To add user input to your C program, you can use the "scanf" function from the standard input library, "stdio.h". This function reads input from the user and stores it in a specified variable. Here's an example of how to modify the "Hello World" program to include user input:

#include <stdio.h>

int main()
{
char name[20];
printf("Enter your name: ");
scanf("%s", name);
printf("Hello %s\n", name);
return 0;
}

In this program, the "scanf" function is used to read a string (up to 19 characters) from the user and store it in the "name" variable. The user is prompted to enter their name with the "Enter your name:" message. Then, the "printf" function is used to print a personalized greeting to the user, using the value stored in the "name" variable.

Compiling and running the program is the same as before. Note that the size of the "name" array should be one more than the maximum number of characters you want to allow, as it needs to store the null terminator character.
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