Understanding memcpy

[Kai Meyer]

Thanks for catching that :) I knew it would be something simple.

On 10/19/2011 10:00 PM, rohan puri wrote:
>
>
> On Thu, Oct 20, 2011 at 3:04 AM, Kai Meyer <kai at gnukai.com 
> <mailto:kai at gnukai.com>> wrote:
>
>     I'm trying to poke around an ext4 file system. I can submit a bio for
>     the correct block, and read in what seems to be the correct
>     information,
>     but when I try to memcpy my char *buffer to a reference to a
>     struct I've
>     made, it just doesn't seem to work. The relevant code looks like this:
>
>     typedef struct ext2_superblock {
>             /* 00-03 */ uint32_t e2sb_inode_count;
>             /* 04-07 */ uint32_t e2sb_block_count;
>             /* 08-11 */ uint32_t e2sb_blocks_reserved;
>             /* 12-15 */ uint32_t e2sb_unallocated_blocks;
>             /* 16-19 */ uint32_t e2sb_unallocated_inodes;
>             /* 20-23 */ uint32_t e2sb_sb_block;
>             /* 24-27 */ uint32_t e2sb_log_block_size;
>             /* 28-31 */ uint32_t e2sb_log_fragment_size;
>             /* 32-35 */ uint32_t e2sb_num_blocks_per_group;
>             /* 36-39 */ uint32_t e2sb_num_frag_per_group;
>             /* 40-43 */ uint32_t e2sb_num_inodes_per_group;
>             /* 44-47 */ uint32_t e2sb_last_mount_time;
>             /* 48-51 */ uint32_t e2sb_last_written_time;
>             /* 52-53 */ uint16_t e2sb_num_mounted;
>             /* 54-55 */ uint16_t e2sb_num_allowed_mounts;
>             /* 56-57 */ uint16_t e2sb_signature;
>             /* 58-59 */ uint16_t e2sb_fs_state;
>             /* 60-61 */ uint16_t e2sb_error_action;
>             /* 62-63 */ uint16_t e2sb_ver_minor;
>             /* 64-67 */ uint32_t e2sb_last_check;
>             /* 68-71 */ uint32_t e2sb_time_between_checks;
>             /* 72-75 */ uint32_t e2sb_os_id;
>             /* 76-79 */ uint32_t e2sb_ver_major;
>             /* 80-81 */ uint16_t e2sb_uid;
>             /* 82-83 */ uint16_t e2sb_gid;
>     } e2sb;
>
>
>     char *buffer;
>     uint32_t *pointer;
>     e2sb sb;
>     buffer = __bio_kmap_atomic(bio, 0, KM_USER0);
>     pointer = (uint32_t *)buffer;
>     printk(KERN_DEBUG "sizeof pbd->sb %lu\n", sizeof(bpd->sb));
>     printk(KERN_DEBUG "Inode Count: %u\n", pointer[0]); /* Works! */
>     printk(KERN_DEBUG "Block Count: %u\n", pointer[1]); /* Works! */
>     printk(KERN_DEBUG "Block Reserved: %u\n", pointer[2]); /* Works! */
>     printk(KERN_DEBUG "Unallocated blocks: %u\n", pointer[3]); /*
>     Works! */
>     printk(KERN_DEBUG "Unallocated inodes: %u\n", pointer[4]); /*
>     Works! */
>     memcpy(buffer, &sb, sizeof(sb));
>
> This should be : -
> memcpy(&sb, buffer, sizeof(sb));
>
>     __bio_kunmap_atomic(bio, KM_USER0);
>     printk(KERN_DEBUG "e2sb_debug: Total number of inodes in file system
>     %u\n", sb->e2sb_inode_count);/* Doesn't work! */
>     printk(KERN_DEBUG "e2sb_debug: Total number of blocks in file
>     system%u\n", sb->e2sb_block_count); /* Doesn't work! */
>
>     My code is actually much more verbose. The values I get from indexing
>     into pointer are correct, and match what I get from dumpe2fs. The
>     values
>     I get from the e2sb struct are not. They are usually 0. I would
>     imagine
>     that memcpy is the fastest way to copy data from buffer instead of
>     casting the pointer to something else, and using array indexing to get
>     the values.
>
>     I struggled to find where ext4 actually does this, so I'm making
>     this up
>     as I go along. Any thing that you see that I should be doing a
>     different
>     way that isn't actually part of my question is welcome too.
>
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>
> Regards,
> Rohan Puri
>
>
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