Problem in sizeof() operator and poiner declaration

问题

This question was asked in my Sem-2 examination. Question asked us to give the desired output.

int main(void)
{
    int a[] = {10,20,30,40,50,60};

    int (*p1)[2]=a , (*p2)[3]= a;

    if(sizeof(p1)==sizeof(p2))
    printf("%d",*(*p1+2));

    if(sizeof(*p1)==sizeof(*p2))
    printf("%d",*(*(p2+1)));

    return(0);
}

Compiler warnings:

Warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
         initialization from incompatible pointer type [-Wincompatible-pointer-types]

Output that I expect: 20

Output that I get when I run it: 30

Using : gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0

回答1:

Let's ignore the undefined behavior to work out what is probably happening.

p1 and p2 are both pointing to a[0] (ignoring the incompatible pointer types).

p1 and p2 are both pointers. Pointers to object types are generally the same size (assume this is the case), so sizeof(p1)==sizeof(p2) will be true.

p1 is of type int (*)[2], so *p1 is of type int[2]. In most expressions, an array will decay to a pointer to its first element, so in the expression *(*p1+2), *p1 will decay to an int * and will be pointing to a[0]. Therefore *p1+2 will be pointing to a[2]. Therefore *(*p1+2) will be the same as a[2], which has the value 30. Therefore the program prints 30.

An array does not decay to a pointer when it is the operand of the sizeof operator. *p1 is of type int[2] and *p2 is of type int[3], so sizeof(*p1)==sizeof(*p2) is equivalent to sizeof(int[2])==sizeof(int[3]), which is false. Therefore the second printf call that prints the value of *(*p2+1) is not evaluated.

(Let's pretend the second printf is called and that *(*p2+1) is evaluated. *p2 is of type int[3] and in this expression it decays to an int * pointing to a[0]. Therefore *p2+1 points to a[1]. Therefore, *(*p2+1) will be the same as a[1], which has the value 20.)

来源：https://stackoverflow.com/questions/60285607/problem-in-sizeof-operator-and-poiner-declaration

标签

pointers

sizeof