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## gdb_learn.md

      
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    set coredump file

echo '/tmp/core.%e.%p.%t' | sudo tee /proc/sys/kernel/core_pattern
special

(gdb) info sharedlibrary
gdb -ex "layout asm" ./foo
gdb -q ./foo -ex "b main" -ex "r"
set history save

set confirm off

set disassemble-next-line on

set disassembly-flavor intel
normal

l : load debug file default
f :  file
detail

1、examine命令（简写是x）来查看内存地址中的值
gdb  x/nfu  ADDR

n 是一个正整数，表示显示内存的长度，也就是说从当前地址向后显示几个地址的内容。
f 表示显示的格式，如果地址所指的是字符串，那么格式可以是s，如果地十是指令地址，那么格式可以是i。
u 表示从当前地址往后请求的字节数，如果不指定的话，GDB默认是4个bytes。u参数可以用下面的字符来代替，b表示单字节，h表示双字节，w表示四字 节，g表示八字节。当我们指定了字节长度后，GDB会从指内存定的内存地址开始，读写指定字节，并把其当作一个值取出来。
举例：
(gdb) x/9i 0x7c00
0x7c00:	cli

=> 0x7c01:	cld

0x7c02:	xor    %ax,%ax
0x7c04:	mov    %ax,%ds
0x7c06:	mov    %ax,%es
0x7c08:	mov    %ax,%ss
0x7c0a:	in     $0x64,%al
0x7c0c:	test   $0x2,%al
0x7c0e:	jne    0x7c0a
gdb Commands

Using gdb:


Compile with debugging symbols using the -g flag in gcc.
Run program with gdb:

$ gdb program_name
$ (gdb) r[un] arg1 "arg2" ...

Stepping through code:

Resuming code:
c[ontinue]

Step into (C):
s[tep]

Step over (C):
n[ext]

Step out of a function (C):
f[inish]

Step into (Assembly):
s[tep]i

Step over (Assembly):
n[ext]i

Breakpoints:

Set breakpoint at a line:
b[reak] file:line_num

Set breakpoint at a function:
b[reak] function_name

Delete all break points:
d[elete]

Delete a specific breakpoint:
d[elete] b[reakpoints] breakpoint_number

Registers

Viewing registers:
i[nfo] r[egisters] register_name

Setting registers:
set $register_name = value

Variables:

View a variable:
p[rint] var

Dereference a variable:
p[rint] *var

View reference:
p[rint] &var

View struct fields:
p[rint] stuct_ptr->field
p[rint] stuct.field

Set variable by variable name:
set var = value

Set variable by address:
set {type}address = value

View address of a symbol:
i[nfo] address symbol_name

Call stack:

Print the call stack:
b[ack]t[race]

View code:

View current and following lines:
l[ist]

View current and previous lines:
l[ist] -

View code centered around line_num:
l[ist] line_num

View code centered around function_name:
l[ist] function_name

Miscellaneous

Stop execution:
kill

Quit gdb
q[uit]

References


Advanced tricks.