c - Unix system call to print numbers in ascending then descending order -

i have small program needs print numbers in following format:

0 1 2 3 4 3 2 1 0 

which try accomplish c code:

int main() {        int i;      (i = 0; < 5 && !fork(); i++) {         printf("%d ", i);     }      wait(null);      return 0; } 

but prints

0 1 2 3 4 0 1 2 3 0 1 2 0 1 0 

and can't figure out how omit digits. can see what's wrong?

each iteration forks new child. parent exits loop , waits child, , child prints iteration number. however, because there no newline, characters remain buffered. however, each fork duplicates buffer in current state. so, each child inherits of printed values ancestors, , waits child die before exiting. so, last child exits, buffers this:

parent:   "" child-0 : "0 " child-1 : "0 1 " child-2 : "0 1 2 " child-3 : "0 1 2 3 " child-4 : "0 1 2 3 4 " 

child-4 exits (because wait not block), causing stdout buffer flushed. causes child-3 return wait , exits, causing stdout buffer flushed. continues until parent exits, see following (all on 1 line because there no newlines anywhere):

0 1 2 3 4 0 1 2 3 0 1 2 0 1 0 

to fix this, flush buffer before printing. forces child's copy of parent's buffer output in sequence order, , children's own output appear exit in reverse order:

int main() {        int i;      (i = 0; < 5 && !fork(); i++) {         fflush(stdout);         printf("%d ", i);     }      wait(null);      return 0; } 

gives:

0 1 2 3 4 3 2 1 0