I have this code:

#include <stdio.h>

void f(int* n){
    *n  ;
}

int main() {
    int n= 1;
    f(&n);
    printf("%d\n", n);
    return 0;
}

The value of n doesn't change. If I want to change the value of n I must do this:

#include <stdio.h>

void f(int* n){
    (*n)  ;
}

int main() {
    int n= 1;
    f(&n);
    printf("%d\n", n);
    return 0;
}

Why are the parentheses so important, and what's the difference between these two lines of code?

*n  ;
(*n)  ;

CodePudding user response：

This is by design. The precedence of postfix operator is higher than the pointer dereference operator *. And so the compiler will interpret *n as *(n ). To change the order in which operators are applied, you must use parentheses.

To be clear about the differences:

• *n increments the pointer, returns the original pointer and dereferences it which results in no change to the integer it points to. It is essentially a "no-op", and any half-decent compiler would optimize your entire function away because it does nothing.
• (*n) dereferences the pointer and increments the integer it was pointing to, which is exactly what you wanted.

You can find a full table here: C Operator Precedence

As always, it usually doesn't hurt to add parentheses tactfully, even if it captures the same ordering as the language specifies. This is a courtesy to anyone reading your code.

CodePudding user response：

Because

void f(int* n){
    *n  ;
}

is the same as

void f(int* n){
    n  ; // increment of local (stack) variable.
    *n; // reads undefined memory which is foolish but harmless in this instance
}

and is obviously quite pointless (if you'll pardon the pun.)