On Tue, 5 Oct 2021 23:09:08 +0200 (CEST) Jan Engelhardt <jengelh@xxxxxxx> wrote: > On Tuesday 2021-10-05 22:37, Steven Rostedt wrote: > > > >Really, thinking about abstraction, I don't believe there's anything wrong > >with returning a pointer of one type, and then typecasting it to a pointer > >of another type. Is there? As long as whoever uses the returned type does > >nothing with it. > > Illegal. > https://en.cppreference.com/w/c/language/conversion > subsection "Pointer conversion" > "No other guarantees are offered" Basically (one alternative I was looking at) was simply passing around a void pointer. Not sure how the RCU macros would handle that. But to completely abstract it out, I was thinking of just returning void * and accepting void *, but I didn't want to do that because now we just lost any kind of type checking done by the compiler. The tricks I was playing was to keep some kind of type checking. > > >struct trace_pid_list *trace_pid_list_alloc(void) > >{ > > struct pid_list *pid_list; > > > > pid_list = kmalloc(sizeof(*pid_list), GFP_KERNEL); > > [..] > > > > return (struct trace_pid_list *)pid_list; > >} > > struct trace_pid_list { void *pid_list; }; > struct trace_pid_list trace_pid_list_alloc(void) > { > struct trace_pid_list t; > t.pid_list = kmalloc(sizeof(t.orig), GFP_KERNEL); > return t; > } > void freethat(struct strace_pid_list x) > { > kfree(x.pid_list); > } > > Might run afoul of -Waggregate-return in C. The above isn't exactly what I was suggesting. And really, not that I'm going to do this, I could have followed the rest of the kernel with: struct trace_pid_list { int max; [..] }; int *trace_pid_list_alloc(void) { struct trace_pid_list *pid_list; pid_list = kmalloc(sizeof(*pid_list), GFP_KERNEL); [..] return &pid_list->max; } void trace_pid_list_free(int *p) { struct trace_pid_list *pid_list = container_of(p, struct pid_list, max); [..] free(pid_list); } Because we do this all over the kernel. Talk about lying to the compiler ;-) -- Steve
