Question

I am aware that in C you can pass (or return) a structure by value but you cannot pass an array by value. What happens when the structure contains an array? Will the array (that is within the structure) be copied as the structure is passed (or returned) by value? I have run a sample at ideone.com and it works, but I would like to know where in the standard this is covered (and yes, I have looked).

http://open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf

typedef struct
{
    float aValue;
    int anArray[5];
} myStruct;

myStruct addValueToArray(myStruct in)
{
    myStruct out = in;

    int i;
    for (i = 0; i < 5; i++)
    {
        out.anArray[i] = in.anArray[i] + in.aValue;
    }

    return out;
}

Answer 1

Yes, it will be copied. The entire structure is a value, so it can be passed to a function, returned, and (many seem to forget this but you use it, good!) assigned.

Note that any padding that might be present need not be copied, which makes it possible for = to be faster than a manual call to memcpy() could be, since it can never do that.

Quite hard to find a single place in the PDF that supports this, but I’m not very experienced in looking. Basically, struct instances are “values” in C’s sense, so most of the talk just automatically covers structs.

Like:

(6.2.5, part 1)
The meaning of a value stored in an object or returned by a function is determined by the
type of the expression used to access it.

(6.2.5, part 20)
A structure type describes a sequentially allocated nonempty set of member objects
(and, in certain circumstances, an incomplete array), each of which has an optionally
specified name and possibly distinct type.

(6.8.6.4, part 3)
If a return statement with an expression is executed, the value of the expression is
returned to the caller as the value of the function call expression.