Difference Malloc and Calloc/Local and Static Variable/Auto and Extern

1.       Difference Between Static Memory Allocation and Dynamic Memory Allocation

2.       Difference Between Malloc and Calloc

3.       Difference Between Global variable and Local variable

4.       Difference Between Auto and Extern

5.       Difference Between Local variable and static variable

1.    Difference Between Static Memory Allocation and Dynamic Memory Allocation

Static Memory Allocation	Dynamic Memory Allocation
Static Memory Allocation is performed at compile time.	Dynamic Memory Allocation is performed at run time.
It uses the concept of stack for allocating memory.	It uses the concept of heap for allocating memory.
Example : int a [5];	Example : int *ptr; ptr=(int *)malloc(sizeof(int)*5);

2.    Difference Between Malloc and Calloc

Malloc()	Calloc()
Malloc stands for memory allocation.	Calloc stands for contiguous memory allocation.
Malloc is faster than calloc.	Calloc is slower than malloc.
Malloc takes number of bytes as an argument. Void *malloc (size_in_bytes);	Calloc takes number of blocks and size of every block as arguments. Void *calloc (number_of_blocks, size_of_each_block_in_bytes);
The allocated region is initialized to zero.	The contents of allocated memory contain unpredictable or garbage values.

3.    Difference Between Global variable and Local variable

Global variable	Local variable
Global variable are declared outside all the functions.	Local variables are declared inside function.
Global variable can be accessed by all the function of the program.	Local variable can be accessed only in the function in which they are declared.

4.    Difference Between Auto and Extern

Auto	Extern
It is used to declare variable as auto.	It is used to declare variable as extern.
The scope of the auto variable is merely within the function during which it's declared as auto.	The scope of the extern variable is global. It means it can be accessed by all the function in a program.
If it is not initialized than it contains garbage value.	If it is not initialized than it contains zero value.

5.    Difference Between Local variable and Static variable

Local variable	Static variable
Local variable can be declared only inside function.	Static variable are often declared inside function also as outside function.
If it's not initialized than it contains garbage value.	It it's not initialized than it contains 0 value by default.
It can be declared using auto Keyword.	It can be declared using static Keyword.
It is initialized each time a function is invoked.	It is initialized only once.
It lost its value upon exit from function.	It retains its value even after exit from function.