Why do std::cout and printf() not print between usleep() delays? -

i have program supposed print "hello, world!" in scrolling text. using unistd.h library usleep() function, , i'm using std::cout print characters standard output:

    #include <iostream>      #include <stdio.h>        #include <unistd.h>     char hello[13]={'h','e','l','l','o',',',' ','w','o','r','l','d'};      int main (){          for(int i=0; i<14; i++){               std::cout<<hello[i]; //it prints entire string after               usleep(100000);      //100000 ms, should print char after               }                    //every 100 ms.     } 

you might need flush output stream.