设置观察点以在 GDB 中输出十六进制列表

Question

我正在使用 GDB 调试 C 程序，并且在变量 uint8_t msg_bin_tst[64]; 上设置了观察点。当观察点被触发时，GDB 输出如下内容：

Watchpoint 4: msg_bin_tst
Old value = "@\001\000\067\335\002\033a32c85ba9dda45823be416246cf8b433baa068d7\000\000\000\000\000\000\000\000\000\017\000\000\000(\000\000"
New value = "@\001\000\067\335\002\033a32c85ba9dda45823be416246cf6cf8b433baa068d7\000\000\000\000\000\000\017\000\000\000(\000\000"

这有点难以解释，有没有办法将输出格式设置为print/x，即类似于：

msg_bin_tst = {0x40, 0x1, 0x0, 0x37, 0xdd, 0x2, 0x1b, 0x61, 0x33, 0x32, 0x63, 0x38, 0x35, 0x62, 0x61, 0x39, 0x64, 0x64, 0x61, 0x34, 0x35, 0x38, 0x32, 0x33, 0x62, 0x65, 0x34, 0x31, 0x36, 0x32, 0x34, 0x36, 0x63, 0x66, 0x38, 0x62, 0x34, 0x33, 0x33, 0x62, 0x61, 0x61, 0x30, 0x36, 0x38, 0x64, 0x37, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xf, 0x0, 0x0, 0x0, 0x28, 0x0, 0x0, 0x0}

Answer 1

使用commands设置watchpoint命中时的命令，例如：

(gdb) commands 2
Type commands for breakpoint(s) 2, one per line.
End with a line saying just "end".
>print /x msg_bin_tst
>end

2 是观察点的数量。然后当观察点被命中时，数组将被自动转储。

您可以参考gdb手册：https://sourceware.org/gdb/onlinedocs/gdb/Break-Commands.html。

Answer 2

gdb 根据 uint8_t[64] 的默认格式打印您的值。但是gdb 还允许您更改打印特定类型的方式。这称为Python Pretty-Printers。你可以在这里阅读它们：

• http://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Developer_Guide/debuggingprettyprinters.html
• http://sourceware.org/gdb/onlinedocs/gdb/Writing-a-Pretty_002dPrinter.html#Writing-a-Pretty_002dPrinter

我没有 c++11，为了演示示例，我使用 unsigned char 作为类型。您的类型的打印机可能如下所示：

$ cat my_priner.py
class CustomPrinter(object):
    def __init__(self, val):
        self.val = val

    def to_string(self):
        res = "{"
        for m in xrange(64):
          res += hex(int(self.val[m]))
          if m != 63:
            res += ", "
        res += " }"
        return res

def lookup_type (val):
    if str(val.type) == 'unsigned char [64]':
        return CustomPrinter(val)
    return None

gdb.pretty_printers.append (lookup_type)

这就是我的变量unsigned char m[64]; 的打印机：

$ gdb  -q -x my_priner.py  -ex "set pagination off" -ex "start" -ex "watch m" -ex "c"  ./main
Reading symbols from /home/main...done.
Temporary breakpoint 1 at 0x400590: file main.cpp, line 7.
Starting program: /home/main

Temporary breakpoint 1, main () at main.cpp:7
7         memcpy(m, "He",2);
Watchpoint 2: m
Continuing.
Watchpoint 2: m

Old value = {0x48, 0x65, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 }
New value = {0x48, 0x65, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 }
0x0000003c41288ac4 in memcpy () from /lib64/libc.so.6
(gdb)