谁作过FAT File system? Fseek（）这个函数看着有点迷糊，请高手指点一下大慨思路。源码如下：
bit fat_fseek (Int32 offset)
{
Uint32 file_pos; /* current position in file */
Uint16 cluster_offset; /* cluster offset in file */
Byte sector_offset; /* sector offset in cluster */
#define i sector_offset /* local variable overlay */
#define byte_offset cluster_offset /* local variable overlay */
/* File size information update for write mode*/
if (fat_open_mode == WRITE)
{
if (fat_cache.current.size.l <= fat_file_size.l)
fat_cache.current.size.l = fat_file_size.l;
}
/* init file pos with the current cluster in the index */
if (fat_fchain_nb_clust != 0)
file_pos = fat_fchain_nb_clust - 1;
else
file_pos = 0;
/* Add the previous cluster number */
for (i = 0; i != fat_fchain_index; i++)
{
file_pos += fclusters.number;
}
/* convert absolute cluster value in byte position */
file_pos *= SECTOR_SIZE * fat_cluster_size;
if ((fat_fchain_nb_clust != 0) && ( fat_fclust_byte_count % (fat_cluster_size * SECTOR_SIZE) == 0))
{
file_pos += (fat_cluster_size * SECTOR_SIZE);
}
else
{
((Byte*)&fat_fclust_byte_count)[0] &= fat_cluster_mask;
file_pos += fat_fclust_byte_count; /* offset in cluster */
}
/* Check range value */
if (((file_pos + offset) < 0) ||
((file_pos + offset) > fat_cache.current.size.l))
{
return KO; /* out of file limits */
}
file_pos += offset; /* new position */
/* Calculate byte position in cluster */
((Byte*)&fat_fclust_byte_count)[1] = ((Byte*)&file_pos)[3];
((Byte*)&fat_fclust_byte_count)[0] = ((Byte*)&file_pos)[2];
/* Calculate the absolute cluster position */
cluster_offset = file_pos / (SECTOR_SIZE * fat_cluster_size);
fat_fchain_index = 0; /* reset fragment number */
/* Determinate the index for the chain cluster */
while (cluster_offset >= fclusters[fat_fchain_index].number)
{
cluster_offset -= fclusters[fat_fchain_index].number;
fat_fchain_index++; /* ome more fragment */
}
/* Determinate the cluster offset value for the selected index */
fat_fchain_nb_clust = cluster_offset;
/* Determinate the sector offset value in the selected cluster */
sector_offset = (fat_fclust_byte_count & ((fat_cluster_size * SECTOR_SIZE) - 1)) / SECTOR_SIZE;
/* seek into sector */
byte_offset = file_pos % SECTOR_SIZE;
/* re-open file in read or write mode */
if (fat_open_mode == READ)
{
Hard_read_close(); /* close reading */
Hard_read_open(((Uint32)(fclusters[fat_fchain_index].cluster + fat_fchain_nb_clust) * fat_cluster_size)
+ sector_offset + fat_ptr_data);
if ((fat_fchain_nb_clust != 0) || (sector_offset != 0) || (byte_offset != 0))
{ /* if no offset, nb_clust incremented in fgetc() function */
fat_fchain_nb_clust++; /* one-based variable */
}
while (byte_offset != 0)
{
Hard_read_byte(); /* dummy read */
byte_offset--;
}
}
else
{
Hard_write_close(); /* close writing */
Hard_write_open(((Uint32)(fclusters[fat_fchain_index].cluster + fat_fchain_nb_clust) * fat_cluster_size)
+ sector_offset + fat_ptr_data);
byte_offset = file_pos % SECTOR_SIZE;
if (byte_offset != 0)
{ /* if no offset, nb_clust incremented in fputc() function */
fat_fchain_nb_clust++; /* one-based variable */
}
fat_file_size.l += offset; /* Update byte position in write mode */
if ((((Byte*)&fat_fclust_byte_count)[1] == 0x00) &&
((((Byte*)&fat_fclust_byte_count)[0] & fat_cluster_mask) == 0x00))
{
if ((fclusters[fat_fchain_index].number == fat_fchain_nb_clust) && (fat_fchain_index == fat_last_clust_index))
{
flag_end_disk_file = TRUE;
}
else
{
flag_end_disk_file = FALSE;
}
}
}
return OK;
} |
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