如何檢查GDB中函數的返回值？

Question

假設返回值爲而不是分配給變量，是否可以檢查gdb函數的返回值？

Answer 1

我想有更好的方法來做到這一點，但finish命令執行到當前堆棧幀被彈出並打印返回值 - 定的程序

int fun() { 
    return 42; 
} 

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

可以調試它是這樣 -

(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) 

Answer 2

是的，只需通過輸入print $eax來檢查EAX寄存器。對於大多數函數，返回值都存儲在該寄存器中，即使它沒有被使用。

該例外的是返回類型大於32位，特別是64位整數（long long），double s和structs或classes功能。

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，你必須找出使用哪個寄存器（如果有的話）。

Answer 3

下面是沒有符號的待辦事項。

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 

格式化有點搞砸了那裏，注意使用「信息框」檢查框，「幀＃」的導航您的上下文到另一個上下文（上下堆疊）

還有Bt show是一個縮略的堆棧來幫助你。