How to Compare Pointers in C: Syntax and Examples

In C, you compare pointers using relational operators like ==, !=, <, >, <=, and >=. These operators compare the memory addresses stored in the pointers, not the values they point to.

Syntax

You can compare two pointers using these operators:

• == checks if two pointers point to the same address.
• != checks if two pointers point to different addresses.
• <, >, <=, and >= compare the relative positions of addresses in memory.

Example syntax:

if (ptr1 == ptr2) { /* same address */ }

c

if (ptr1 == ptr2) {
    // pointers point to the same memory location
}

if (ptr1 != ptr2) {
    // pointers point to different memory locations
}

if (ptr1 < ptr2) {
    // ptr1 points to a lower memory address than ptr2
}

Example

This example shows how to compare two pointers to integers and print the result.

c

#include <stdio.h>

int main() {
    int a = 5, b = 10;
    int *ptr1 = &a;
    int *ptr2 = &b;
    int *ptr3 = &a;

    if (ptr1 == ptr3) {
        printf("ptr1 and ptr3 point to the same address.\n");
    } else {
        printf("ptr1 and ptr3 point to different addresses.\n");
    }

    if (ptr1 != ptr2) {
        printf("ptr1 and ptr2 point to different addresses.\n");
    }

    if (ptr1 < ptr2) {
        printf("ptr1 points to a lower memory address than ptr2.\n");
    } else {
        printf("ptr1 points to a higher or equal memory address than ptr2.\n");
    }

    return 0;
}

Output

ptr1 and ptr3 point to the same address.
ptr1 and ptr2 point to different addresses.
ptr1 points to a lower memory address than ptr2.

Common Pitfalls

Common mistakes when comparing pointers include:

• Comparing pointer values instead of the data they point to. To compare data, dereference pointers with *ptr.
• Comparing pointers that do not belong to the same array or object can lead to undefined behavior when using relational operators like < or >.
• Assuming pointer comparison checks the content rather than the address.

c

#include <stdio.h>

int main() {
    int a = 5, b = 5;
    int *ptr1 = &a;
    int *ptr2 = &b;

    // Wrong: compares addresses, not values
    if (ptr1 == ptr2) {
        printf("Pointers are equal (wrong assumption).\n");
    } else {
        printf("Pointers are not equal (correct).\n");
    }

    // Correct: compare values pointed to
    if (*ptr1 == *ptr2) {
        printf("Values pointed to are equal.\n");
    }

    return 0;
}

Output

Pointers are not equal (correct).
Values pointed to are equal.

Quick Reference

Tips for comparing pointers in C:

• Use == and != to check if pointers point to the same or different addresses.
• Use relational operators only when pointers point within the same array.
• To compare the data, dereference pointers with *.
• Never compare pointers from unrelated objects with < or > to avoid undefined behavior.

Key Takeaways

Use relational operators like == and != to compare pointer addresses directly.

Relational comparisons (<, >) are safe only for pointers within the same array.

To compare the actual data, dereference pointers with * before comparing.

Avoid comparing pointers from unrelated memory areas with < or > to prevent undefined behavior.

Pointer comparison checks addresses, not the content they point to.