Comment by psewell

Indeed. The long version is at [pdf] http://www.cl.cam.ac.uk/~pes20/cerberus/notes30-full.pdf; it has 85 questions supported by concrete code examples and experimental data, e.g. (one of several questions that refine the above):

Q64. After an explicit write of zero to a padding

byte followed by a write to adjacent members of

the structure, does the padding byte hold a

well-defined zero value? (not an unspecified

value)

  #include <stdio.h>

  #include <stddef.h>

  typedef struct { char c; float f; int i; } st;

  int main() {

    // check there is a padding byte between c and f

    size_t offset_padding = offsetof(st,c)+sizeof(char);

    if (offsetof(st,f)>offset_padding) {

        st s; 

        unsigned char *p = 

          ((unsigned char*)(&s)) + offset_padding;

        *p = 0;

        s.c = 'A';

        s.f = 1.0;

        s.i = 42;

        unsigned char c3 = *p; 

        // does c3 hold 0, not an unspecified value?

        printf("c3=0x%x\n",c3);

    }

    return 0;

  }

Some of the questions have clear answers with respect to either the ISO or de facto standards, but many do not - that's the point.