Deep copying structs with char arrays in C (How to copy the arrays?)

Question

I have the following struct in my C program

struct person {
    char key[50];
    char color[20];
    int age;
};

I want to make a deep copy of this struct. I've got my deep copy function setup however I'm a bit confused about how to deep copy strings. I've heard of people using strcpy and others using strdup.

What I want in my program is for the deep copied person's key and color not to be affected if the original person is freed. Once set, the key and color cannot change. For my purpose, should I be using the strcpy or strdup function?

Answer 1

Post-K&R (i.e. in standard C) you can just assign them. The function below is just to make the example clear, you would always just assign in-place:

void deepCopyPerson(struct person *target, struct person *src)
{
    *target = *src;
}

To elaborate: The char arrays are part of your struct object (true arrays, not only pointers!), and as such are allocated and copied with the object.

In order to satisfy the disbeliefers ;-) I dug around in the standard draft 1570 :

6.5.16 Assignment operators

Semantics

An assignment operator stores a value in the object designated by the left operand. [Followed by type conversion and sequencing considerations which are not relevant here.]

[...]

6.5.16.1 Simple assignment

Constraints

One of the following shall hold:

• [...]

• the left operand has an atomic, qualified, or unqualified version of a structure or union type compatible with the type of the right;

[...]

Semantics

In simple assignment (=), the value of the right operand is converted to the type of the assignment expression and replaces the value stored in the object designated by the left operand.