Question

How to compare the two pointer in c

Answer 1

A pointer is ultimately a memory address of another variable.

E.g.

#include <stdio.h>

int main()
{
    double salary = 120000.0;
    double * pSalary = &salary;
    printf("Pointer to (the address of) the salary is: %p\n", pSalary);
    printf("The value stored at %p is %lf\n", pSalary, *pSalary);
    return 0;
}

Being a memory address, it really means, it's an integral number depending on the processor architecture. For a 64-bit system, the pointers are stored in a 64-bit unsigned integral type (uint64_t as defined in cstdint.h).

Now, that we know that a memory address is just an integral number, we can do any arithmetic with memory addresses as we'd do with integral numbers, including comparisons, such as ==, <, <=, >=, >, etc. We can not only compare them but do addition and subtraction as well, as in +1 points to the next element of the same size in memory, based on the size of the variable pointed (double in our case below, where double takes up 8 bytes).

#include <stdio.h>

void Compare(double * address1, double * address2)
{
    if (address1 == address2)
        printf("The two addresses are the same: %p\n", address1);
    else if (address1 < address2)
        printf("%p < %p\n", address1, address2);
    else
        printf("%p > %p\n", address1, address2);
}

int main()
{
    double salaries[] = { 120000.0, 100000.0 };
    double * pSalary0 = &salaries[0];
    double * pSalary1 = &salaries[1];
    double * pSalary2 = salaries;
    double * pSalary3 = salaries + 1; // step to the next element from the pointer of the array
    double * pSalary4 = pSalary0 + 1; // step to the next double after a pointer double
    Compare(pSalary0, pSalary1); // <
    Compare(pSalary0, pSalary2); // ==
    Compare(pSalary1, pSalary3); // ==
    Compare(pSalary3, pSalary4); // ==

    return 0;
}

If this is not what you're after, could you be more specific what you'd like to know?