(source)

        /** world.c */
    1   #include <stdio.h>
    2   #include <stdlib.h>
    3   
    4   int main(int argc, char *argv[])
    5   {
    6     printf("world begins\n");
    7     printf("Hello, World!\n");
    8     printf("world ends\n");
    9     return 0;
   10   }

Output:
world begins
Hello, World!
world ends

(source)

        /** exec01.c */
    1   #include <stdio.h>
    2   #include <stdlib.h>
    3   #include <unistd.h>
    4   
    5   int main(int argc, char *argv[]) 
    6   {
    7     printf("exec01 begins!\n");
    8     execlp("world", "world", (char *) 0);
    9     printf("exec01 ends!\n");
   10     return 0;
   11   }

Output:
exec01 begins
world begins
Hello, World!
world ends