What is Free Pool of Memory in C?

What is the Free Pool of Memory?

In C programming, the free pool of memory refers to the part of memory available for dynamic allocation during runtime. This memory is usually located in the heap and can be used when a program needs memory but doesn’t know the size in advance (at compile time).

The programmer manages this memory using specific functions from the C standard library (stdlib.h). The free pool is called “free” because it is available for use until the programmer allocates or deallocates it.

Memory Layout of a C Program

To understand the free pool better, let’s take a quick look at the typical memory layout of a C program:

+--------------------+
|      Stack         | <- Local variables, function calls
+--------------------+
|      Heap          | <- Dynamic memory (Free pool)
+--------------------+
|   Uninitialized    |
|     Data (BSS)     |
+--------------------+
|   Initialized Data |
+--------------------+
|     Text (Code)    |
+--------------------+

• The heap section (highlighted above) is where the free pool of memory exists.
• It grows upward as more memory is allocated.
• The stack grows downward, and if both grow too much, they may collide and cause a segmentation fault.

Why Use a Memory Pool?

• Performance Enhancement: Allocating memory from a pool is faster than using malloc(), as it avoids system calls.​
• Reduced Fragmentation: By reusing fixed-size blocks, memory pools minimize fragmentation, leading to more predictable memory usage.​
• Deterministic Behavior: Memory pools offer consistent allocation and deallocation times, which is crucial for real-time systems.

Functions for Memory Management

1. malloc():

• Allocates a specified number of bytes in memory.
• Returns a pointer to the allocated memory.

int *ptr = (int*)malloc(sizeof(int));  // Allocates memory for one integer

2. calloc():

• Allocates memory for an array and initializes it to zero.
• Takes two arguments: the number of elements and the size of each element.

int *ptr = (int*)calloc(5, sizeof(int));  // Allocates memory for 5 integers and sets them to zero.

3. realloc():

• Resizes previously allocated memory to a new size.
• If the new size is smaller, the memory is freed; if larger, new memory is added.

ptr = (int*)realloc(ptr, 10 * sizeof(int));  // Resizes the memory to hold 10 integers

4. free():

Frees the dynamically allocated memory and returns it to the free pool of memory.

free(ptr);  // Frees the memory allocated to ptr

Example of Using the Free Pool of Memory

#include <stdio.h>
#include <stdlib.h>
 
int main()
{
    // Dynamically allocates memory for storing 3 test scores
    int *scorePtr = (int*)malloc(3 * sizeof(int));
 
    // Check if allocation from the free pool was successful
    if (scorePtr == NULL) {
        printf("Memory allocation from pool failed!\n");
        return 1;  // Exit due to memory failure
    }
 
    // Assigning scores for certification tests
    scorePtr[0] = 85;  // C Programming
    scorePtr[1] = 90;  // Data Structures
    scorePtr[2] = 88;  // Algorithms
 
    // Display the stored scores
    printf("Sanfoundry Test Scores:\n");
    printf("C Programming : %d\n", scorePtr[0]);
    printf("Data Structures: %d\n", scorePtr[1]);
    printf("Algorithms     : %d\n", scorePtr[2]);
 
    // Return the memory to the free pool
    free(scorePtr);
 
    return 0;
}

Output:

Sanfoundry Test Scores:
C Programming : 85
Data Structures: 90
Algorithms     : 88

The program allocates memory for three test scores using malloc(). It checks if the allocation is successful. If successful, it stores and prints scores for C Programming, Data Structures, and Algorithms. After use, it frees the memory to avoid leaks.

Common Mistakes to Avoid

• Memory Leaks: Failing to free dynamically allocated memory.
• Double Free: Freeing the same memory block more than once.
• Dangling Pointers: Using a pointer after its memory has been freed.
• Unchecked Allocation: Not checking if malloc() or calloc() returned NULL.

Advanced Memory Pool Techniques

• Multiple Pools: Use different memory pools for different object sizes. For example, have one pool for small objects and another for large ones. This makes memory use more efficient.
• Dynamic Pool Expansion: Allow the memory pool to grow when needed. This keeps the program running smoothly without wasting memory.
• Garbage Collection Integration: Use garbage collection to automatically free memory that is no longer in use.

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