C++ 我可以在 GDB 中的“内存访问”上设置断点吗？
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Can I set a breakpoint on 'memory access' in GDB?
提问by TJ Seabrooks
I am running an application through gdb and I want to set a breakpoint for any time a specific variable is accessed / changed. Is there a good method for doing this? I would also be interested in other ways to monitor a variable in C/C++ to see if/when it changes.
我正在通过 gdb 运行一个应用程序，我想在任何时候访问/更改特定变量时设置一个断点。有没有好的方法可以做到这一点？我也对在 C/C++ 中监视变量以查看它是否/何时更改的其他方法感兴趣。
回答by asksol
watchonly breaks on write, rwatchlet you break on read, and awatchlet you break on read/write.
watch只在写时中断，rwatch让你在读时中断，awatch让你在读/写时中断。
You can set read watchpoints on memory locations:
您可以在内存位置设置读取观察点：
gdb$ rwatch *0xfeedface
Hardware read watchpoint 2: *0xfeedface
but one limitation applies to the rwatch and awatch commands; you can't use gdb variables in expressions:
但是一个限制适用于 rwatch 和 awatch 命令；你不能在表达式中使用 gdb 变量：
gdb$ rwatch $ebx+0xec1a04f
Expression cannot be implemented with read/access watchpoint.
So you have to expand them yourself:
所以你必须自己扩展它们：
gdb$ print $ebx
= 0x135700
gdb$ rwatch *0x135700+0xec1a04f
Hardware read watchpoint 3: *0x135700 + 0xec1a04f
gdb$ c
Hardware read watchpoint 3: *0x135700 + 0xec1a04f
Value = 0xec34daf
0x9527d6e7 in objc_msgSend ()
Edit:Oh, and by the way. You need either hardware or software support. Software is obviously much slower. To find out if your OS supports hardware watchpoints you can see the can-use-hw-watchpointsenvironment setting.
编辑：哦，顺便说一句。您需要硬件或软件支持。软件显然要慢得多。要了解您的操作系统是否支持硬件观察点，您可以查看can-use-hw-watchpoints环境设置。
gdb$ show can-use-hw-watchpoints
Debugger's willingness to use watchpoint hardware is 1.
回答by Paolo M
What you're looking for is called a watchpoint.
您要查找的内容称为watchpoint。
Usage
用法
(gdb) watch foo: watch the value of variablefoo
(gdb) watch foo: 观察变量的值foo
(gdb) watch *(int*)0x12345678: watch the value pointed by an address, casted to whatever type you want
(gdb) watch *(int*)0x12345678: 观察地址指向的值，转换为你想要的任何类型
(gdb) watch a*b + c/d: watch an arbitrarily complex expression, valid in the program's native language
(gdb) watch a*b + c/d: 观看任意复杂的表达式，在程序的母语中有效
Watchpoints are of three kinds:
观察点分为三种：
- watch: gdb will break when a writeoccurs
- rwatch: gdb will break wnen a readoccurs
- awatch: gdb will break in both cases
- watch: gdb 会在写发生时中断
- rwatch: gdb 将在发生读取时中断
- awatch：gdb 在这两种情况下都会中断
You may choose the more appropriate for your needs.
您可以选择更适合您的需求。
For more information, check thisout.
有关更多信息，请查看此内容。
回答by Smirnov
Assuming the first answer is referring to the C-like syntax (char *)(0x135700 +0xec1a04f)then the answer to do rwatch *0x135700+0xec1a04fis incorrect. The correct syntax is rwatch *(0x135700+0xec1a04f).
假设第一个答案是指类似 C 的语法，(char *)(0x135700 +0xec1a04f)那么 do 的答案rwatch *0x135700+0xec1a04f是不正确的。正确的语法是rwatch *(0x135700+0xec1a04f).
The lack of ()s there caused me a great deal of pain trying to use watchpoints myself.
()那里缺少s 给我自己尝试使用观察点带来了很大的痛苦。
回答by mweerden
I just tried the following:
我只是尝试了以下方法：
$ cat gdbtest.c
int abc = 43;
int main()
{
abc = 10;
}
$ gcc -g -o gdbtest gdbtest.c
$ gdb gdbtest
...
(gdb) watch abc
Hardware watchpoint 1: abc
(gdb) r
Starting program: /home/mweerden/gdbtest
...
Old value = 43
New value = 10
main () at gdbtest.c:6
6 }
(gdb) quit
So it seems possible, but you do appear to need some hardware support.
所以这似乎是可能的，但您似乎确实需要一些硬件支持。
回答by higgs241
Use watch to see when a variable is written to, rwatch when it is read and awatch when it is read/written from/to, as noted above. However, please note that to use this command, you must break the program, and the variable must be in scope when you've broken the program:
使用 watch 查看何时写入变量，读取时使用 rwatch，读取/写入时使用 awatch，如上所述。但是，请注意，要使用此命令，您必须破坏程序，并且在破坏程序时变量必须在范围内：
Use the watch command. The argument to the watch command is an expression that is evaluated. This implies that the variabel you want to set a watchpoint on must be in the current scope. So, to set a watchpoint on a non-global variable, you must have set a breakpoint that will stop your program when the variable is in scope. You set the watchpoint after the program breaks.
使用监视命令。watch 命令的参数是一个被评估的表达式。这意味着您要设置观察点的变量必须在当前范围内。因此，要在非全局变量上设置观察点，您必须设置一个断点，当变量在范围内时，该断点将停止您的程序。在程序中断后设置观察点。
