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. -- Steve
