有时候我们需要知道搜索的内容x位于文本文件的哪一行，然后需要查看这一行附近的内容，典型的例子有：

- 查看kernel的dmesg信息时

- 阅读函数实现代码时

可以简单的通过两个命令行解决，举例如下：

$grep -n 'BUG' snow_dmesg.log
10:<4>[    0.000000] BUG: mapping for 0x00b00000 at 0xc0900000 overlaps vmalloc space
664:<2>[   23.566966] kernel BUG at /work2/ICS_8x25/kernel/fs/ext4/extents.c:1996!
665:<0>[   23.573651] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP

然后我希望查看这附件的所有内容，此时可以使用sed命令达到目的：

$sed -n '650,700p' snow_dmesg.log
<3>[   16.800376] init: starting 'atfwd_log'
<3>[   16.800983] init: starting 'atfwd'
<3>[   21.821501] init: starting 'atfwd_log'
<3>[   21.822424] init: starting 'atfwd'
<2>[   23.529703] EXT4-fs error (device mmcblk0p13): ext4_valid_block_bitmap:282: comm er.ServerThread: Invalid block bitmap - block_group = 10, block = 327681
<3>[   23.530108] Aborting journal on device mmcblk0p13-8.
<2>[   23.531563] EXT4-fs (mmcblk0p13): Remounting filesystem read-only
<2>[   23.531753] EXT4-fs error (device mmcblk0p13) in ext4_free_blocks:4643: Journal has aborted
<2>[   23.533381] EXT4-fs error (device mmcblk0p13) in ext4_reserve_inode_write:5641: Journal has aborted
<2>[   23.535013] EXT4-fs error (device mmcblk0p13) in ext4_ext_remove_space:2669: Journal has aborted
<2>[   23.537513] EXT4-fs error (device mmcblk0p13) in ext4_reserve_inode_write:5641: Journal has aborted
<2>[   23.546488] EXT4-fs error (device mmcblk0p13) in ext4_orphan_del:2120: Journal has aborted
<2>[   23.554731] EXT4-fs error (device mmcblk0p13) in ext4_reserve_inode_write:5641: Journal has aborted
<0>[   23.564124] ------------[ cut here ]------------
<2>[   23.566966] kernel BUG at /work2/ICS_8x25/kernel/fs/ext4/extents.c:1996!
<0>[   23.573651] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
<4>[   23.579118] Modules linked in: sm_event_driver sm_event_log
<4>[   23.584676] CPU: 0    Tainted: G        W    (3.0.21-perf #1)
<4>[   23.590413] PC is at ext4_ext_put_in_cache+0x14/0x48
<4>[   23.595353] LR is at ext4_ext_map_blocks+0x2e0/0x14bc
<4>[   23.600386] pc : [<c01ff260>]    lr : [<c0202a08>]    psr: 60000113
<4>[   23.600391] sp : df647a10  ip : 00000000  fp : df232254
<4>[   23.611848] r10: 00000000  r9 : 00000000  r8 : 00000000
<4>[   23.617053] r7 : 00000000  r6 : df71ac20  r5 : df647b80  r4 : 00000000
<4>[   23.623561] r3 : 00000000  r2 : 00000000  r1 : 00000000  r0 : df2322d0

__iomem是2.6.9中加入的特性。是用来个表示指会指向一个I/O的内存空间。主要是为了driver的通用性考虑。由于不同的CPU体系结构对 I/O空间的表示可能不同。当使用__iomem时，compiler会忽略对变量的检查（因为用的是void __iomem）。但sparse会对它进行检查，当__iomem的指针和正常的指针混用时，就会发出一些warnings