请教一下linux下如何直接定位和读取文件在磁盘上的位置

failno

我最近打算做个搜索引擎，涉及到通过散列表快速定位文件在磁盘中位置的问题，为此需要了解linux下如何直接定位和读取文件在磁盘上的位置
为此我查找了一下并找到了linux提供的两个struct:
//file 每个打开的文件， socket 接口都对应一个文件数据结构。
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struct file
{
        mode_t f_mode;
        loff_t f_pos;
        unsigned short f_flags;
        unsigned short f_count;
        unsigned long f_reada, f_ramax, f_raend, f_ralen,f_rawin;
        struct file *f_next, *f_prev;
        int f_owner;
        /* pid or -pgrp where SIGIO should be sent */
        struct inode * f_inode;
        //~~~~~~~~~~~~~~~~
        struct file_operations * f_op;
        unsigned long f_version;
        void *private_data;
        /* needed for tty driver, and maybeothers */
};
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//inode VFSinode 数据结构包含一个磁盘中的文件或目录的信息。(从逻辑概念 “文件到物理上的”数据块“映射。)
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struct inode
{
        kdev_t i_dev;
        unsigned long i_ino;
        umode_t i_mode;
        nlink_t i_nlink;
        user_id_t i_uid;
        gid_t i_gid;
        kdev_t i_rdev;
        off_t i_size;
        time_t i_atime;
        time_t i_mtime;
        time_t i_ctime;
        unsigned longi_blksize;
        unsigned long i_blocks;
        ~~~~~~~~~~~~~~~~~~~
        unsigned long i_version;
        unsigned long i_nrpages;
        struct semaphore i_sem;
        struct inode_operations *i_op;
        struct super_block *i_sb;
        struct wait_queue *i_wait;
        struct file_lock *i_flock;
        structvm_area_struct *i_mmap;
        struct page *i_pages;
        struct dquot *i_dquot[MAXQUOTAS];
        struct inode *i_next, *i_prev;
        struct inode *i_hash_next, *i_hash_prev;struct inode *i_bound_to,*i_bound_by;
        struct inode *i_mount;
        unsigned shorti_count;
        unsigned char i_lock;
        unsigned char i_dirt;
        unsigned char i_pipe;
        unsigned char i_sock;
        unsigned char i_seek;
        unsigned char i_update;
        unsigned short i_writecount;

        union
        {
                struct pipe_inode_info pipe_i;
                struct minix_inode_info minix_i;
                struct ext_inode_info ext_i;
                struct ext2_inode_info ext2_i;
                struct hpfs_inode_info hpfs_i;
                struct msdos_inode_info msdos_i;
                struct umsdos_inode_info umsdos_i;
                struct iso_inode_info isofs_i;
                struct nfs_inode_info nfs_i;
                struct xiafs_inode_info xiafs_i;
                struct sysv_inode_infosysv_i;
                struct affs_inode_info affs_i;
                struct ufs_inode_info ufs_i;
                struct socket socket_i;
                void *generic_ip;
        } u;
};
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我个人以为这个struct file可能是<stdio.h>里提供的FILE,这样通过访问FILE::f_inode->l_blocks就可以记录文件在磁盘中的位置，然后就可以直接访问文件了
但是写了一个测试的代码 :
[code:1]
/*test.cpp*/
#include<stdio.h>
#include<iostream.h>
int main()
{
        FILE* fp;
        fp=fopen("a.txt","w");
        fputs("hello",fp);
        fclose(fp);
        fp=fopen("a.txt","r");
        struct inode * inode;
        inode=fp->f_inode;
        unsigned long blksize=inode->i_blksize;
        unsigned long blocks=inode->i_blocks;
        struct inode_operations *op=inode->i_op;
        cout<<"block size:"<<blksize<<endl;
        return 1;
}
[/code:1]
发现有点问题
编译器报错：
test.cpp: In function `int main()':
test.cpp:11: `struct _IO_FILE' has no member named `f_inode'
test.cpp:12: invalid use of undefined type `struct inode'
test.cpp:10: forward declaration of `struct inode'
test.cpp:13: invalid use of undefined type `struct inode'
test.cpp:10: forward declaration of `struct inode'
test.cpp:14: invalid use of undefined type `struct inode'
test.cpp:10: forward declaration of `struct inode'
发现原来的猜想可能有点错误，就是说struct file 和 struct inode 如何能访问到的问题
不知道哪位做过类似的工作，可否提示一二