Why are the outputs of the parent process and child process intertwined in xv6?

I am learning about xv6 and I create a program like below from a xv6 document:

#include"types.h"
#include"user.h"
#include"param.h"

int main(){
    int pid;
    pid=fork();
    if(pid > 0){
            printf(0,"parent: child=%d\n", pid);
            pid = wait();
            printf(0,"child %d is done\n", pid);
    }
    else if(pid == 0){
            printf(0,"child: exiting\n");
            exit();
    }
    else {
            printf(0,"fork error\n");
    }
}

I use the wait() syscall to make sure the output of the two processes separated but they're not:

$ test
cphiarlde:n tex:iti ncg
hild=4
child 4 is done
pid 3 test: trap 14 err 5 on cpu 0 eip 0xffffffff addr 0xffffffff--kill proc

What should I do to fix this problem?