15.13 Structure Assignment

Assignment operating on a structure type copies the structure. The left and right operands must have the same type. Here is an example:

#include <stddef.h>  /* Defines NULL. */
#include <stdlib.h>  /* Declares malloc.  */
…

struct point { double x, y; };

struct point *
copy_point (struct point point)
{
  struct point *p
    = (struct point *) malloc (sizeof (struct point));
  if (p == NULL)
    fatal ("Out of memory");
  *p = point;
  return p;
}

Notionally, assignment on a structure type works by copying each of the fields. Thus, if any of the fields has the const qualifier, that structure type does not allow assignment:

struct point { const double x, y; };

struct point a, b;

a = b;            /* Error! */

See Assignment Expressions.

When a structure type has a field which is an array, as here,

struct record
  {
    char *name;
    int data[4];
  };

struct record r1, r2;

structure assigment such as r1 = r2 copies array fields’ contents just as it copies all the other fields.

This is the only way in C that you can operate on the whole contents of a array with one operation: when the array is contained in a struct. You can’t copy the contents of the data field as an array, because

r1.data = r2.data;

would convert the array objects (as always) to pointers to the zeroth elements of the arrays (of type struct record *), and the assignment would be invalid because the left operand is not an lvalue.