malloc function - Step-by-Step Execution

Concept Flow - malloc function

Call malloc(size)

↓

Check if memory available

↓

Return pointer

↓

Use allocated memory

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Free memory when done

The malloc function requests memory from the system. If available, it returns a pointer to the memory; otherwise, it returns NULL.

Execution Sample

C

#include <stdlib.h>

int *p = malloc(sizeof(int));
if (p != NULL) {
  *p = 10;
  free(p);
}

This code allocates memory for one int, stores 10 in it, then frees the memory.

Execution Table

Step	Action	Evaluation	Result
1	Call malloc(sizeof(int))	Request memory for 4 bytes (typical int size)	Pointer p assigned to allocated memory address (e.g., 0x1000)
2	Check if p != NULL	Pointer is valid (not NULL)	Condition true, proceed
3	Assign *p = 10	Store value 10 at memory address p	Memory at p now holds 10
4	Call free(p)	Release allocated memory back to system	Memory freed, p becomes dangling pointer
5	End	Program ends or continues	Memory properly managed

💡 malloc returns NULL if memory is unavailable; here allocation succeeds.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4
p	uninitialized	0x1000 (allocated address)	0x1000 (points to int with value 10)	0x1000 (dangling pointer, memory freed)

Key Moments - 3 Insights

Why do we check if malloc returned NULL?

Because malloc returns NULL if it cannot allocate memory. Checking prevents using invalid memory, as shown in step 2 of the execution_table.

What happens if we forget to call free?

The allocated memory stays reserved and is not returned to the system, causing a memory leak. Step 4 shows freeing memory to avoid this.

Is the pointer p still valid after free?

No, after free(p), p points to freed memory and should not be used. This is shown in step 4 where p becomes a dangling pointer.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of pointer p after step 1?

A An allocated memory address like 0x1000

B NULL

C Value 10

D Undefined

💡 Hint

Check the 'Result' column in step 1 of the execution_table.

At which step does the program store the value 10 in allocated memory?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in the execution_table.

If malloc fails and returns NULL, which step would change in the execution_table?

A Step 3 would assign 10 to *p

B Step 1 would assign NULL to p

C Step 4 would free memory

D Step 2 condition would be true

💡 Hint

Refer to step 1 and step 2 in the execution_table about malloc return values.

Concept Snapshot

malloc(size) requests memory from the system.
Returns pointer to allocated memory or NULL if unavailable.
Always check if pointer is NULL before use.
Use free(pointer) to release memory when done.
Avoid using pointer after free (dangling pointer).

Full Transcript

The malloc function in C asks the system for a block of memory of a given size. If the system has enough memory, malloc returns a pointer to that memory. If not, it returns NULL. It is important to check if the pointer is NULL before using it to avoid errors. After using the allocated memory, you must call free to give the memory back to the system. Using the pointer after freeing it is unsafe because it points to memory that is no longer yours. This example shows allocating memory for an int, storing 10 in it, and then freeing it properly.