Re: break when opening certain file

[Doug Evans <dje at google dot com>]

On Mon, Jan 12, 2009 at 6:11 PM, peter360 <peter360@fastmail.us> wrote:
>
> I want to set a break point when the program I am debugging opens a certain
> file.  I can do "break open", but how do I make gdb stop only when file
> "foo" is opened?  Thanks!

You want a "conditional breakpoint".

Assuming you're on i386-linux or similar (adjustments are
straightforward for most other platforms),

#include <fcntl.h>

int
main (int argc, char *argv[])
{
  int i;

  for (i = 1; i < argc; ++i)
    open (argv[i], O_RDONLY);

  return 0;
}

bash$ gcc -g foo.c
bash$ gdb --args a.out foo bar baz
(gdb) start
(gdb) break open
(gdb) condition 2 strcmp (((char**)$esp)[1], "bar") == 0
(gdb) c
Continuing.

Breakpoint 2, 0x42ce0e80 in open () from /lib/tls/i686/cmov/libc.so.6
(gdb) x/s ((char**)$esp)[1]
0xffffdafe:      "bar"
(gdb) c
Continuing.

Program exited normally.
(gdb)

You kinda have to know the i386 calling convention to know that on
entry to open the file name is at ((char**)$esp)[1].

The use of "start" above simplifies a few things, it runs the program
to the start of main().  At this point libc is loaded and any open
calls made while trying to get to main() are skipped.

One caveat is that specifying the condition this way will call
malloc() to allocate space for "bar" so that the call to strcmp will
work.  If you don't want to call malloc when testing the breakpoint
condition then one alternative is to manually compare the characters.
A real pain, but if you're debugging a problem and you just need to
have gdb stop at the right file you often don't need to compare every
character in the file name, just enough to get you close.  You can
keep hitting continue until the breakpoint hits the right one.

Starting over,

bash$ gdb --args a.out foo bar baz
(gdb) start
(gdb) break open
(gdb) condition 2 ((char**)$esp)[1][0] == 'b'
(gdb) commands 2
x/s ((char**)$esp)[1]
end
(gdb) c
Continuing.

Breakpoint 2, 0x42ce0e80 in open () from /lib/tls/i686/cmov/libc.so.6
0xffffdafe:      "bar"
(gdb) c
Continuing.

Breakpoint 2, 0x42ce0e80 in open () from /lib/tls/i686/cmov/libc.so.6
0xffffdb02:      "baz"
(gdb) c
Continuing.

Program exited normally.
(gdb)