fat16.c

AVR读取SD卡里FAT格式内容的源程序

        return 0;

    uint32_t fat_offset = fs->header.fat_offset;
    uint8_t buffer[2];
    while(cluster_num)
    {
        if(!fs->partition->device_read(fat_offset + 2 * cluster_num, buffer, 2))
            return 0;

        /* get next cluster of current cluster before freeing current cluster */
        uint16_t cluster_num_next = ((uint16_t) buffer[0]) |
                                    ((uint16_t) buffer[1] << 8);

        if(cluster_num_next == FAT16_CLUSTER_FREE)
            return 1;
        if(cluster_num_next == FAT16_CLUSTER_BAD ||
           (cluster_num_next >= FAT16_CLUSTER_RESERVED_MIN &&
            cluster_num_next <= FAT16_CLUSTER_RESERVED_MAX
           )
          )
            return 0;
        if(cluster_num_next >= FAT16_CLUSTER_LAST_MIN && cluster_num_next <= FAT16_CLUSTER_LAST_MAX)
            cluster_num_next = 0;

        /* free cluster */
        buffer[0] = FAT16_CLUSTER_FREE & 0xff;
        buffer[1] = (FAT16_CLUSTER_FREE >> 8) & 0xff;
        fs->partition->device_write(fat_offset + 2 * cluster_num, buffer, 2);

        /* We continue in any case here, even if freeing the cluster failed.
         * The cluster is lost, but maybe we can still free up some later ones.
         */

        cluster_num = cluster_num_next;
    }

    return 1;
#else
    return 0;
#endif
}

/**
 * \ingroup fat16_fs
 * Frees a part of a cluster chain and correctly terminates the rest.
 *
 * Marks the specified cluster as the new end of a cluster chain and
 * frees all following clusters.
 *
 * \param[in] fs The filesystem on which to operate.
 * \param[in] cluster_num The new end of the cluster chain.
 * \returns 0 on failure, 1 on success.
 * \see fat16_free_clusters
 */
uint8_t fat16_terminate_clusters(const struct fat16_fs_struct* fs, uint16_t cluster_num)
{
#if FAT16_WRITE_SUPPORT
    if(!fs || cluster_num < 2)
        return 0;

    /* fetch next cluster before overwriting the cluster entry */
    uint16_t cluster_num_next = fat16_get_next_cluster(fs, cluster_num);

    /* mark cluster as the last one */
    uint8_t buffer[2];
    buffer[0] = FAT16_CLUSTER_LAST_MAX & 0xff;
    buffer[1] = (FAT16_CLUSTER_LAST_MAX >> 8) & 0xff;
    if(!fs->partition->device_write(fs->header.fat_offset + 2 * cluster_num, buffer, 2))
        return 0;

    /* free remaining clusters */
    if(cluster_num_next)
        return fat16_free_clusters(fs, cluster_num_next);
    else
        return 1;
#else
    return 0;
#endif
}

/**
 * \ingroup fat16_fs
 * Clears a single cluster.
 *
 * The complete cluster is filled with zeros.
 *
 * \param[in] fs The filesystem on which to operate.
 * \param[in] cluster_num The cluster to clear.
 * \returns 0 on failure, 1 on success.
 */
uint8_t fat16_clear_cluster(const struct fat16_fs_struct* fs, uint16_t cluster_num)
{
#if FAT16_WRITE_SUPPORT
    if(cluster_num < 2)
        return 0;

    uint32_t cluster_offset = fs->header.cluster_zero_offset +
                              (uint32_t) (cluster_num - 2) * fs->header.cluster_size;
    uint8_t zero[16];
    return fs->partition->device_write_interval(cluster_offset,
                                                zero,
                                                fs->header.cluster_size,
                                                fat16_clear_cluster_callback,
                                                0
                                               );
#else
    return 0;
#endif
}

/**
 * \ingroup fat16_fs
 * Callback function for clearing a cluster.
 */
uint16_t fat16_clear_cluster_callback(uint8_t* buffer, uint32_t offset, void* p)
{
#if FAT16_WRITE_SUPPORT
    memset(buffer, 0, 16);
    return 16;
#else
    return 0;
#endif
}

/**
 * \ingroup fat16_file
 * Opens a file on a FAT16 filesystem.
 *
 * \param[in] fs The filesystem on which the file to open lies.
 * \param[in] dir_entry The directory entry of the file to open.
 * \returns The file handle, or 0 on failure.
 * \see fat16_close_file
 */
struct fat16_file_struct* fat16_open_file(struct fat16_fs_struct* fs, const struct fat16_dir_entry_struct* dir_entry)
{
    if(!fs || !dir_entry || (dir_entry->attributes & FAT16_ATTRIB_DIR))
        return 0;

#if USE_DYNAMIC_MEMORY
    struct fat16_file_struct* fd = malloc(sizeof(*fd));
    if(!fd)
        return 0;
#else
    struct fat16_file_struct* fd = fat16_file_handlers;
    uint8_t i;
    for(i = 0; i < FAT16_FILE_COUNT; ++i)
    {
        if(!fd->fs)
            break;

        ++fd;
    }
    if(i >= FAT16_FILE_COUNT)
        return 0;
#endif
    
    memcpy(&fd->dir_entry, dir_entry, sizeof(*dir_entry));
    fd->fs = fs;
    fd->pos = 0;
    fd->pos_cluster = dir_entry->cluster;

    return fd;
}

/**
 * \ingroup fat16_file
 * Closes a file.
 *
 * \param[in] fd The file handle of the file to close.
 * \see fat16_open_file
 */
void fat16_close_file(struct fat16_file_struct* fd)
{
    if(fd)
#if USE_DYNAMIC_MEMORY
        free(fd);
#else
        fd->fs = 0;
#endif
}

/**
 * \ingroup fat16_file
 * Reads data from a file.
 * 
 * The data requested is read from the current file location.
 *
 * \param[in] fd The file handle of the file from which to read.
 * \param[out] buffer The buffer into which to write.
 * \param[in] buffer_len The amount of data to read.
 * \returns The number of bytes read, 0 on end of file, or -1 on failure.
 * \see fat16_write_file
 */
int16_t fat16_read_file(struct fat16_file_struct* fd, uint8_t* buffer, uint16_t buffer_len)
{
    /* check arguments */
    if(!fd || !buffer || buffer_len < 1)
        return -1;

    /* determine number of bytes to read */
    if(fd->pos + buffer_len > fd->dir_entry.file_size)
        buffer_len = fd->dir_entry.file_size - fd->pos;
    if(buffer_len == 0)
        return 0;
    
    uint16_t cluster_size = fd->fs->header.cluster_size;
    uint16_t cluster_num = fd->pos_cluster;
    uint16_t buffer_left = buffer_len;
    uint16_t first_cluster_offset = fd->pos % cluster_size;

    /* find cluster in which to start reading */
    if(!cluster_num)
    {
        cluster_num = fd->dir_entry.cluster;
        
        if(!cluster_num)
        {
            if(!fd->pos)
                return 0;
            else
                return -1;
        }

        if(fd->pos)
        {
            uint32_t pos = fd->pos;
            while(pos >= cluster_size)
            {
                pos -= cluster_size;
                cluster_num = fat16_get_next_cluster(fd->fs, cluster_num);
                if(!cluster_num)
                    return -1;
            }
        }
    }
    
    /* read data */
    do
    {
        /* calculate data size to copy from cluster */
        uint32_t cluster_offset = fd->fs->header.cluster_zero_offset +
                                  (uint32_t) (cluster_num - 2) * cluster_size + first_cluster_offset;
        uint16_t copy_length = cluster_size - first_cluster_offset;
        if(copy_length > buffer_left)
            copy_length = buffer_left;

        /* read data */
        if(!fd->fs->partition->device_read(cluster_offset, buffer, copy_length))
            return buffer_len - buffer_left;

        /* calculate new file position */
        buffer += copy_length;
        buffer_left -= copy_length;
        fd->pos += copy_length;

        if(first_cluster_offset + copy_length >= cluster_size)
        {
            /* we are on a cluster boundary, so get the next cluster */
            if((cluster_num = fat16_get_next_cluster(fd->fs, cluster_num)))
            {
                first_cluster_offset = 0;
            }
            else
            {
                fd->pos_cluster = 0;
                return buffer_len - buffer_left;
            }
        }

        fd->pos_cluster = cluster_num;

    } while(buffer_left > 0); /* check if we are done */

    return buffer_len;
}

/**
 * \ingroup fat16_file
 * Writes data to a file.
 * 
 * The data is written to the current file location.
 *
 * \param[in] fd The file handle of the file to which to write.
 * \param[in] buffer The buffer from which to read the data to be written.
 * \param[in] buffer_len The amount of data to write.
 * \returns The number of bytes written, 0 on disk full, or -1 on failure.
 * \see fat16_read_file
 */
int16_t fat16_write_file(struct fat16_file_struct* fd, const uint8_t* buffer, uint16_t buffer_len)
{
#if FAT16_WRITE_SUPPORT
    /* check arguments */
    if(!fd || !buffer || buffer_len < 1)
        return -1;
    if(fd->pos > fd->dir_entry.file_size)
        return -1;

    uint16_t cluster_size = fd->fs->header.cluster_size;
    uint16_t cluster_num = fd->pos_cluster;
    uint16_t buffer_left = buffer_len;
    uint16_t first_cluster_offset = fd->pos % cluster_size;

    /* find cluster in which to start writing */
    if(!cluster_num)
    {
        cluster_num = fd->dir_entry.cluster;
        
        if(!cluster_num)
        {
            if(!fd->pos)
            {
                /* empty file */
                fd->dir_entry.cluster = cluster_num = fat16_append_clusters(fd->fs, 0, 1);
                if(!cluster_num)
                    return -1;
            }
            else
            {
                return -1;
            }
        }

        if(fd->pos)
        {
            uint32_t pos = fd->pos;
            uint16_t cluster_num_next;
            while(pos >= cluster_size)
            {
                pos -= cluster_size;
                cluster_num_next = fat16_get_next_cluster(fd->fs, cluster_num);
                if(!cluster_num_next && pos == 0)
                    /* the file exactly ends on a cluster boundary, and we append to it */
                    cluster_num_next = fat16_append_clusters(fd->fs, cluster_num, 1);
                if(!cluster_num_next)
                    return -1;

                cluster_num = cluster_num_next;
            }
        }
    }
    
    /* write data */
    do
    {
        /* calculate data size to write to cluster */
        uint32_t cluster_offset = fd->fs->header.cluster_zero_offset +
                                  (uint32_t) (cluster_num - 2) * cluster_size + first_cluster_offset;
        uint16_t write_length = cluster_size - first_cluster_offset;
        if(write_length > buffer_left)
            write_length = buffer_left;

        /* write data which fits into the current cluster */
        if(!fd->fs->partition->device_write(cluster_offset, buffer, write_length))
            break;

        /* calculate new file position */
        buffer += write_length;
        buffer_left -= write_length;
        fd->pos += write_length;

        if(first_cluster_offset + write_length >= cluster_size)
        {
            /* we are on a cluster boundary, so get the next cluster */
            uint16_t cluster_num_next = fat16_get_next_cluster(fd->fs, cluster_num);
            if(!cluster_num_next && buffer_left > 0)
                /* we reached the last cluster, append a new one */
                cluster_num_next = fat16_append_clusters(fd->fs, cluster_num, 1);
            if(!cluster_num_next)
            {
                fd->pos_cluster = 0;
                break;
            }

            cluster_num = cluster_num_next;
            first_cluster_offset = 0;
        }

        fd->pos_cluster = cluster_num;

    } while(buffer_left > 0); /* check if we are done */

    /* update directory entry */
    if(fd->pos > fd->dir_entry.file_size)
    {
        uint32_t size_old = fd->dir_entry.file_size;

        /* update file size */
        fd->dir_entry.file_size = fd->pos;
        /* write directory entry */
        if(!fat16_write_dir_entry(fd->fs, &fd->dir_entry))
        {
            /* We do not return an error here since we actually wrote
             * some data to disk. So we calculate the amount of data
             * we wrote to disk and which lies within the old file size.
             */
            buffer_left = fd->pos - size_old;
            fd->pos = size_old;
        }
    }

    return buffer_len - buffer_left;

#else
    return -1;
#endif
}

/**
 * \ingroup fat16_file
 * Repositions the read/write file offset.
 *
 * Changes the file offset where the next call to fat16_read_file()
 * or fat16_write_file() starts reading/writing.
 *
 * If the new offset is beyond the end of the file, fat16_resize_file()
 * is implicitly called, i.e. the file is expanded.
 *
 * The new offset can be given in different ways determined by
 * the \c whence parameter:
 * - \b FAT16_SEEK_SET: \c *offset is relative to the beginning of the file.
 * - \b FAT16_SEEK_CUR: \c *offset is relative to the current file position.
 * - \b FAT16_SEEK_END: \c *offset is relative to the end of the file.
 *
 * The resulting absolute offset is written to the location the \c offset
 * parameter points to.
 * 
 * \param[in] fd The file decriptor of the file on which to seek.
 * \param[in,out] offset A pointer to the new offset, as affected by the \c whence
 *                   parameter. The function writes the new absolute offset
 *                   to this location before it returns.