open在使用O_RDWR时

    我在使用管道时候，有一段是关于一个进程在创建子进程后，子进程execl另一个已编写的pipe4，pipe4中read设备文件符file_descriptors[0]以获取数据，我在想关于'\n'是否会被识别到pipe4中（实际上能传入就能识别）。于是就想做个从标准端口输入，并写入文件test.txt中，再读出的实验。

   len1.c如下：

#include<string.h>

#include<stdio.h>

#include<stdlib.h>

#include<unistd.h>

#include<fcntl.h>

#include<sys/types.h>

#include<sys/stat.h>

int main()

{        int file;

        int res;

        char list[1024];

        memset(list,'\0',sizeof(list));

        file = open("test.txt",O_RDWR|O_CREAT,S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH);

        res = read(0,list,1024);

        printf("input %d characters\n",res);

        res = write(file,list,res);

        printf("write %d characters into test.txt .\n",res);

        res = read(file,list,1024);

        printf("read %d characters from test.txt \n%s\n", res ,list);

        exit(EXIT_SUCCESS);

}

结果：

I love you

input 11 characters

write 11 characters into test.txt .

read 0 characters from test.txt

I love you

最后res变成了0，说明最后读了test.txt，但是读出0个字符。list值没有变，大概是应为没有读出，维持原值。

（后来我在read操作前加入了：sprintf(list,"I like this game");。最后一行也变成了I like this game）

我改变源代码,len2.c:

#include<string.h>

#include<stdio.h>

#include<stdlib.h>

#include<unistd.h>

#include<fcntl.h>

#include<sys/types.h>

#include<sys/stat.h>

int main()

{

        int file;

        int res;

        char list[1024];

        memset(list,'\0',sizeof(list));

        file = open("test.txt",O_WRONLY|O_CREAT,S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH);

        res = read(0,list,1024);

        printf("input %d characters\n",res);

        res = write(file,list,res);

        printf("write %d characters into test.txt .\n",res);

        close(file);

        file = open("test.txt",O_RDONLY);

        res = read(file,list,1024);

        printf("read %d characters from test.txt \n%s\n", res ,list);

        exit(EXIT_SUCCESS);

}

      结果如下：

I love you

input 11 characters

write 11 characters into test.txt .

read 11 characters from test.txt

I love you

结果正常，我想open的读写模式是否是只能先读后写（做实验成功，可行），而不能先写后读？又或者在open的O_RDWR后面还要与上一些其他的标示符？