Question

Specifically, I was wondering if I could make something like this:

typedef struct {
    char *s; /* still a cstr, with '0' bit at end */
    size_t len;
} str;
str *newstr(char *s) {/*...*/};
void freestr(str *s) {/*...*/};

and do things like this (treat it as a cstr with stdlib/string functions):

int main() {
    str *s = newstr("hello");
    printf("The first character of '%s' is '%c'", *s, (*s)[0]);
    freestr(s);
}

If not, it’s not a big deal—and, of course, I’m not really concerned about wasting a byte.

Answer 1

No, that’s a place where the standard explicitly forbids placing any padding. The address of the first member of a struct and the address of the struct must be the same.

Section 6.7.2.1 (15) in the n1570 draft of the C2011 standard states:

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

(emphasis mine)