debugging - 如何检查 GDB 中函数的返回值？

Question

假设返回值未分配给变量，是否可以检查 gdb 中函数的返回值？

Answer 1

我想有更好的方法可以做到这一点，但是完成命令会一直执行，直到当前堆栈帧被弹出并打印返回值——给定程序

int fun() {
    return 42;
}

int main( int argc, char *v[] ) {
    fun();
    return 0;
}

你可以这样调试它——

(gdb) r
Starting program: /usr/home/hark/a.out 

Breakpoint 1, fun () at test.c:2
2               return 42;
(gdb) finish
Run till exit from #0  fun () at test.c:2
main () at test.c:7
7               return 0;
Value returned is $1 = 42
(gdb) 

该finish命令可简写为fin。不要使用命令f的缩写frame！

Answer 2

是的，只需EAX通过键入来检查寄存器print $eax。对于大多数函数，返回值存储在该寄存器中，即使它没有被使用。

例外情况是返回大于 32 位的类型的函数，特别是 64 位整数 ( long long)、doubles 和structsor classes。

另一个例外是如果您不是在英特尔架构上运行。在这种情况下，您必须弄清楚使用了哪个寄存器（如果有）。

Answer 3

这是不使用符号的方法。

gdb ls
This GDB was configured as "ppc64-yellowdog-linux-gnu"...
(no debugging symbols found)
Using host libthread_db library "/lib64/libthread_db.so.1".

(gdb) break __libc_start_main
Breakpoint 1 at 0x10013cb0
(gdb) r
Starting program: /bin/ls
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
Breakpoint 1 at 0xfdfed3c
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread 4160418656 (LWP 10650)]
(no debugging symbols found)
(no debugging symbols found)
[Switching to Thread 4160418656 (LWP 10650)]

Breakpoint 1, 0x0fdfed3c in __libc_start_main () from /lib/libc.so.6
(gdb) info frame
Stack level 0, frame at 0xffd719a0:
 pc = 0xfdfed3c in __libc_start_main; saved pc 0x0
 called by frame at 0x0
 Arglist at 0xffd71970, args:
 Locals at 0xffd71970, Previous frame's sp is 0xffd719a0
 Saved registers:
  r24 at 0xffd71980, r25 at 0xffd71984, r26 at 0xffd71988, r27 at 0xffd7198c,
  r28 at 0xffd71990, r29 at 0xffd71994, r30 at 0xffd71998, r31 at 0xffd7199c,
  pc at 0xffd719a4, lr at 0xffd719a4
(gdb) frame 0
#0  0x0fdfed3c in __libc_start_main () from /lib/libc.so.6
(gdb) info fr
Stack level 0, frame at 0xffd719a0:
 pc = 0xfdfed3c in __libc_start_main; saved pc 0x0
 called by frame at 0x0
 Arglist at 0xffd71970, args:
 Locals at 0xffd71970, Previous frame's sp is 0xffd719a0
 Saved registers:
  r24 at 0xffd71980, r25 at 0xffd71984, r26 at 0xffd71988, r27 at 0xffd7198c,
  r28 at 0xffd71990, r29 at 0xffd71994, r30 at 0xffd71998, r31 at 0xffd7199c,
  pc at 0xffd719a4, lr at 0xffd719a4

格式有点乱，注意使用“信息框架”来检查框架，以及“框架#”将你的上下文导航到另一个上下文（在堆栈上下）

bt 还显示了一个简短的堆栈来提供帮助。