For next technboard meeting. On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote: > On Sun, 7 Apr 2024 13:07:06 +0200 > Morten Brørup <m...@smartsharesystems.com> wrote: > > > > From: Mattias Rönnblom [mailto:hof...@lysator.liu.se] > > > Sent: Sunday, 7 April 2024 11.32 > > > > > > On 2024-04-04 19:15, Tyler Retzlaff wrote: > > > > This series is not intended for merge. It insteat provides examples > > > of > > > > converting use of VLAs to alloca() would look like. > > > > > > > > what's the advantages of VLA over alloca()? > > > > > > > > * sizeof(array) works as expected. > > > > > > > > * multi-dimensional arrays are still arrays instead of pointers to > > > > dynamically allocated space. this means multiple subscript syntax > > > > works (unlike on a pointer) and calculation of addresses into > > > allocated > > > > space in ascending order is performed by the compiler instead of > > > manually. > > > > > > > > > > alloca() is a pretty obscure mechanism, and also not a part of the C > > > standard. VLAs are C99, and well-known and understood, and very > > > efficient. > > > > The RFC fails to mention why we need to replace VLAs with something else: > > > > VLAs are C99, but not C++; VLAs were made optional in C11. > > > > MSVC doesn't support VLAs, and is not going to: > > https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-arriving-in-msvc/#variable-length-arrays > > > > > > I dislike alloca() too, and the notes section in the alloca(3) man page > > even discourages the use of alloca(): > > https://man7.org/linux/man-pages/man3/alloca.3.html > > > > But I guess alloca() is the simplest replacement for VLAs. > > This RFC patch series opens the discussion for alternatives in different > > use cases. > > > > The other issue with VLA's is that if the number is something that can be > externally > input, then it can be a source of stack overflow bugs. That is why the Linux > kernel > has stopped using them; for security reasons. DPDK has much less of a security > trust domain. Mostly need to make sure that no data from network is being > used to compute VLA size. >
Looks like we need to discuss this at the next techboard meeting. * MSVC doesn't support C11 optional VLAs (and never will). * alloca() is an alternative that is available on all platforms/toolchain combinations. * it's reasonable for some VLAs to be turned into regular arrays but it would be unsatisfactory to be stuck waiting discussions of defining new constant expression macros on a per-use basis. * there is resistance to using alloca() vs VLA so my proposal is to change only the code that is built to target windows.
