Understanding the Importance of int** ar vs int* ar in C Pointers

Explore why `int** ar` is essential for memory allocation in C programming, and learn how it can help prevent memory leaks efficiently. --- This video is based on the question https://stackoverflow.com/q/71446627/ asked by the user 'HTMT' ( https://stackoverflow.com/u/18444028/ ) and on the answer https://stackoverflow.com/a/71447250/ provided by the user 'H.S.' ( https://stackoverflow.com/u/8542346/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why use int** ar and not int* ar? Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Understanding the Importance of int** ar vs int* ar in C Pointers When it comes to programming in C, pointers play a crucial role in managing memory. However, many beginners often find themselves puzzled by the intricacies of pointer types. One common question that arises is: Why should we use int** ar instead of int* ar when dealing with dynamic memory allocation? This question is particularly relevant when you're trying to create and manipulate dynamic arrays in C. In this guide, we will break down the difference between int** and int*, specifically in the context of memory allocation, and explain why it is essential to understand this difference to avoid common pitfalls like memory leaks. The Problem at Hand Imagine you have a function that needs to allocate memory for an array based on user input. The confusion arises when you try to pass the pointer to this function. The original code snippet suggests using int** ar instead of int* ar. Let’s clarify why this is necessary. What Happens in the Code? In the provided code example, we see a well-structured approach to handle dynamic memory allocation, which does the following: Declare/define a pointer in the main() function. Allocate memory for that pointer in the input() function based on user input. Print the contents of that memory using another function called output(). Finally, deallocate the memory and exit the program. The Core of the Confusion Let's focus on why int** ar is utilized in the input function. Many might think they could simply switch it to int* ar and expect it to work, but they are likely to encounter problems. Here’s why: Memory Allocation Requires a Pointer to a Pointer Accessing the Original Pointer: To modify the pointer ar defined in main(), the input() function needs access to the address of ar. Thus, you pass the address of ar (i.e., &ar). Doing so allows the input function to alter the original pointer's address in main(). [[See Video to Reveal this Text or Code Snippet]] Dereferencing: Inside the input() function, you can dereference input_ar (which is essentially ar passed from main()) to allocate memory: [[See Video to Reveal this Text or Code Snippet]] Here, *input_ar points to ar in main(), thus allowing memory allocation directly to it. What Happens If You Use int* ar? If you try to declare ar as int* instead of int**, and pass it to the input() function, you won’t be able to store the allocated memory address back into the original pointer ar defined in main(). Instead, you would have created a separate pointer in the input() function that points to a different memory location after the allocation, leading to: Memory Leaks: Because the original pointer now simply points to the newly allocated memory in input(), and after it exits, this reference is lost. Undefined Behavior: Your program may crash, or you may end up with invalid memory access when trying to deallocate non-allocated pointers. The Correct Approach Structure of the Input Function Here’s a quick outline of how to correctly define the functions: [[See Video to Reveal this Text or Code Snippet]] Use int** input_ar to access and modify the original pointer ar. Use malloc correctly with dereferencing to populate the values efficiently. Conclusion Understanding the distinction between int** ar and int* ar in C is essential for effective memory management when crafting programs. It ensures proper memory allocation and deallocation without leaks or undefined behaviors. Always remember: if you need to modify a pointer in another function, pass it as a pointer to a pointer. By grasping these concepts, you can enhance your skill set and become more proficient in handling pointers in C. Happy coding!