在C中使用char数组深度复制结构（如何复制数组？）

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 (ie 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.

Answer 2

To perform a deep copy of a struct that contains arrays (without any pointers), a deep copy is simple

struct person x = {"Key", "Color", 42};   /*  initialise to something */
struct person y = x;

This doesn't work if the "strings" are pointers though. It is necessary then to allocate new strings, and then use a function like strcpy() to copy members.

#include <stdlib.h>
#include <string.h>

struct pointer_person
{
    char *key;
    char *color;
    int age;
};

struct pointer_person deep_copy(struct pointer_person p)
{
     struct pointer_person retval;
     retval.key = malloc(strlen(p.key) + 1);
     strcpy(retval.key, p.key);
     retval.color = malloc(strlen(p.color) + 1);
     strcpy(retval.color, p.color);
     retval.age = p->age;
     return retval;
}

int main()
{
   struct pointer_person p;
   struct pointer_person pc;

   p.key = malloc(50);
   strcpy(p.key, "A key");
   p.color = malloc(20);
   strcpy(p.color, "A colour");
   p.key = 42;

   pc = deep_copy(p);

   /* do stuff with pc and c */

   free(p.key);
   free(p.color);
   free(pc.key);
   free(pc.color);
   return 0;
}

Some error checking left out of the above (eg need to check that malloc() succeeds before copying).

Answer 3

If you use strcpy() you have to allocate the memory yourself. strdup() will do that for you. You can use either to create a new memory block which is separate from the original, but naturally strdup() is simpler since it doesn't require a separate malloc(strlen()) call.

Answer 4

There is a difference between strcpy() and strdup() .

• strdup() allocates space and returns a pointer to the copy of the string, you also have to free() the returned pointer.

• strcpy() takes the allocated space and copies the string into it.

It seems that in your case it's strcpy() because the fields of your structure are not pointers so you can't assign to them a pointer to allocated space which is what strdup() returns.

However, as explained in this answer you actually don't need to.