On Wed, 27 Oct 2021, 00:24 Andrew Makhorin via Gcc-help, < gcc-help@xxxxxxxxxxx> wrote: > Hi, > > Could anyone tell me which gcc options can control layout of bit > fields? > > The problem I encountered is that gcc for Cygwin doesn't follow > System V ABI for i386. I think that's intentional. Have you tried -mabi=sysv ? https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html Namely, ABI says: > > A bit-field must entirely reside in a storage unit appropriate for > its declared type. Thus a bit-field never crosses its unit boundary. > > But in the code generated by gcc under Cygwin bit fields are allocated > contiguously and may cross the unit boundary. On the other hand, gcc > under Linux follows the ABI conventions. > > Example: > > struct { int a; char b; int c:14, d:14; int e; } > s = {0xAAAAAAAA, 0xBB, 0xCCC, 0xDDD, 0xEEEEEEEE}; > > static int *p = (int *)&s; > > int main(void) > { > printf("0x%08X 0x%08X 0x%08X 0x%08X\n", p[0], p[1], p[2], p[3]); > > return 0; > } > > Under Cygwin s.d is splitted between p[1] and p[2]: > > 0xAAAAAAAA 0x774CCCBB 0x00000003 0xEEEEEEEE > > Under Linux all is okay: > > 0xAAAAAAAA 0x000CCCBB 0x00000DDD 0xEEEEEEEE > > > Thank you, > > Andrew Makhorin >
