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How To Use Malloc() And Free() Functions In C/C++

How can we use memory in C and C++? How can I allocate something in memory? How can I manage memory dynamically? What kind of memory methods or functions are used for the Dynamic Memory Management? How can I use malloc()? How can I use free()? What is Static Memory Allocation? What is Dynamic Memory Allocation? What is difference between Static Memory Allocation and Dynamic Memory Allocation? Let’s answer these questions for you.

In programs all data and operations during runtime is stored in the memory of our computers, including IoT or other microdevices. This memory is generally a RAM (Random Access Memory) that allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory.

In general, in programming, there are two kinds of memory allocations, Static Memory Allocation, and Dynamic Memory Allocation.

What does static memory allocation mean?

Static Memory Allocation is a memory allocation method that is defined by variable definitions when programming and it has a fixed size which can not be changed at run-time. These variables are defined in our programs using items like constants, strings, pointers, arrays, and record structures. When a program is compiled, the compiler allocates part of the memory to store data. This is called Static Memory Allocation or Compile-time Memory. There are limitations in such static memory allocation to use these kinds of variables.

Because these allocations are done in memory exclusively allocated to a program, we can’t increase the size of static allocations to handle more new elements. Thus, this may result in declaring larger data arrays than required which ultimately means a waste of memory usage. If we used less data than expected static memory Allocations don’t allow us to reduce array size to save memory. It is hard to create advanced dynamic data structures which can be deleted, reallocated variables, linked lists, trees, and other data, which are essential in most real-life programming situations.

What does dynamic memory allocation mean?

Dynamic Memory Allocation is a memory allocation method in which the memory is allocated during the execution of a program (at run-time). Dynamic Memory Management functions/methods involve the use of pointers and standard library functions. Sometimes we use pointers to point to the address of blocks of memory which is allocated dynamically. Using pointers we can easily access or operate on those dynamic memory allocations.

Note that malloc, calloc, realloc functions comes from C language included in the <alloc.h> and it can be used with C++ included in the <cstdlib> library. These functions might be very dangerous in Modern C++ thus using new and delete operations are higher level memory management operations which are a better choice than these ones.

Here is the Comparison Table of new and delete methods with malloc and free methods in C++,

Memory Management FeatureUsing new and delete methodsUsing malloc and free methods
Memory allocated fromfree storeheap
Use of constructor / destructorYesNo
ReturnsFully typed pointervoid* pointer
On failurenever returns NULL, ThrowsReturns NULL
Memory size requiredCalculated by compilerMust be specified in bytes
Handling arraysHas an explicit versionRequires manual calculations
ReallocatingNot handled intuitivelySimple (no copy constructor)
Call of reverseImplementation definedNo
Low memory casesCan add a new memory allocatorNot handled by user code
OverridableYesNo

The C++ language is a great programming language with its ancestor C programming language. C programming language has both Static Memory Allocation and Dynamic Memory Allocation methods. Most used Dynamic Memory Allocation functions are defined in header <stdlib.h> and <cstdlib> mostly we use malloc(), calloc(), realloc() and free().

FunctionSyntax Description
mallocvoid* malloc( size_t size );allocates a block of from memory heap and returns a pointer
callocvoid* calloc( size_t num, size_t size );allocates a block of from memory heap, initializes it to zero and returns a pointer
reallocvoid* realloc( void *ptr, size_t new_size );re-allocates the size of the allocated memory block,copies the contents to a new location
freevoid free( void* ptr );Free block of memory blk allocated from memory heap

Let’s see how we use malloc() and free() functions.

How do we use the malloc() function?

malloc() function is a Dynamic Memory Allocation function that allocates a block of size bytes from the memory heap. It allows a program to allocate memory explicitly as it is needed, and in the exact amounts needed. The allocation is from the main memory. The heap is used for dynamic allocation of variable-sized blocks of memory. Many data structures, for example, trees and lists, naturally employ heap memory allocation.

In the large data models, all the space beyond the program stack to the end of available memory is available for the heap.

Syntax:

If successful, malloc returns a pointer to the newly allocated block of memory. If not enough space exists for the new block, it returns NULL. The contents of the block are left unchanged. If the argument size == 0, malloc returns NULL.

For example we can allocate char array as below,

How can I use malloc() and free() functions?

The free() function is a Dynamic Memory Allocation function that frees allocated block. Free() deallocates a memory block allocated by a previous call to callocmalloc, or realloc.

Syntax:

Is there a full C example of using the C and C++ malloc() and free() functions?

Here is a full C++ example about to use malloc() and free() functions

Note that malloc() is faster than calloc() function because it doesn’t initialize all bytes in the allocated storage to zero. Be sure that you filled with correct data before you output or operate with your dynamic data. In modern C++ we should use std::unique_ptr (Since C++11) and other smart pointers.


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About author

33+ years of coding with more than 30+ programming languages, mostly C++ on Windows, Android, Mac-OS, iOS, Linux and some other operating systems. Dr. Yilmaz Yoru was born in 1974, Eskisehir-Turkey. He graduated from the department of Mechanical Engineering of Eskisehir Osmangazi University in 1997. One year later he started to work in the same university as an assistant. He received his MSc and PhD degrees from the same department of the same university. He has married and he is a father of a son. Some of his interests are Programming, Thermodynamics, Fluid Mechanics and Artificial Intelligence. He also likes the graphical 2D & 3D design and high-end innovations.
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