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)); __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.