Parameter Passing in C - Pointers, Addresses, Aliases

Question

Could someone please explain the difference between parameter passing in C please? According to professor notes there are 4 different ways to pass parameters

1. Call-by-value
2. Call-by-address (pointer)
3. Call-by-alias
4. Global variable / Static variable

If you could please give an example, I would greatly appreciate that, and your work would be commended.

Answer 1

1. Call-by-value

   Passing the value to a function as a parameter. If the function modifies the variable, the actual variable won't get changed.

   void fun1(int myParam)
   {
       myParam = 4;
   }
   
   void main()
   {
       int myValue = 2;
       fun1(myValue);
       printf("myValue = %d",myValue);
   }
   

   myValue will always be 2.

2. Call-by-address (pointer)

   void fun1(int *myParam)
   {
       *myParam = 4;
   }
   void main()
   {
       int myValue = 2;
       fun1(&myValue);
       printf("myValue = %d",myValue);
   }
   

   Here we are passing the address of myValue to fun1. So the value of myValue will be 4 at the end of main().

3. Call-by-alias

   There is no alias in C as per my understanding. It should be the C++ reference mechanism.

4. Global variable / Static variable

   Global and static variables are variables stored in a common places, accessible by the caller and callee functions. So both caller and callee will be able to access and modify them.

   int myValue = 2;
   void fun1()
   {
       myValue = 4;
   }
   void main()
   {
       myValue = 2
       fun1();
       printf("myValue = %d",myValue);
   }
   

   As you can guess, the value of myValue will be 4 at the end of main().

Hope it helps.

Answer 2

C passes all function parameters by value, period; the formal parameter (in the definition) is a separate object in memory from the actual parameter (in the call). Any updates to the formal parameter have no effect on the actual parameter. You can fake pass-by-reference semantics by using a pointer, but the pointer is passed by value.

True pass-by-reference means that the formal and actual parameters refer to the same object in memory, so any changes to the formal parameter also affect the actual parameter. In practice, a pointer-like object is passed to the subroutine, but that's hidden from the programmer.

C does not support pass-by-reference. C++ supports pass-by-reference with special operators. Old-school Fortran was pass-by-reference.

Global variables are simply visible to both the caller and callee.

Can't speak to pass-by-name or pass-by-alias; never worked with a language that used that mechanism.