Sven Eckelmann <sven@xxxxxxxxxxxxx> writes: > On Wednesday, 17 November 2021 09:12:55 CET Kalle Valo wrote: >> > https://www.kernel.org/doc/html/v5.15/process/coding-style.html#using-bool > [...] >> >> Yeah, I have been worried about this as well and we should fix this. But >> instead of u8 I would prefer to use bool like mt76 uses: > [...] >> I didn't even know using bool is legal until I saw it in mt76. > > Interesting, I was also not aware of it. And it also seems to have some > interesting implications when assigning values to it (example 4): > > #include <stdbool.h> > #include <stdint.h> > #include <stdio.h> > > struct test { > uint8_t u:1; > uint8_t u2:1; > bool b:1; > bool b2:1; > }; > > int main(void) > { > struct test x; > > x.u = false; > x.b = false; > printf("u %u b %u\n", x.u, x.b); > > x.u = true; > x.b = true; > printf("u %u b %u\n", x.u, x.b); > > x.u = 0; > x.b = 0; > printf("u %u b %u\n", x.u, x.b); > > x.u = 8; > x.b = 8; > printf("u %u b %u\n", x.u, x.b); > > return 0; > } > > > Result: > > u 0 b 0 > u 1 b 1 > u 0 b 0 > u 0 b 1 > > > The last example is basically the reason we see stuff like > > boolean_like_value = !!(some_retrieved_value); > > when using unsigned bitfields instead of bool (bitfields). > > > And the memory layout (on x86-64): > > $ pahole test.o > struct test { > uint8_t u:1; /* 0: 0 1 */ > uint8_t u2:1; /* 0: 1 1 */ > _Bool b:1; /* 0: 2 1 */ > _Bool b2:1; /* 0: 3 1 */ > > /* size: 1, cachelines: 1, members: 4 */ > /* bit_padding: 4 bits */ > /* last cacheline: 1 bytes */ > }; > > > To my surprise, it was already mentioned in one of the discussions [1]. > Was there anything in the discussion which I might have missed and > is a good reason to not use "bool ...:1" in structs? No responses so must be safe to use ;) Changing the subject to gain more visibility. But IMHO we should just switch using boolean bitfields as I would expect mt76 to have noticed any problems by now. -- https://patchwork.kernel.org/project/linux-wireless/list/ https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches
