Question

can anyone explain why the output of

main()   
{   
    printf("hello ");   
    fork();   
    printf("hello ");   
}

is:

hello hello hello hello

and the output of:

main()   
{   
    printf("hello\n");   
    fork();   
    printf("hello ");   
}

is:

hello
hello hello

what difference does \n make w.r.t to buffer?

Answer 1

When you fork the memory of the process is copied. This includes stdio buffers, so if the hello stays in the buffer it will be printed by both processes. Both processes go on about their business and eventually flush their buffers and you see “hello” twice.

Now on most implementations stdout is line-buffered which means a \n triggers a flush. So when the fork happens the buffer is empty. A sure fire way to prevent this would be to flush everything before forking.

EDIT

so why does the hello appears twice in the second line of second
output

There are now two processes (parent & child) executing the same code so that printf is executed twice.