// Save to Cdemo.c,  compile with gcc -o Cdemo Cdemo.c, run with ./Cdemo
#include<stdlib.h>
#include<stdio.h>

// something like a class, but no methods or access control, just data
typedef struct {
  int first;
  int second;
} mystruct;

// Oops -  parameters are call-by-value, so x and y are integer values
void badswap (int x, int y) {
  int temp;
  temp = x;
  x = y;
  y = temp;
}

// Pass pointers to the variables we want to swap so x and y 
// are pointer values (ie addresses of integers)
void goodswap (int *x, int *y) {
  int temp;

  temp = *x;  // *x means "deference" x, ie get the value 
              // from the location pointed to by x
  *x = *y;    // copy the value pointed to by y, into the location
              // pointed to by x
  *y = temp;
}

// Everything starts here - like a Java  public static void main(String[] args)
// but giving the argument count explicitly and returning an integer code 
// indicating successful completion (or otherwise) to the calling environment
int main(int argc, char *argv[])
{
  int a, b;     // just like Java
  mystruct *s;  // a pointer to a structure, like a Java reference variable

  a = 1; b = 2;

  // C's print statement. The string is printed, with  values filled 
  // in from the subsequent parameters to the %d placeholders in the string 
  // (%d means print as a decimal)
  printf("a is %d and b is %d\n\n", a, b);

  badswap (a, b);
  printf("a is %d and b is %d\n\n", a, b);

  goodswap (&a, &b);  // pass in the addresses, obtained with &
  printf("a is %d and b is %d\n\n", a, b);

  // create a structure -  like s = new mystruct();
  // malloc allocates the space and returns a generic
  // pointer (ie a "void *"). Its "prototype" is 
  //
  //      void *  malloc  (size_t size);
  //
  // The "(mystruct *)" casts this value to a "mystruct pointer" 
  // to convince the compiler that it can be safely assigned to s

  s = (mystruct *) malloc (sizeof(mystruct));  
   
  s->first  = 3;   s->second = 4; 
  printf("s->first is %d and s->second is %d\n\n", s->first, s->second);

  // see that (*p).f is the same as p->f
  goodswap(&s->first, &s->second);
  printf("(*s).first is %d and (*s).second is %d\n\n", (*s).first, (*s).second);
}