c - sizeof struct inside struct -

struct s1 {     char c;     int i; };  struct s3 {     char c1;     struct s1 s;     double c2; }; 

i trying calculate sizeof(s3), program 24. don't know offset of struct s1 inside s3, there rule offset? thought "the offset of s1 must n times of size of itself, i.e., n * sizeof(s1)", however, found not true. help.

the c standard intentionally flexible on this.

all guarantees (i) address of first element in struct same address of struct, , (ii) data members appear in order declared in struct. (iii) empty struct have non-zero sizeof. last 1 means pointer arithmetic valid on null structures.

in particular, padding (of amount) allowed inserted between structure member, , @ end.