How to do it…

Follow these steps to create a recipe that registers a function that executes automatically when a program terminates normally:

1. Register a function using the atexit function.
2. Allocate some memory dynamically and allow that memory to be pointed to by a pointer.
3. Ask the user to enter a string and assign the string to the dynamically allocated memory block.
4. Display the entered string on the screen.

5. When the program terminates, the function registered via the atexit function is invoked automatically.
6. The registered function simply frees up the dynamically allocated memory so that it can be used by other applications.

The program for registering a function that automatically executes when a program terminates is as follows (atexistprog1.c):

#include <stdio.h> 
#include <stdlib.h> 

char *str; 
void freeup() 
{ 
    free(str); 
    printf( "Allocated memory is freed  \n"); 
} 

int main() 
{ 
    int retvalue; 
    retvalue = atexit(freeup); 
    if (retvalue != 0) { 
        printf("Registration of function for atexit () function 
          failed\n"); 
        exit(1); 
    } 
    str = malloc( 20 * sizeof(char) ); 
    if( str== NULL ) 
    { 
        printf("Some error occurred in allocating memory\n"); 
        exit(1); 
    } 
    printf("Enter a string "); 
    scanf("%s", str); 
    printf("The string entered is %s\n", str); 
}

Now, let's go behind the scenes to understand the code better.