On Fri, Aug 11, 2023 at 03:28:17PM -0400, Steven Rostedt wrote: > On Thu, 10 Aug 2023 16:17:57 +0200 > Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote: > > > > Btw, it's easy to enforce IIUC, i.e. by dropping > > > > > > #ifndef _FOO_H > > > #define _FOO_H > > > #endif > > > > > > mantra from the headers. > > > > No, you can't do that, because some headers legitimately include other > > headers, often for type definitions. Say some struct definition where > > one of the members is another struct (struct list_head being an obvious > > example). Or a static inline function. > > > > We _also_ don't want to force everybody who includes a.h to ensure that > > they first include b.h because something in a.h needs stuff from b.h. > > > > So include guards must be used. They are a so well-known idiom that gcc > > even has special code for handling them: If everything in a foo.h file > > except comments is inside an ifndef/define/endif, gcc remembers that > > that foo.h file has such an include guard, so when gcc then encounters > > some #include directive that would again resolve to that same foo.h, and > > the include guard hasn't been #undef'ed, it doesn't even do the syscalls > > to open/read/close the file again. > > I hope Andy was just joking with that recommendation. Too radical to be true to implement. But it's always good to have a rationale (thanks Rasmus) behind existing approach. -- With Best Regards, Andy Shevchenko
