关于linux中truncate函数的问题。

Question

为什么我使用ftruncate截断文件后文件大小确实改变了，但是用read读取文件的内容还是截断之前的呢？

 int main(void)
{
    //创建文件
    int new_fd = open("file/a",O_RDWR|O_CREAT|O_EXCL,0644);
    if(-1==new_fd)
    {
        printf("%d: ",__LINE__-3);
        perror("creat");
        exit(-1);
    }
    //打开文件
    int old_fd = open("file/file_test",O_RDWR);
    if(-1==old_fd)
    {
        printf("%d: ",__LINE__-3);
        perror("open");
        exit(-1);
    }
    //复制文件
    char buff[1024];
    while(read(old_fd,buff,sizeof(buff)))
    {
        int res = write(new_fd,buff,strlen(buff));
        if(-1==res)
        {
            printf("%d: ",__LINE__-3);
            perror("write");
            close(new_fd);
            close(old_fd);
            exit(-1);
        }
        memset(buff,sizeof(buff),0);
    }
    memset(buff,sizeof(buff),0);
    close(old_fd);

    //读出修改后的文件
    lseek(new_fd,0,SEEK_SET);
    while(read(new_fd,buff,sizeof(buff)))
    {   
        printf("%s",buff);
        memset(buff,sizeof(buff),0);
    }
    printf("\n\n");
    //获得修改过后的文件的信息
    struct stat st;
    if(-1==fstat(new_fd,&st))
    {
        printf("%d: ",__LINE__-3);
        perror("stat");
        close(new_fd);
        exit(-1);
    }
    printf("文件大小为%d\n",st.st_size);
    printf("文件最后修改时间为%s",ctime(&st.st_mtime));
    printf("文件权限为%o\n\n",st.st_mode&0777);
    //截取前100个字节
    if(-1==ftruncate(new_fd,100))
    {
        printf("%d: ",__LINE__-3);
        perror("truncate");
        close(new_fd);
        exit(-1);
    }

    close(new_fd);   //先关闭文件描述符
    new_fd = open("file/a",O_RDWR);//再次打开
    if(-1==new_fd)
    {
        printf("%d: ",__LINE__-3);
        perror("creat");
        exit(-1);
    }

#ifdef DE
    if(-1==fstat(new_fd,&st))
    {
        printf("%d: ",__LINE__-3);
        perror("stat");
        close(new_fd);
        exit(-1);
    }
    printf("文件大小为%d\n",st.st_size);
    printf("文件最后修改时间为%s",ctime(&st.st_mtime));
    printf("文件权限为%o\n\n",st.st_mode&0777);
#endif
    //读出文件
    memset(buff,sizeof(buff),0);
    lseek(new_fd,0,SEEK_SET);
    while(read(new_fd,buff,sizeof(buff)))
    {   
        printf("%s",buff);
        memset(buff,sizeof(buff),0);
    }
    printf("\n\n");
    //改变新建文件的权限

    close(new_fd);
    remove("file/4_file");
    return 0;
}

Answer 1

http://blog.csdn.net/tototuzuoquan/article/details/39271757