linux c 关于vfork正确执行的问题

Question

在编译下述代码的时候

如果已经宏定义了DEBUG
产生如下结果：

child : process id is 4287
child : parent process id is 4286

havent run strcpy -> buf:THIS IS parent process line
have run strcpy -> buf:parent : process line
parent : process id is 4286

如果没有宏定义DEBUG
产生如下结果：

child : process id is 4223
child : parent process id is 4222

havent run strcpy -> buf:child : process write
have run strcpy -> buf:parent : process line
parent : process id is 4222

point is ： 为什么 “havent run strcpy ->” 所在行的显示会有不同 ？
编译环境：ubuntu 10.04.3, gcc
代码如下

#include <sys/types.h>
#include <unistd.h>

#include <sys/stat.h>
#include <fcntl.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define DEBUG

char  buf[100];

int main(int argc, char **argv)
{
    int   fd;

#ifdef DEBUG
    pid_t pid = vfork();
#else
    pid_t pid = -1;
#endif

    if((fd = open("temp", O_CREAT | O_TRUNC | O_RDWR, S_IRWXU)) == -1)
    {
        perror("open file error !\n");
        return 0;
    }

    strcpy(buf, "THIS IS parent process line\n");

#ifdef DEBUG
    if(pid == 0)
#else
    if( (pid = vfork()) == 0)
#endif
    {
        strcpy(buf, "child : process write\n");

        printf("\n");
        printf("child : process id is %d\n", getpid());
        printf("child : parent process id is %d\n\n", getppid());

        write(fd, buf, strlen(buf));
        close(fd);

        exit(0);    //child process must exit while work done
    }
    else
    {
        printf("havent run strcpy -> buf:%s", buf);
        strcpy(buf, "parent : process line\n");
        printf("have   run strcpy -> buf:%s", buf);

        printf("parent : process id is %d\n", getpid());
        write(fd, buf, strlen(buf));

        printf("\n");
        close(fd);
    }
    return 0;
}