Impara Pointer Declaration and Dereferencing

In C, understanding the difference between a value and an address is essential when working with pointers. A variable holds a value directly, such as int age = 30;, where age contains the value 30, while a pointer holds the memory address of a variable, such as int *p;, where p can store the address of an int variable. Suppose you have this code:

int number = 42;
int *ptr = &number;

Dereferencing means accessing the value stored at the memory address held by a pointer. Use the * operator to do this:

int value = *ptr; // value is now 42

Here, *ptr gives you the value stored at address 0x1000, which is 42.

main.c


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#include <stdio.h>

int main() {
    int num = 42;
    int *ptr;

    // Assign the address of num to ptr
    ptr = &num;

    // Print the address stored in ptr
    printf("Address of num: %p\n", (void*)ptr);

    // Print the value of num using the pointer
    printf("Value of num via ptr: %d\n", *ptr);

    // Modify the value of num through the pointer
    *ptr = 100;

    // Print the modified value of num
    printf("Modified value of num: %d\n", num);
    return 0;
}

Uninitialized pointers are a frequent source of bugs in C. If you declare a pointer but do not assign it a valid address, it points to an unknown memory location. Using such a pointer can cause your program to crash or behave unpredictably.

int *ptr; // Declared but not initialized
*ptr = 42; // Dangerous: writing to an unknown address

To avoid this mistake always initialize pointers when you declare them. Assign them either the address of a variable or NULL if you do not have an address yet.

int value = 10;
int *ptr = &value; // Safe: initialized with address

Note

The * operator declares a pointer and also dereferences it to access the value stored at its address.

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Sezione 1. Capitolo 2

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int number = 42;
int *ptr = &number;