A field in a packed struct will often not have the normal alignment for its type. So taking the address of such a field may yield a misaligned pointer, but gcc does not warn about that. This is similar to casting a pointer from a less aligned type to a more aligned type, which does give a warning (with -Wcast-align).
Example: > cat aopf.c #include <stdio.h> void __attribute__((noinline)) f(int *p) { printf("%p\n", p); } struct s { int x; char c; } __attribute__((__packed__)); struct s A[10]; int main(void) { unsigned int i; for (i = 0; i < sizeof(A)/sizeof(A[0]); ++i) { f(&A[i].x); f((int*)(char*)&A[i].x); } return 0; } > gcc -Wall -Wextra -Wcast-align -O aopf.c aopf.c: In function 'main': aopf.c:21:4: warning: cast increases required alignment of target type Ideally both calls to f() should trigger alignment warnings. -- Summary: escaping address of packed field should trigger warning Product: gcc Version: 4.5.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: other AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: mikpe at it dot uu dot se http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41809