from this are very hard to track down. >> >> I suppose we could check the available stack space. However I don't >> really see a clear stack bottom in U-Boot - I think it is set up to >> grow downwards as much as needed. I can certainly add sanity checks on >> the input values. > > There is no way to check stack usage from C. > >>> If the size is somehow limited to a safe value, it is more efficient to >>> simply allocate this maximum size statically. >> >> Yes although this does waste BSS. > > Sorry, I meant a statically sized stack allocation. But, there's also no way to detect failure in that case either.