C_135 Dynamic Memory Allocation using realloc() | C Language Tutorials

Summary

In this video, the focus is on dynamic memory allocation in C, specifically explaining the `realloc` function. The script covers how `realloc` resizes a previously allocated memory block, either increasing or decreasing its size. The function is demonstrated with a practical example of reallocating memory for integers based on user input. Key concepts like memory block expansion, memory address checking, and proper memory deallocation using `malloc`, `realloc`, and `free` are discussed. This informative video provides a thorough understanding of memory management, highlighting important points that developers need to know when working with dynamic memory in C.

Takeaways

• 😀 Dynamic memory allocation in C allows you to allocate memory at runtime, using functions like malloc and calloc.
• 😀 The realloc function in C is used to resize previously allocated memory, allowing you to either increase or decrease the memory block size.
• 😀 The realloc function requires two arguments: the pointer to the previously allocated memory and the new desired size.
• 😀 Realloc can return the same pointer if the memory block is successfully expanded. If the memory block cannot be expanded, realloc allocates a new block and returns the new address.
• 😀 If realloc successfully reallocates memory, it retains the previously stored data in the new memory block.
• 😀 If the new memory block cannot be allocated due to space constraints, realloc will allocate a new block elsewhere, copy the existing data, and free the original block.
• 😀 If realloc is called with a new size of 0, it functions like free() and deallocates the memory.
• 😀 If the requested memory block size is smaller than the original size, realloc will shrink the memory, potentially losing some data.
• 😀 The realloc function returns a void pointer, which requires typecasting to the correct pointer type (e.g., int*, float*) for the specific data type being used.
• 😀 It’s essential to check if realloc returns NULL, indicating a failure to reallocate memory, which could lead to memory issues in the program.

Q & A

What is the purpose of the `realloc` function in C programming?

The `realloc` function is used to resize a previously allocated memory block. It can either increase or decrease the size of the memory block while preserving the original data (unless the size is reduced).

Can `realloc` be used without previously allocated memory?

No, `realloc` should only be used if memory has been previously allocated using `malloc` or `calloc`. Using `realloc` on a pointer that hasn't been allocated memory will result in undefined behavior.

What are the two main arguments that `realloc` takes?

The two main arguments that `realloc` takes are the previously allocated memory pointer and the new size (which could either be larger or smaller than the original allocation).

What happens if the `realloc` function is unable to resize the memory block?

If `realloc` cannot resize the memory block (due to insufficient memory), it will allocate a new memory block elsewhere, copy the old data to the new location, free the original memory, and return the address of the new block.

What is the return type of `realloc`, and how should it be handled?

The return type of `realloc` is a void pointer. Depending on the type of data being stored, it should be typecasted to the appropriate pointer type (e.g., `int*`, `char*`, etc.) before being used.

What does it mean if `realloc` fails, and how can we handle it?

If `realloc` fails, it returns `NULL`, indicating that the memory allocation was unsuccessful. It is important to check for `NULL` before using the pointer. If `realloc` fails, the original pointer remains unchanged and the previously allocated memory is still valid.

What happens when you decrease the size of a memory block using `realloc`?

When the size of a memory block is decreased using `realloc`, data beyond the new size is lost. The remaining data will be preserved, but any elements exceeding the new size will be discarded.

What happens when you use `NULL` as the pointer argument in `realloc`?

If `NULL` is passed as the pointer argument in `realloc`, it behaves like `malloc` and allocates new memory. In this case, `realloc` does not attempt to resize a previously allocated block but instead allocates fresh memory of the given size.

Can you use `realloc` to free a memory block?

Yes, if `realloc` is passed `NULL` as the new size, it behaves like the `free` function and will deallocate the memory previously allocated by `malloc` or `calloc`.

What is the importance of checking the addresses before and after calling `realloc`?

By checking the addresses before and after calling `realloc`, you can verify whether the memory block was resized in place (i.e., the address remains the same) or if it was moved to a new location. This helps ensure that the original data is intact and that the new memory location is correctly referenced.