Copy character using pointers with post-increment

Question

Recently I've been learning C and pointers.

In the book Programming in C by Stephen G. Kochan, I've met an example which I have difficulties of fully understand it.

To copy string from to string to using pointers, the example indicates:

void copyString (char *to, char *from) {
    while ( *from ) 
         *to++ = *from++;

    *to = '\0'; 
}

In my understanding, *from++ is a post-increment of *from ; thus the value of *to++ should be *from only.

For instance, if

`*from` is in the position 1.

`*from++` is in position 2

`*to++` in position 2, 

But: *from++ = *to++ should return values of *from as *to position 1, not 2.

The compiler said it's position 2, the book also said it's position 2.

I'm a little bit confused here. Do you have any feasible explanation for this case?

Answer 1

When using the postfix ++ unary operator, the increment is sequenced after the computation of the value of the operand. So the expression is equivalent to:

*to = *from;
to++ ;
from++ ;

In your example: *to++ = *from++; , the values of *to and *from are obtained and then the value of *from is assigned to *to , then both pointers are incremented.

Answer 2

*to++ = *from++; both to and from have post increment.

You can read like,

1. copy the contains of *from to *to
2. increment to and from .

The postfix operators are evaluated and the operation of increment (decrement) is performed once the evaluation of assignment operator = has finished. So, first the values are copied, then both the pointers are incremented.

Answer 3

As a supplyment to previous answer.

In C, suffix increment ++ has higher precedence than dereference operator * , which means *ptr++ is equivalent to *(ptr++). Check this for more info about operator precedence of C.