GDB

This is to keep all possible stuffs related to GDB

打印函数里面的static variable

(gdb) p 'longc_perf_test::longc_perf_cnt_7()::cnt'

这里longc_perf_test是namespace，longc_perf_cnt_7()是函数，cnt是函数longc_perf_cnt_7()中的static变量，注意单引号必须加上

查看变量的类型:

https://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_109.html

whatis variable_name

ptype variable_name

查看gdb是否在编译时期设置了python support

(gdb) gdb --configuration

如何加载core dump文件

http://www.yolinux.com/TUTORIALS/GDB-Commands.html#STLDEREF

Gdbinit file example

https://gist.github.com/CocoaBeans/1879270

加载需要读入stdin的程序

  • 开两个terminal，一个运行程序，直到运行到等待stdin的时候阻塞，另一个terminal在此时用gdb通过pid加载进程，然后再在等待stdin的terminal中输入

  • Write the desired input to a file “input.txt”, then redirect in gdb

    (gdb) r program-arg-list < input.txt

    https://github.com/cgdb/cgdb/issues/36

GDB 查看所加载程序的参数

(gdb) show args

GDB check vtable from a pointer/reference to base class object

(gdb) run
#Starting program: /home/bazis/test

Program received signal SIGTRAP, Trace/breakpoint trap.
main (argc=1, argv=0xbffff064) at test.cpp:23
23 delete pObject;

(gdb) print pObject
$1 = (BaseClass *) 0x804b008

(gdb) info vtbl pObject
vtable for 'BaseClass' @ 0x80486c8 (subobject @ 0x804b008):
[0]: 0x80485f4 <ChildClass::Test()>

(gdb) info symbol 0x80486c8
vtable for ChildClass + 8 in section .rodata of /home/bazis/test

GDB Prints char* to a file

https://stackoverflow.com/questions/14609577/print-character-array-to-file-in-gdb

(gdb) pi open("myoutput_data.log","w").write(gdb.execute("print jclGlobals->pydata.source",to_string=True))

https://stackoverflow.com/questions/233328/how-do-i-print-the-full-value-of-a-long-string-in-gdb

This is even better, because it honors the newline/carriage return chars instead of escaping them

call (void)puts(your_string)

Create std::string in GDB

link

可以在GDB里面生成一个堆上的变量，然后赋值

(gdb) call malloc(sizeof(std::string))
$1 = (void*) 0x91a6a0
(gdb) call((std::string*)0x91a6a0)->basic_string()
(gdb) call((std::string*)0x91a6a0)->assign("Hello, World")
$2= (..., _M_p = 0x91a6f8"Hello, World"}}
(gdb) call SomeFunctionThatTakesAConstStringRef(*(const std::string*)0x91a6a0)

Watch array element changes

GDB watch point

使用 watch 命令来观察一个数组当中某个元素值的变化，并在变化的时候添加断点。

watch -l array_name[array_index]

这里array_name就是数组的名字，array_index是某个元素的索引，可以是变量名。

选项-l告诉编译器对变量名（或表达式）去求值（evaluate）。

GDB save breakpoints

Save current breakpoints as a file

save breakpoints my_bp_fname.brk

Load a breakpoint file into current gdb sesseion

source my_bp_fname.brk

Start GDB with a program and its arguments

gcc is the name of the program, and everything after it is the arguments, because --args is specified.

gdb --args gcc -O2 -c foo.c

Run gdb without printing the front material

gdb --silent
gdb --quiet
gdb -q

Run gdb with a core dump file

gdb <program_name> <core_dump_file>

GDB uses a file as interactive input

If a program asks for user’s input after launched, the user’s input can be written into a file, and let gdb read that file as user’s input, thus interactive input in gdb can be avoided.

For example, when running corexec, it might ask for user’s confirmation and then continue,

If you wish to ignore this warning, type Y.
Typing anything else will terminate.
>

In this circumstances, you can type Y, and then hit Enter key to let it continue.

If you wish to ignore this warning, type Y.
Typing anything else will terminate.
> Y

But if the binary is attached into gdb, and then it asks for user’s input. But there’s no way to type that as gdb session doesn’t know how to read it from stdin.

So write Y to a file (for example, gdbStdin.in), and start gdb as below,

corexec simp1_PBCSIMPLE_NDF.pjx 331 331 -color 1 -no_output < ./gdbStdin.in