gdb：显示一些数据的类型信息

Question

基本上，我想得到typeid(*this).name()，即this的真实类型。

我想在 GDB 中得到这个（不修改源代码）。我试过print typeid(*this)，但它说typeid 是未知的（因为我没有在源文件中包含它）。

Answer 1

使用ptype 命令，像这样：

(gdb) ptype 42
type = int

Answer 2

ptype [ARG] 命令将打印类型。

Answer 3

这个问题可能相关：vtable in polymorphic class of C++ using gdb:

(gdb) help set print object
Set printing of object's derived type based on vtable info. 

它不完全是 typeid() 但它应该在检查多态指针时显示真实的对象类型（例如基类中的this）。自然只适用于具有 vtable（即至少一个虚拟方法）的类，但 typeid 也是如此。

Answer 4

使用ptype、whatis 和explore（我的最爱！）：

您有一个名为 value 的变量，其定义为：

uint32_t value = 1234;

...以下所有工作：

1. ptype value 显示unsigned int
2. whatis value 显示uint32_t
3. explore value（我最喜欢的！）显示：

   The value of 'value' is of type 'uint32_t' which is a typedef of type 'unsigned int'
   'value' is a scalar value of type 'unsigned int'.
   value = 1234
   

例子：

(gdb) ptype value
type = unsigned int
(gdb) ptype &value
type = unsigned int *
(gdb) whatis value
type = uint32_t
(gdb) explore value
The value of 'value' is of type 'uint32_t' which is a typedef of type 'unsigned int'
'value' is a scalar value of type 'unsigned int'.
value = 1234

感谢@o11c's comment below 指出whatis 命令的存在。

我通过在 gdb 中运行 help all 发现了 explore 命令。请参阅下面“参考”部分中的评论。

自己试试吧：

# download the file "type_punning.c"
wget https://raw.githubusercontent.com/Generalsimus/eRCaGuy_hello_world/master/c/type_punning.c

# build it with optimization OFF (`-O0`) and debugging symbols ON (`-ggdb`), 
# and output all intermediary files (`-save-temps=obj`), and run it in the 
# gdb debugger (`gdb bin/type_punning`)
mkdir -p bin && gcc -Wall -Wextra -Werror -O0 -ggdb -std=c11 -save-temps=obj \
type_punning.c -o bin/type_punning && gdb bin/type_punning

现在，gdb 正在运行，请执行以下操作：

# set breakpoint to a line just after `u.value = 1234;`
b type_punning.c:52
# run to that point
r
# Now run these various commands to see what type `u.value` is:
ptype u.value
whatis u.value 
explore u.value  

完整的示例命令和输出：

eRCaGuy_hello_world/c$ mkdir -p bin && gcc -Wall -Wextra -Werror -O0 -ggdb -std=c11 -save-temps=obj type_punning.c -o bin/type_punning && gdb bin/type_punning
GNU gdb (Ubuntu 8.1.1-0ubuntu1) 8.1.1
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from bin/type_punning...done.
(gdb) b type_punning.c:52
Breakpoint 1 at 0x70b: file type_punning.c, line 52.
(gdb) r
Starting program: /home/gabriel/GS/dev/eRCaGuy_hello_world/c/bin/type_punning 
Type punning and ptr-based serialization demo
TECHNIQUE 1: union-based type punning:

Breakpoint 1, main () at type_punning.c:53
53          printf("1st byte = 0x%02X\n", (u.bytes)[0]);
(gdb) ptype u.value
type = unsigned int
(gdb) whatis u.value 
type = uint32_t
(gdb) explore u.value  
The value of 'u.value' is of type 'uint32_t' which is a typedef of type 'unsigned int'
'u.value' is a scalar value of type 'unsigned int'.
u.value = 1234
(gdb) 

我的旧/原始答案

作为@Star Brilliant says here，这个：

ptype my_var

返回type = unsigned short 之类的东西，但我希望它返回type = uint16_t，这样我才能真正知道检查内存时它有多少字节。我能想到的最好的方法是：

print &my_var

它打印(uint16_t *) 0x7ffffffefc2c，从而表明它的指针类型是uint16_t*，意味着它的类型是uint16_t。

我发现这比ptype my_var 更有用，但是如果您有任何建议，需要更直接的方法来获得这种效果。

示例 gdb 命令和输出：

(gdb) ptype my_var
type = unsigned short
(gdb) print &my_var
$27 = (uint16_t *) 0x7ffffffefc2c

同样，注意ptype my_var 显示它是unsigned short，而print &my_var 显示更详细和期望的答案是它是uint16_t。

参考资料：

1. @o11c's comment below
2. @Star Brilliant's answer
3. help all - 我在运行 gdb 时使用了此命令，将输出全部复制粘贴到文本编辑器，然后搜索“类型”以发现 explore 命令。

另见：

1. [我的答案]"gdb" debugger skips a break point weirdly
2. [我的问答]What's the difference between a compiler's `-O0` option and `-Og` option?
3. [我的问答]https://askubuntu.com/questions/1349047/where-do-i-find-core-dump-files-and-how-do-i-view-and-analyze-the-backtrace-st

_{关键词：如何运行gdb；如何在 gdb 中查看变量类型和值；如何构建和编译gdb调试符号}