Local & Global Functions

* Before starting to learn about local and global functions in C, you must have a clear concept of local and global variables.

Local & Global Variables

Variables can be categorized into two main types: local variables and global variables. These two types of variables have different scopes and lifetimes, which determine where and how they can be accessed within a program.

Local Variables:

• Local variables are declared within a specific block of code, such as a function or a compound statement (enclosed within curly braces {}).
• They have a limited scope, meaning they are only accessible within the block in which they are declared. Outside of this block, the variable is not visible or usable.
• Local variables are typically used for temporary storage of data within a specific function or block. They are only alive (exist in memory) while the block in which they are declared is executing.

Example of a local variable:

#include ＜stdio.h>

int main() {
    int x = 10; // x is a local variable
    printf("The value of x is %d\n", x);
    return 0;
}

The value of x is 10

* Local variables are automatically initialized with garbage values if not explicitly initialized by the programmer.

Global Variables

• Global variables are declared outside of any function or block, typically at the beginning of a C program or in a header file that is included in multiple source files.
• They have a global scope, meaning they can be accessed from any part of the program, including multiple functions.
• Global variables have a lifetime equal to the entire duration of the program. They are initialized once at the start of the program and remain in memory until the program terminates.

Example of a global variable:

#include ＜stdio.h>

int x = 10; // x is a global variable

void func1(){
    printf("The value of x is %d\n", x);
}
void func2(){
    x = 20; // Modifying the global variable
}

int main() {
    func1();
    func2();
    printf("Now The value of x is %d\n", x);
    return 0;
}

The value of x is 10    
Now The value of x is 20

* It's considered good practice to declare global variables as 'static' if they should only be accessible within the current source file to limit their scope.

#include ＜stdio.h>

static int add(int a, int b) {
    return a + b;
}

int main() {
    int result = add(5, 3);
    printf("Result: %d\n", result);
    return 0;
}

Result: 8

Global Functions:

• Global functions are functions that are not restricted to a single source file; they are declared outside of any specific block or function.
• Global functions are accessible from any part of the program, including other source files.
• These functions are typically declared in header files and defined in separate source files.
• Global functions are commonly used for providing reusable functionality that can be used throughout a program.

Here's an example of a global function in C:

math.h

 #ifndef MATH_H
 #define MATH_H
 
 int add(int a, int b);
 
 #endif

math.c

#include "math.h"

int add(int x, int y)
{
    return x + y;
}

main.c

#include ＜stdio.h>
#include "math.h"

int main()
{
    int result = add(10, 15);
    printf("Result: %d", result);
    return 0;
}

Result: 25

In this example, 'add' is a global functions declared in the header file 'math.h' and defined in the source file 'math.c'. It can be used in other source files, such as 'main.c'.