lvella lvella - 1 year ago 84
C Question

struct similarity in C

Consider the two

s below:

struct A {
double x[3];
double y[3];
int z[3];

struct A *a;
int b;
struct A *c;
unsigned d[10];

struct B {
double x[3];
double y[3];
int z[3];

Notice that
struct B
is a strict subset of
struct A
. Now, I want to copy the members
from an instance of
struct A
to an instance of
struct B
. My question is: according to the standards, is it valid to do:

struct A s_a = ...;
struct B s_b;
memcpy(&s_b, &s_a, sizeof s_b);

I.e. is it guaranteed that the paddings for the members, in their sequence of appearance, will be the same, so that I can "partially"
struct A
struct B

Answer Source

It is not guaranteed that struct A's layout starts off the same as struct B's layout.

However, if and only if they were both members of a union:

union X
    struct A a; 
    struct B b;

then it is guaranteed that the common initial sequence has the same layout.

I've never heard of any compiler that would lay out a struct differently if it detected that the struct were a member of a union, so in practice you should be safe!

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