why after the exit(0) is called, but the child process still remains?

Question

in the parent process I want to start a daemon to do a long time work, so I use the double fork() to create the grandchild process to start the daemon. The question is every once in a while, the child process does not exit successfully, I am sure that the printf() before the exit(0) is called, but when I use the "ps" I can see the child process pid listed, and it never exit. Do you know why this happened, should I call _exit(0) instead of exit(0) to make the child process exit?

startProcess(const char *pidfile, const char *command)
    pid_t   pid;
    char    cmd[1024];
    char    *head = cmd;
    char    *end;
    char    *argv[128] = {0};
    int     argc = 0;

    if( 0 == (pid=fork()) ) { //fork a child 
        if( 0 > setpriority(PRIO_PROCESS, 0, 0) ) {
            perror();
            return;
        }
        pid = fork(); //fork a grandchild
        if(-1 == pid){
            exit(-1);
        }else if(0 == pid){ //grandchild process
            if (snprintf(cmd, sizeof(cmd), "%s", command) <= sizeof(cmd) - 1) {
                /* change command line to argc/argv format */
                do {
                    if (argc >= sizeof(argv)/sizeof(argv[0]) - 1)
                        break;
                    for (; (*head == ' ' || *head == '\t'); head++); //remove space or TAB in the header
                    if (*head == '\0')
                        break;
                    if (NULL != (end = strpbrk(head, " \t"))) {
                        *end = '\0';
                        argv[argc++] = head;
                        head = end + 1;
                    } else {
                        argv[argc++] = head;
                        break;
                    }
                } while (1);
                argv[argc] = NULL; // Maximal value of argc is ARRAY_SIZE(argv) - 1.

                /* execute command to start a daemon*/
                execvp(argv[0], argv);
           }
           //should not enter here
           exit(-1);
        }else{ //still in child process
            printf("child process exit\n");
            exit(0); //child process exit, but sometimes this call seems fail
        }
    }else if(-1 == pid){
        return;
    }else{ //parent process
        int     status = 0;
        int     rv
        pid_t   r_pid = waitpid(pid, &status, WNOHANG);
        if (pid == r_pid) {
            if (WIFEXITED(status) && (0 == WEXITSTATUS(status))) {
                rv = 0;
            } else {
                rv = -1;
            }
        }
        return rv;
    }

Answer 1

Using WNOHANG instead of 0 is surely wrong. If the parent calls waitpid (with WNOHANG) before the child exits, the child will remain in existence as a zombie waiting to be reaped until the parent exits too.