signal 0 command

signal 0 command

[Seong-Kook Shin]

Hi.

While reading the gdb info manual, I found the  paragraph saying:

     Alternatively, if SIGNAL is zero, continue execution without
     giving a signal.  This is useful when your program stopped on
     account of a signal and would ordinary see the signal when resumed
     with the `continue' command; `signal 0' causes it to resume
     without a signal.

If I understand correctly, I can issue "signal 0" to ignore the current
signal GDB caught, right?  So I made a simple program to raise SIGSEGV:

void
foo(const char *str)
{
  char *p = 0;     /* This should be "char *p = str" */

  while (*p != '\0') {
    /* do something */
    p++;
  }
}

After reading the manual, I thought that it is possible to
undo the generation of SIGSEGV after modifying the value `p'.
But when I set the value of `p' corrently, and execute "signal 0",
I can still see the SIGSEGV is generated.

$ gdb -q a.out
Using host libthread_db library "/lib/libthread_db.so.1".
(gdb) r
Starting program: /home/cinsk/pesticide/a.out

Program received signal SIGSEGV, Segmentation fault.
0x08048399 in foo (str=0x80484b8 "hello, world") at segv.c:8
8	  while (*p != '\0') {
(gdb) p p
$1 = 0x0
(gdb) p p = str
$2 = 0x80484b8 "hello, world"
(gdb) signal 0
Continuing with no signal.

Program received signal SIGSEGV, Segmentation fault.
0x08048399 in foo (str=0x80484b8 "hello, world") at segv.c:8
8	  while (*p != '\0') {
(gdb) _

Am I missing something?  If I misunderstand the meaning of "signal 0" command,
please let me know.  (A short example session would be great!!)

Thanks in advance.

-- 
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Korean: http://pcrc.hongik.ac.kr/~cinsk/cfaqs/

Re: signal 0 command

[Andreas Schwab]

"Seong-Kook Shin" <cinsky@gmail.com> writes:

> If I understand correctly, I can issue "signal 0" to ignore the current
> signal GDB caught, right?  So I made a simple program to raise SIGSEGV:
>
> void
> foo(const char *str)
> {
>   char *p = 0;     /* This should be "char *p = str" */
>
>   while (*p != '\0') {
>     /* do something */
>     p++;
>   }
> }
>
> After reading the manual, I thought that it is possible to
> undo the generation of SIGSEGV after modifying the value `p'.
> But when I set the value of `p' corrently, and execute "signal 0",
> I can still see the SIGSEGV is generated.

Modifying the variable does not necessarily mean that the current
instruction can see the change.  The value may be been loaded into a
register which is used instead by the insn.  You may have to modify the
program counter to move back to where the variable's value is loaded.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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