如何释放内存时在gdb中得到通知？

Question

I'm debugging a memory issue in C. The piece of memory I'm accessing has been accidentally free() :d by somebody else's module. Is there a way in gdb to get notified when a piece of memory is free() :d?

Answer 1

Suppose your libc's free 's argument is called mem .

Then, you can print out everything that is freed:

(gdb) break __GI___libc_free # this is what my libc's free is actually called
Breakpoint 2 at 0x7ffff7af38e0: file malloc.c, line 3698.
(gdb) commands 2
Type commands for when breakpoint 2 is hit, one per line.
End with a line saying just "end".
>print mem
>c
>end

Now, every time anyone frees anything, you will get a little printout (you can omit c if you want it to stop every time free occurs):

Breakpoint 2, *__GI___libc_free (mem=0x601010) at malloc.c:3698
3698    malloc.c: No such file or directory.
    in malloc.c
$1 = (void *) 0x601010

Or, if you already know what memory address you are interested in, use cond to break when someone tries to free that address:

(gdb) cond 2 (mem==0x601010)
(gdb) c
Breakpoint 3, *__GI___libc_free (mem=0x601010) at malloc.c:3698
3698    malloc.c: No such file or directory.
    in malloc.c
(gdb) 

Answer 2

In order to get info about memory leaks the following tools will be really helpful.

1. Valgrind

2. Google Perf Tools

And it doesn't take very long to get used to working with these - definitely worth giving a try.

Or using a hardware watch point to keep track of certain addresses might help - the debugger gets control whenever a read or write happens to the addresses you are watching - but I'm not sure if this offers the exact solution to your problem.