A global variable in C programming is a variable that is declared outside of any function. Global variables are accessible from any part of the program, including inside functions. They have global scope, which means they can be accessed from any part of the program.

Here is an example of declaring and using a global variable:

#include <stdio.h>

int num = 10; // declaring and initializing a global variable

void myFunction() {
  printf("The value of num is %d\n", num); // accessing the value of the global variable
}

int main() {
  printf("The value of num is %d\n", num); // accessing the value of the global variable
  myFunction(); // calling a function that uses the global variable
  return 0;
}

In this example, we declared a global variable named num outside of any function. The variable is accessible from any part of the program, including inside functions. We then accessed the value of the global variable from both the main() function and the myFunction() function.

Global variables can be used to store values that are needed throughout the program. They are created when the program starts and they are destroyed when the program ends.

It is generally not recommended to use global variables unless they are absolutely necessary. Global variables can make the program more difficult to understand and maintain because they can be accessed from any part of the program. Instead, it is generally better to use local variables and pass values between functions as parameters.

If a local variable has the same name as a global variable, the local variable takes precedence within the function or block of code. However, the global variable is still accessible from other parts of the program.

In summary, a global variable in C programming is a variable that is declared outside of any function. Global variables have global scope, which means they can be accessed from any part of the program. Global variables can be used to store values that are needed throughout the program, but it is generally better to use local variables and pass values between functions as parameters.