There are three types of Variables in C Program

1. Local variable
2. Global variable
3. Environment variable

Local variable in C Progarm 

• The scope of local variables will be within the function only.
• These variables are declared within the function and can’t be accessed outside the function.
• In the below example, m and n variables are having scope within the main function only. These are not visible to test function.
• Like wise, a and b variables are having scope within the test function only. These are not visible to main function.

Global variable in C Program

• The scope of global variables will be throughout the program. These variables can be accessed from anywhere in the program.
• This variable is defined outside the main function. So that, this variable is visible to main function and all other sub functions.

Enviornment variable in C Program

• Environment variable is a variable that will be available for all C  applications and C programs.
• We can access these variables from anywhere in a C program without declaring and initializing in an application or C program.
• The inbuilt functions which are used to access, modify and set these environment variables are called environment functions.
• There are 3 functions which are used to access, modify and assign an environment variable in C.
  1. setenv()
  2. getenv()
  3. putenv()