realloc function - Step-by-Step Execution

Concept Flow - realloc function

Start with pointer p

↓

Call realloc(p, new_size)

↓

Check if realloc returned NULL?

Yes→Handle error, keep old p

No↓

p now points to resized memory

↓

Use resized memory

↓

Free memory when done

The realloc function tries to resize a memory block pointed by p. If successful, it returns a new pointer; if it fails, it returns NULL and the old pointer remains valid.

Execution Sample

C

int *p = malloc(2 * sizeof(int));
p[0] = 10; p[1] = 20;
int *temp = realloc(p, 4 * sizeof(int));
if (temp != NULL) {
    p = temp;
    p[2] = 30; p[3] = 40;
} else {
    // Handle realloc failure
}

This code allocates memory for 2 integers, stores values, then resizes to hold 4 integers and adds more values safely by checking realloc's return value.

Execution Table

Step	Action	Pointer p	Memory Size	Result/Notes
1	malloc(2 ints)	p -> block1	2 ints	Memory allocated for 2 ints
2	Assign p[0]=10, p[1]=20	p -> block1	2 ints	Values stored in memory
3	realloc(p, 4 ints)	p -> block2 or block1	4 ints	Memory resized, pointer updated if successful
4	Assign p[2]=30, p[3]=40	p -> block2 or block1	4 ints	New values stored in resized memory

💡 Execution ends after storing new values in resized memory block

Variable Tracker

Variable	Start	After malloc	After realloc	Final
p	NULL	Address block1	Address block2 or block1	Address block2 or block1
Memory Size	0	2 ints	4 ints	4 ints

Key Moments - 3 Insights

What happens if realloc returns NULL?

If realloc returns NULL (see step 3 in execution_table), the original pointer p is still valid and points to the old memory. You must not overwrite p without checking, or you lose the reference to allocated memory causing a memory leak.

Does realloc always move memory to a new location?

No, realloc may resize the memory in place (p points to same block) or move it to a new block (p points to new block). This is why p must be assigned the return value of realloc.

Why assign p = realloc(p, new_size) instead of just calling realloc(p, new_size)?

Because realloc returns the new pointer to the resized memory. If you don't assign it, you won't know the new location and may access invalid memory.

Visual Quiz - 3 Questions

Look at the variable_tracker table, what is the value of p after realloc?

A Same as after malloc or new address

B NULL

C Unchanged NULL

D Undefined

💡 Hint

Check 'After realloc' column for variable p in variable_tracker

At which step in execution_table is the memory resized?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for realloc call in execution_table

If realloc fails and returns NULL, what should you do to avoid losing the original memory?

A Assign p = realloc(p, new_size) without check

B Free p immediately

C Use a temporary pointer to store realloc result first

D Ignore the return value

💡 Hint

Refer to key_moments about handling realloc failure

Concept Snapshot

realloc(pointer, new_size) resizes memory block.
Returns new pointer or NULL if fails.
Always assign result to a temp pointer first.
If NULL, old pointer is still valid.
Use realloc to grow or shrink allocated memory.

Full Transcript

The realloc function in C changes the size of a memory block pointed to by a pointer. First, you allocate memory with malloc. Then you can call realloc to resize that memory. realloc returns a new pointer to the resized memory or NULL if it fails. If it returns NULL, the original pointer remains valid and must not be overwritten to avoid memory leaks. Always check realloc's return value before assigning it to your pointer. This example shows allocating memory for 2 integers, storing values, then resizing to hold 4 integers and adding more values. The execution table traces each step, showing how the pointer and memory size change. Key moments clarify common confusions about realloc's behavior and safe usage.