hello
i had inserted my module to the kernel.The symbol table of this is

0000000000000000 a hello.c [hello]
ffffffff88515000 t hello_exit [hello]
ffffffff8851500e t hello_init [hello]
ffffc2000013e0a0 ? __mod_license3 [hello]
0000000000000000 a hello.mod.c [hello]
ffffc2000013e0c0 ? __mod_srcversion30 [hello]
ffffc2000013e0e3 ? __module_depends [hello]
ffffffff88515060 r ____versions [hello]
ffffc2000013e100 ? __mod_vermagic5 [hello]
ffffffff88515480 d __this_module [hello]
ffffffff88515000 t cleanup_module [hello]
ffffffff8851500e t init_module [hello]
ffffffff80233e64 u printk [hello]
ffffffff884c3b5d t br_hello_timer_expired [bridge]
ffffffff884c4930 t show_hello_timer [bridge]
ffffffff884c4a43 t store_hello_time [bridge]
ffffffff884c4d86 t set_hello_time [bridge]
ffffffff884c4b57 t show_hello_time [bridge]
ffffffff884ccf40 d dev_attr_hello_time [bridge]
ffffffff884cd200 d dev_attr_hello_timer [bridge]

my questions are:
1>At first row why the hello.c file is mensioned in the symbol
table,similarly hello.mod.c file also.
2>why _mod_license3 is of type "?".
3>why printk is of type "u".

I 'm just guessing. There segments loaded while you load module in kernel. They are .bss, .data, .text. You can find them in /sys/modules/<module-name>/sections/.*
So the symbols with d before them belongs to data and t before them belong to text.

The symbols listed as undefined are those that the kernel will supply, e.g. printk. It's pretty much the same as linking to a shared library to use a particular function such as printf.
ta0kira

If you want to know exactly what those initials mean, look at