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有时system.map文件可以帮助我们理解内核编译，它记录了所有代码的运行地址。

下面一起看看如何读懂它

system.map内容格式为：线性地址 类型 符号

具体内容如下：
00100000 A phys_startup_32
c0100000 T startup_32
c0100000 A _text                             注:表示内核代码第一个字节的地址
c01000c6 t checkCPUtype
c0100147 t is486
c010014e t is386
c0100199 t L6
c010019b t check_x87
c01001c2 t setup_idt
c01001df t rp_sidt
c01001ec t ignore_int
c0100220 t rest_init
c0100220 T stext
c0100220 T _stext
c0100252 t do_pre_smp_initcalls
c0100257 t run_init_process
c0100283 t init
c01003a8 t try_name
c0100529 T name_to_dev_t
c0100790 t calibrate_delay_direct

...

c02f95d7 T register_kretprobe
c02f96cd T unregister_kretprobe
c02f9760 t .text.lock.kprobes
c02f97b0 T __kprobes_text_end
c02f9abe t iret_exc
c02fa1af A _etext                              注:内核代码结束的位置，之后为内核初始化的数据
c02fa1b0 A __start___ex_table
c02facc0 A __stop___ex_table
c02fb000 r __func__.12
c02fb000 A __start_rodata
c02fb00c r __func__.13
c02fb020 r __func__.2
c02fb02c r __func__.3
c02fb040 R linux_banner
c02fb0bb r __func__.0
c02fb0c7 r __func__.1
c02fb0e0 r __func__.0
c02fb0ec r __func__.1
c02fb100 r border
c02fb160 r cplens

...

c03e1b08 D ip_statistics
c03e1b10 D tcp_statistics
c03e1b18 D udp_statistics
c03e1b20 D icmp_statistics
c03e1b28 D net_statistics
c03e1b30 d fn_hash_kmem
c03e1b34 d fn_alias_kmem
c03e1b38 d xfrm_dst_cache
c03e1b3c d secpath_cachep
c03e1b40 A _edata                       注:内核初始化数据结束，之后为未初始化数据
c03e2000 D init_thread_union
c03e4000 A __init_begin
c03e4000 t no_halt
c03e4000 T _sinittext
c03e400d t mca_pentium
c03e401d t no_387
c03e4033 t check_fpu
...

c043a058 b pfkey_table
c043a05c b pfkey_table_lock
c043a05c b pfkey_table_users
c043a060 b pfkey_socks_nr
c043a064 b acqseq.4
c043a068 b acqseq_lock.5
c043a068 A __bss_stop
c043a068 A _end                         注:内核未初始化数据结束
c043b000 A pg0
ffffe400 A __kernel_vsyscall
ffffe410 A SYSENTER_RETURN
ffffe420 A __kernel_sigreturn
ffffe440 A __kernel_rt_sigreturn

符号类型.

小写字母表示局部; 大写字母表示全局(外部).
A
The symbol's value is absolute, and will not be changed by further linking.

B
The symbol is in the uninitialized data section (known as BSS).

C
The symbol is common. Common symbols are uninitialized data. When linking, multiple common symbols may appear with the same name. If the symbol is defined anywhere, the common symbols are treated as undefined references. For more details on common symbols, see the discussion of -warn-common in Linker options.

D
The symbol is in the initialized data section.

G
The symbol is in an initialized data section for small objects. Some object file formats permit more efficient access to small data objects, such as a global int variable as opposed to a large global array.

I
The symbol is an indirect reference to another symbol. This is a GNU extension to the a.out object file format which is rarely used.

N
The symbol is a debugging symbol.

R
The symbol is in a read only data section.

S
The symbol is in an uninitialized data section for small objects.

T
The symbol is in the text (code) section.

U
The symbol is undefined.

V
The symbol is a weak object. When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error. When a weak undefined symbol is linked and the symbol is not defined, the value of the weak symbol becomes zero with no error.

W
The symbol is a weak symbol that has not been specifically tagged as a weak object symbol. When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error. When a weak undefined symbol is linked and the symbol is not defined, the value of the weak symbol becomes zero with no error.

-
The symbol is a stabs symbol in an a.out object file. In this case, the next values printed are the stabs other field, the stabs desc field, and the stab type. Stabs symbols are used to hold debugging information. For more information, see Stabs.

?
The symbol type is unknown, or object file format specific.