Question

It is an academic question so the reason is to understand the output.

I have a code:

int main(int argc, char **argv) {
            int k ;
            while(*(++argv)) {
                    k = fork();
                    printf("%s ",*argv);
            }
    return 0;
    }

running the program with : prog a b
The output is :

  a b a b a b a b

Why do I get this result?

Answer 1

As suggested by Chris Lutz in the comments, you are observing the effect of a static buffer used by printf being duplicated by the fork() call. The two processes created by the first fork() do not print b (as you could expect, and as happens if you force a flush). They both print a b because they both have a pending, unflushed a in their respective buffers.

There are 4 processes (2^2, including the initial one), they all only really print at exit when the buffer is flushed, and they all have a b in their respective buffers at that time.