如何在GDB中檢查函數的返回值？

Question

假設返回值未分配給變量，是否可以在gdb中檢查函數的返回值？

Answer 1

我想有更好的方法可以做到這一點，但是執行完成命令直到彈出當前的堆棧幀並打印返回值 - 給定程序

int fun() {
    return 42;
}

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

你可以這樣調試 -

(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) 

finish命令可以縮寫為fin 。 不要使用f ，這是frame命令的縮寫！

Answer 2

是的，只需輸入print $eax來檢查EAX寄存器。 對於大多數函數，返回值存儲在該寄存器中，即使它未被使用。

例外情況是函數返回大於32位的類型，特別是64位整數（ long long ）， double s和structs或classes 。

另一個例外是如果您沒有在英特爾架構上運行。 在這種情況下，您必須確定使用哪個寄存器，如果有的話。

Answer 3

這是沒有符號的todo。

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

格式化有點混亂，注意使用“信息框架”檢查幀，並使用“幀＃”將您的上下文導航到另一個上下文（在堆棧中上下）

bt還顯示了一個縮寫堆棧來幫助。