Common Errors - Dynamic Memory Allocation - C Tutorials

What is Dynamic Memory Allocation in C?

Dynamic memory allocation refers to the process of allocating memory manually during runtime using the following standard library functions:

Function	Purpose
malloc()	Allocates a block of memory (uninitialized).
calloc()	Allocates and initializes memory to zero.
realloc()	Resizes previously allocated memory block.
free()	Frees previously allocated memory.

Common Errors During Dynamic Memory Allocation in C

1. Not Checking for NULL After Allocation

When memory allocation fails (e.g., due to insufficient memory), functions like malloc() return NULL. If you blindly use the pointer without checking, it may lead to a segmentation fault.

Incorrect Code:

int *score = (int *)malloc(5 * sizeof(int));
score[0] = 10;  // May crash if malloc fails

Correct Code:

int *score = (int *)malloc(5 * sizeof(int));
if (score == NULL) {
    printf("Memory allocation failed\n");
    return 1;
}
score[0] = 10;

2. Memory Leak (Forgetting to free() Memory)

If you allocate memory and don’t release it using free(), your program may consume more memory over time, leading to a memory leak.

Incorrect Code:

int *quiz = (int *)malloc(5 * sizeof(int));
// ... used but never freed

Correct Code:

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int *quiz = (int *)malloc(5 * sizeof(int));
// ... use quiz
free(quiz);  // Prevents memory leak

3. Dangling Pointer (Access After Free)

Accessing memory after it has been freed leads to undefined behavior and often a crash.

Incorrect Code:

int *data = (int *)malloc(3 * sizeof(int));
free(data);
printf("%d", data[0]);  // Dangling pointer access

Fix:

int *data = (int *)malloc(3 * sizeof(int));
free(data);
data = NULL;  // Safe: avoids accidental access

4. Double Free

Freeing the same memory twice can cause undefined behavior, crashes, or vulnerabilities.

Incorrect Code:

int *marks = (int *)malloc(4 * sizeof(int));
free(marks);
free(marks);  // Error: double free

Fix:

int *marks = (int *)malloc(4 * sizeof(int));
free(marks);
marks = NULL;  // Safe: avoids accidental access

5. Memory Leak due to Reassignment

Reassigning a pointer before freeing it causes a memory leak.

int *list = (int *)malloc(5 * sizeof(int));
list = (int *)malloc(10 * sizeof(int));  // First block is leaked

Fix:

int *list = (int *)malloc(5 * sizeof(int));
free(list);  // Free before reassigning
list = (int *)malloc(10 * sizeof(int));

6. Improper malloc() Size Calculation

Using incorrect size for data type causes insufficient or excessive allocation.

int *arr = (int *)malloc(10);  // May not allocate enough for 10 integers

Correct:

int *arr = (int *)malloc(10 * sizeof(int));

7. Accessing Uninitialized Memory

Using memory before assigning values leads to garbage data or bugs.

int *quiz = (int *)malloc(3 * sizeof(int));
printf("%d", quiz[0]);  // Undefined value

Correct:

int *quiz = (int *)malloc(3 * sizeof(int));
if (quiz == NULL) {
    printf("Memory allocation failed\n");
    return 1;
}
 
// Initialize memory before use
for (int i = 0; i < 3; i++) {
    quiz[i] = 0;
}
 
printf("%d", quiz[0]);  // Now safe and predictable
free(quiz);

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