GDB Objective-c debugging (no symbol table)

Question

I have an executable and I am debugging it using gdb. This is my first time using gdb so bear with me please.

I want to set a breakpoint at a function and I know the name of the function using class dump. Now it won't let me add breakpoint to that function because it say's that there's no symbol table. I tried adding the symbol table but it still complains that no symbol table loaded.

So, is there's anyway I can set a breakpoint at this method? It's an objective c method, not c (If that makes a difference). All I need to do is examine the argument of this method.

How are you invoking GDB? Command line?

Kirby Todd
– Kirby Todd

2011-03-26 07:03:01 +00:00
Commented Mar 26, 2011 at 7:03 — Kirby Todd
– Kirby Todd, Commented Mar 26, 2011 at 7:03

kennytm · Accepted Answer · 2011-03-26 07:03:08Z

8

In class-dump there is an -A option will can print the function's address, e.g.

@interface FooObject : NSObject
{
}

- (void)y;  // IMP=0x100000d54

@end

With this you can set a break point using the address:

(gdb) b *0x100000d54
Breakpoint 1 at 0x100000d54

Note that, unless you have stripped the executable, you should always be possible to set a break point using the method's name

(gdb) b -[FooObject y]
Breakpoint 2 at 0x100000d60

(The address isn't the same as the latter skips some frame set-up code.)

answered Mar 26, 2011 at 7:03

kennytm

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4 Comments

user635064 Over a year ago

Thanks, just did that, and although I set the breakpoint at 0x00420d10, it stopped at "Breakpoint 1, 0x35af7d10". This is not what I set the break point to? Thanks again

kennytm Over a year ago

@user: 0x3xxxxxxx would be the location of some Apple libraries. Are you sure you have cleared all previous break points?

jim.huang Over a year ago

With class-dump-z -A i only ge the VM offset, e.g.0x97179, which i think need to add with application's base address, how can i get the base address when an application get loaded?

Paul Semionov Over a year ago

@jim.huang: base address is 0x1000, but with ASLR (iOS 6): in gdb console prompt use command "info sharedLibrary" and grab the address at the very beginning of the output.

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