sparse file

windows - chsize
unix    - truncate
unix    - lseek

二进制，填充0

chsize/truncate比write快很多很多

空洞文件　又叫　稀疏文件  Sparse File

http://space.itpub.net/8242091/viewspace-619756
http://hi.baidu.com/holylizejin/blog/item/5c8072885f67be739f2fb420.html
http://blog.chinaunix.net/u3/104183/showart_2277274.html
http://blog.csdn.net/BABY313/archive/2010/01/18/5208836.aspx

linux 命令
ls -l  file.txt　　　//　文件大小
du -sk file.txt　　　//　文件实际分配的空间
fileplace file.txt   {必须安装perfagent.tools包才能使用}

#include <iostream>
#include <windows.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

using namespace std;

int main(int argc,char argv[])
{
    char buf[] = "0123456789";

    int fd = open("G:/data", O_RDWR | O_CREAT, S_IREAD | S_IWRITE);
    if (-1 == fd) return 0;

    cout << write(fd, buf, sizeof(buf)) << endl;

    chsize(fd, 20000);

    close(fd);
    cout << GetLastError() << endl;
    return 0;
}

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

#define MAXSIZE 1024

int main(int argc, char *argv[])
{    
    int fd = open("file.txt",O_RDWR);
    printf("fd = %d \n",fd);
    
    char buf[MAXSIZE];
    memset(buf, 0, MAXSIZE);
    int size = read(fd,buf ,MAXSIZE);
    printf("read size = %d \n",size);
    printf("conten = %s \n",buf);
    
    int lse = lseek(fd, 4096, SEEK_END);
    printf("lseek size= %d \n",lse);
    
    char s[] = "0123"; 
    int ok = write(fd, s, sizeof(s));
    printf("write size = %d \n",ok);
    
    close(fd);
    
    return 0;
}

int createSparseFile(char *path, long size)
{
    if (access(path, F_OK) == 0)    return -1;

    int fd = creat(path, O_WRONLY);
    if (fd < 0)     return -2;

    lseek(fd, size-1, SEEK_SET);
    write(fd, 0, 1);
    close(fd);

    return 0;
}