On Fri, Jan 05, 2024 at 08:55:13AM +0100, Stefano Brivio wrote: > On Thu, 4 Jan 2024 20:51:19 +1100 > David Gibson wrote: > > > On Wed, Jan 03, 2024 at 08:08:34AM +0100, Stefano Brivio wrote: > > > On Wed, 3 Jan 2024 14:54:27 +1100 > > > David Gibson wrote: > > > > > > > I'm not sure where to get the actual text of the standards > > > > > > Let me answer this first: one (the?) trick is to use so-called final > > > drafts, which are made freely available (same as working drafts) by the > > > Working Group. > > > > > > Those are not the same as the standards, but differences from the final > > > draft are also published... and they are usually not substantial. > > > > > > This is _very_ informative: > > > https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-standard-documents > > > > Ah, thanks. > > > > > Wikipedia also has the links, by the way. Anyway, in practice: > > > > > > - C11: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf > > > - C99: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf > > > - C89: > > > https://web.archive.org/web/20200909074736if_/https://www.pdf-archive.com/2014/10/02/ansi-iso-9899-1990-1/ansi-iso-9899-1990-1.pdf > > > > > > > On Tue, Jan 02, 2024 at 07:13:35PM +0100, Stefano Brivio wrote: > > > > > On Sun, 31 Dec 2023 16:58:39 +1100 > > > > > David Gibson wrote: > > > > > > > > > > > On Thu, Dec 28, 2023 at 07:25:18PM +0100, Stefano Brivio wrote: > > > > > > > On Thu, 21 Dec 2023 17:15:46 +1100 > > > > > > > David Gibson wrote: > > > > > > > > > > > > > > > In general, the passt code is a bit haphazard about what's a true global > > > > > > > > variable and what's in the quasi-global 'context structure'. The > > > > > > > > flow_count field is one such example: it's in the context structure, > > > > > > > > although it's really part of the same data structure as flowtab[], which > > > > > > > > is a genuine global. > > > > > > > > > > > > > > Well, the reason is that flow_tab[FLOW_MAX] might be problematically > > > > > > > too big to live on the stack, unlike flow_count. > > > > > > > > > > > > > > But anyway, as far as thoughts of multithreading are concerned, both > > > > > > > should probably be global. And sure, it's more consistent this way. > > > > > > > > > > > > > > > Move flow_count to be a regular global to match. For now it needs to be > > > > > > > > public, rather than static, but we expect to be able to change that in > > > > > > > > future. > > > > > > > > > > > > > > If it's not static, it should be initialised, and that's not done here. > > > > > > > > > > > > Uh... what? "static" here is meaning module-global rather than > > > > > > global-global, which has no bearing on initialisation. AFAIK globals > > > > > > are zero-initialised whether they're static or not. > > > > > > > > > > ...and to my utter surprise, I just discovered that if you talk C11, > > > > > you're right. From the N1570 draft (ISO/IEC 9899:201x), Section 6.7.9 > > > > > "Initialization", clause 10: > > > > > > > > > > If an object that has automatic storage duration is not initialized > > > > > explicitly, its value is indeterminate. If an object that has static > > > > > or thread storage duration is not initialized explicitly, then: > > > > > > > > > > [...] > > > > > > > > > > — if it has arithmetic type, it is initialized to (positive or > > > > > unsigned) zero; > > > > > > > > > > And 'flow_count' has thread storage duration. > > > > > > > > No.. I don't think it does. AFAICT only thread-local variables have > > > > thread storage duration. As a global flow_count will have static > > > > storage duration, even without the static keyword. > > > > > > So, C11 defines static storage duration here: > > > > > > 6.2.4 Storage durations of objects > > > > > > [...] > > > > > > 3 An object whose identifier is declared without the storage-class > > > specifier _Thread_local, and either with external or internal linkage > > > or with the storage-class specifier static, has static storage > > > duration. Its lifetime is the entire execution of the program and its > > > stored value is initialized only once, prior to program startup. > > > > > > do we have any linkage here? I would have said no -- but, going back > > > to C99 for this, "6.2.2 Linkages of identifiers": > > > > > > 5 [...] If the declaration of an identifier for an object has file > > > scope and no storage-class specifier, its linkage is external. > > > > > > which supports your paragraph below. > > > > Right. > > > > > By the way, C11 now says: > > > > > > 6.11.2 Linkages of identifiers > > > > > > 1 Declaring an identifier with internal linkage at file scope without > > > the static storage-class specifier is an obsolescent feature > > > > Ok. I'm not even sure how you would do that. > > By doing what I *thought* you were doing (see below): "int x" at file > scope (outside functions), no static, no extern declaration, nothing. Oh, ok. Even without that, I think the behaviour has to be the same. Plain "int x" at file scope is a public variable, and a linked module could "extern" it, even if that extern isn't visible while we're compiling this module. > > > > > In C99, however (draft > > > > > N1256), Section 6.7.8 "Initialization", clause 10: > > > > > > > > > > If an object that has automatic storage duration is not initialized > > > > > explicitly, its value is indeterminate. If an object that has static > > > > > storage duration is not initialized explicitly, then: > > > > > > > > > > [...] > > > > > > > > > > note the missing "or thread storage duration". > > > > > > > > > > C89, the one I was actually basing my observation on, says, at 3.5.7 > > > > > "Initialization": > > > > > > > > > > If an object that has static storage duration is not initialized > > > > > explicitly, it is initialized implicitly as if every member that has > > > > > arithmetic type were assigned 0 and every member that has pointer type > > > > > were assigned a null pointer constant. If an object that has > > > > > automatic storage duration is not initialized explicitly, its value is > > > > > indeterminate. > > > > > > > > > > so... um. We won't go back to C99. But to me, and maybe others, not > > > > > having a "= 0;" for a "global" means pretty much that we don't rely on > > > > > any particular initial value. > > > > > > > > Again, I'm pretty sure that's not true, even for C99 and C89. AIUI, > > > > 'static' locals and *all* globals have "static storage diration". > > > > > > > > I'm not sure where to get the actual text of the standards but see for > > > > example > > > > > > > > https://en.cppreference.com/w/c/language/static_storage_duration > > > > > > > > Here 'flow_count' has external linkage, thus satisfying the conditions > > > > for static storage duration. > > > > > > Right. Well, for C99 and C11 at least. For C89 things are slightly > > > different: > > > > > > 6.1.2.4 Storage durations of objects > > > > > > [...] > > > > > > An object whose identifier is declared with external or internal > > > linkage. or with the storage-class specifier static has static storage > > > duration. > > > > > > [...] > > > > > > An object whose identifier is declared with no linkage and without the > > > storage-class specifier static has automatic storage duration. > > > > > > You might say it has external linkage. But it was not *declared with* > > > external linkage -- it just happens to have it (C89 and C99 don't > > > differ here). > > > > Hrm. We do have: > > extern unsigned flow_first_free; > > in flow_table.h. Does that cound as declaring with external linkage? > > Gosh, sorry... yes, it also counts as me completely missing it. > > > > > Fwiw, I'm pretty sure the kernel has relied on zero-initialization of > > > > non-static globals for many years. > > > > > > True, and the opposite is even considered as a style issue since 2007, > > > commit f0a594c1c74f ("update checkpatch.pl to version 0.08"). I also > > > found a discussion similar to this one: > > > https://lore.kernel.org/all/20201102184147.GA42288@localhost/#r > > > > > > Anyway... a couple of years before that, it must have been a gcc version > > > in the late 2.x, I actually hit an issue with it. Was it a compiler > > > issue, or the correct interpretation of C89? Or maybe something on the > > > lines of: > > > https://www.thegoodpenguin.co.uk/blog/u-boot-relocation-bss-hang/ > > > > If it was an embedded setup, that last one is certainly possible. > > Zeroing the BSS is typically the loader's job, and I've certainly seen > > loader implementations - particularly in embedded firmware - that got > > this wrong. > > Embedded setup, yes, but in a Linux kernel (early 2.4.x). No 'extern' > there, though. Right. Depending on platform, the kernel sometimes has a loader for a later stage so does need to clear BSS. Or it might need to as a workaround for a broken firmware loader. I'm pretty sure I've also seem Linux bugs on some embedded platforms that involved failing to clear the BSS. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson