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15.12 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.