/* structs.c: study the struct in C */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct a_tag { /* does not declare storage; just a pattern for a struct */
   char c;
   int i;
   char *w;
};

void print_a(int test_num, struct a_tag  a); /* pass a struct (copy) */
void print_p(int test_num, struct a_tag *p); /* pass a pointer */
struct a_tag  return_struct(void);           /* return a struct (copy) */
struct a_tag *return_ptr(void);              /* return a pointer */
struct a_tag *return_bad(void);              /* return a pointer */

int main() {
   struct a_tag a1, a2, a3; /* "empty" structs */
   struct a_tag *p1 = &a1;  /* p1 points to a1 */
   struct a_tag *p3, *p4;   /* pointers */
   /* initialize a1 */
   a1.c = 'X'; a1.i = 99;
   a1.w = (char *) malloc(20);
   strcpy(a1.w, "Now is the time");
   print_a(1, a1);          /* print a1 (make copy) */
   print_p(2, p1);          /* print a1 (without copy) */
   print_a(3, a1);          /* print a1 (make copy) */
   print_p(4, p1);          /* print a1 (without copy) */
   a2 = a1;                 /* copy right struct into left struct */
   print_a(5, a2);          /* print a2 (without copy) */
   strcpy(a1.w, "Quick brown foxes");     /* change a1 */
   print_a(6, a2);          /* a2 gets changed also! */
   a3 = return_struct();    /* return a struct */
   print_a(7, a3);          /* print a3 (without copy) */
   p3 = return_ptr();       /* return pointer to a struct */
   print_p(8, p3);          /* print returned struct */
   p4 = return_bad();       /* return a pointer to local auto storage */
   print_p(9, p4);          /* print  returned struct */
   return 0;
}

/* print_a: print a struct passed by value (copied) */
void print_a(int test_num, struct a_tag a) {
   printf("Test %i, ", test_num);
   printf("Passing struct,  fields: c:%c, i:%3i, w:\"%s\"\n",
      a.c, a.i, a.w);
   /* increment below has no effect back in main */
   a.i++;  /* same as (a.i)++ */
}

/* print_p: print a struct where pointer is passed */
void print_p(int test_num, struct a_tag *p) {
   printf("Test %i, ", test_num);
   printf("Passing pointer, fields: c:%c, i:%3i, w:\"%s\"\n",
      (*p).c, p -> i, p -> w);  /* (*p).c same as p -> c */
   /* increment below changes struct back in main */
   p->i++; /* same as (p->i)++ */
}

/* return_struct: return a struct by value (copied) */
struct a_tag return_struct(void) {
   struct a_tag a;
   a.c = 'Y';
   a.i = 88;
   a.w = "My dog has fleas";
   /* can return local auto storage, because it is copied */
   return a;
}

/* return_ptr: return a pointer to a struct */
struct a_tag *return_ptr(void) {
   struct a_tag *p;
   p = (struct a_tag *) malloc(sizeof(struct a_tag));
   p -> c = 'Z';
   (*p).i = 77; /* same as p -> i = 77 */
   p -> w = "Yours does too";
   return p;
}

/* return_bad: return local auto storage through */
/*   a pointer to a struct--a serious error! */
struct a_tag *return_bad(void) {
   struct a_tag *p;
   struct a_tag a;
   p = &a;
   p -> c = 'U';
   (*p).i = 666; /* same as p -> i = 66 */
   p -> w = "Should screw up";
   return p;
}
% lint -m -u structs.c

function returns value which is always ignored
    printf              strcpy          
% cc -o structs structs.c
% structs
Test 1, Passing struct,  fields: c:X, i: 99, w:"Now is the time"
Test 2, Passing pointer, fields: c:X, i: 99, w:"Now is the time"
  (The i field is still 99, so in Test 1, the struct was copied.)
Test 3, Passing struct,  fields: c:X, i:100, w:"Now is the time"
  (The i field is 100, so in Test 2, the struct was changed.)
Test 4, Passing pointer, fields: c:X, i:100, w:"Now is the time"
Test 5, Passing struct,  fields: c:X, i:101, w:"Now is the time"
Test 6, Passing struct,  fields: c:X, i:101, w:"Quick brown foxes"
  (This shows that a change to a1 changed a2 also.)
Test 7, Passing struct,  fields: c:Y, i: 88, w:"My dog has fleas"
Test 8, Passing pointer, fields: c:Z, i: 77, w:"Yours does too"
Test 9, Passing pointer, fields: c:ï, i: 85, w:"ïÿüp"
  (Return value invalid: c should be 'U', i should be 666.)
% CC -o structscc structs.c
% structscc  (same output)
% gcc -o structsgcc structs.c
% structsgcc
. . .
Test 9, Passing pointer, fields: c:, i:68608, w:""
  (Return value invalid: c should be 'U', i should be 666.)