On Sat, Mar 17, 2018 at 11:40 AM, Jonathon Reinhart <jonathon.reinh...@gmail.com> wrote: > This was originally posted on Stack Overflow: > https://stackoverflow.com/a/49339771/119527 > > The following program: > > #include <stdio.h> > > static void pshort(short val) > { > printf("0x%hx ", val); > } > > int main(void) > { > short A[] = {1, 2, 3, 4, 5, 6}; > > #define EXP ((short*)((char*)A + 7)) > > short *p = EXP; > short q = *EXP; > > pshort(*p); > pshort(q); > pshort(*EXP); > printf("\n"); > > return 0; > } > > ...when compiled on x86-64 with > > gcc -O0 -fno-strict-aliasing -g -Wall -Werror endian.c > > ...produces the following unexpected output > > 0x500 0x500 0x4 > > ...on all available versions of GCC, from 4.9 thru 7.3.1. > > It appears that GCC is actually generating different code when the > expression is used directly as an argument, versus when used with > intermediate variables. > > The language lawyers declared that UB is UB. But -fno-strict-aliasing > seems to be ineffective here, and this feels like a bug.
As mentioned this is the right mailing list for this kind of question. But this is undefined not due to aliasing but alignment reasons. short has an alignment requirement of 2 while you made it unaligned. Thanks, Andrew > > Jonathon Reinhart
