在假设返回值未分配给变量的情况下,是否可以使用gdb检查函数的返回值?
我想象有更好的方法来完成它,但是finish 命令会执行直到当前的栈帧弹出并打印返回值--在给定的程序中。
int fun() {
return 42;
}
int main( int argc, char *v[] ) {
fun();
return 0;
}
你可以这样进行调试--
(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
命令可以缩写为fin
。不要使用f
,它是frame
命令的缩写!
是的,只需要输入print $eax
来检查EAX
寄存器。对于大多数函数,即使它没有被使用,返回值也存储在该寄存器中。
唯一的例外是返回比32位更大的数据类型,特别是64位整数(long long
)、double
和structs
或classes
。
另一个例外是如果您不是在Intel架构上运行,则必须找出使用的寄存器(如果有)。
以下是如何在不使用符号的情况下完成此操作。
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
格式有些混乱,请注意使用“info frame”来检查帧,以及“frame#”以导航您的上下文到另一个上下文(上下堆栈)。
bt还显示了一个缩写堆栈以帮助解决问题。