在std :: cerr上打印某些內容時設置gdb斷點

Question

我使用qt-creator作爲gdb的IDE和前端。如何在std :: cerr變量上調用運算符< <時如何設置斷點？是否有可能從qt-creator或我將不得不運行gdb獨立？

Answer 1

如何設置在性病斷點:: CERR

你的問題沒有任何意義：std::cerr是一個全局變量。您只能在函數上設置斷點。你也可以在變量上設置觀察點，所以當變量被修改時GDB停止，但它可能不是你想要的。

你是什麼可能是問的是：「如果有什麼東西寫到STDERR_FILENO文件描述符？」，我該怎麼辦？

如果是這樣，catch syscall write可能是答案（但真正的答案取決於你的操作系統，你沒有透露）。

更新：

捕捉系統調用寫是不是一種選擇，我相信，因爲我經常寫入文件

可以使系統調用，捕獲點條件上寫STDERR_FILENO（在Linux上是2）。示例（這將在Linux/x86_64上工作，但需要針對Linux/ix86進行調整）：

cat t.cc 
#include <iostream> 
using namespace std; 

int main() 
{ 
    cout << "Hello,"; 
    cerr << "error 1" << endl; 
    cout << " World!" << endl; 
    cerr << "error 2" << endl; 
} 

gcc -g t.cc 
gdb -q ./a.out 
Reading symbols from /tmp/a.out...done. 

(gdb) catch syscall write 
Catchpoint 2 (syscall 'write' [1]) 

# Make the catchpoint conditional on writing to stderr: 
(gdb) cond 2 $rdi == 2 

# By default, "catch syscall" will stop both before and after the actual syscall 
# Ignoring the catchpoint once will skip past the "after" stop. 
(gdb) command 2 
Type commands for breakpoint(s) 2, one per line. 
End with a line saying just "end". 
>ignore 2 1 
>end 

(gdb) r 
Starting program: /tmp/a.out 
Hello, 
Catchpoint 2 (call to syscall write), 0x00007ffff7383ff0 in __write_nocancel() at ../sysdeps/unix/syscall-template.S:82 
82 ../sysdeps/unix/syscall-template.S: No such file or directory. 
(gdb) x/s $rsi 
0x400a83: "error 1" # Good: we caught our first write to std::cerr 

(gdb) c 
Continuing. 
error 1 
Catchpoint 2 (call to syscall write), 0x00007ffff7383ff0 in __write_nocancel() at ../sysdeps/unix/syscall-template.S:82 
82 in ../sysdeps/unix/syscall-template.S 
(gdb) x/s $rsi 
0x7ffff76298e3 <_IO_2_1_stderr_+131>: "\n" # I didn't know endl gets a separate write syscall. 
(gdb) c 
Continuing. 

World! 

Catchpoint 2 (call to syscall write), 0x00007ffff7383ff0 in __write_nocancel() at ../sysdeps/unix/syscall-template.S:82 
82 in ../sysdeps/unix/syscall-template.S 
(gdb) x/s $rsi 
0x400a93: "error 2" 
(gdb) c 
Continuing. 
error 2 
Catchpoint 2 (call to syscall write), 0x00007ffff7383ff0 in __write_nocancel() at ../sysdeps/unix/syscall-template.S:82 
82 in ../sysdeps/unix/syscall-template.S 
(gdb) x/s $rsi 
0x7ffff76298e3 <_IO_2_1_stderr_+131>: "\n" 
(gdb) c 
Continuing. 

[Inferior 1 (process 17291) exited normally]