segmentation fault

segmentation fault

[John]

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Hi all:
 
I encounter a segmentation fault probelm. When I use
"gdb core", gdb shows that the program stops at the
following line of my code:
sprintf(pt_->buffer() + offset,
 
I think that line cause segmentation fault. 
I want to set a breakpoint at that line and run the
code to see what happens before segmetation fault. How
can I make the program stop right before segmentation
fault happens? 
I also attach some information from gdb. 
 
Thanks a lot.
 
John
 
---------------------------------------------------------------------------------
Loaded symbols for /lib/ld-linux.so.2
#0  0x4026ecef in vfprintf () from /lib/libc.so.6
(gdb) bt
#0  0x4026ecef in vfprintf () from /lib/libc.so.6
#1  0xbfffdd04 in ?? ()
#2  0x40286fcb in vsprintf () from /lib/libc.so.6
#3  0x4027492d in sprintf () from /lib/libc.so.6
#4  0x0812226b in CMUTrace::format_mac_common(Packet*,
char const*, int) (this=0x97528b8, p=0x12ef75a0,
why=0x82cc85c "---", offset=0)
    at trace/cmu-trace.cc:228
 .........
 
(gdb) frame 4
#4  0x0812226b in CMUTrace::format_mac_common(Packet*,
char const*, int) (this=0x97528b8, p=0x12ef75a0,
why=0x82cc85c "---", offset=0)
    at trace/cmu-trace.cc:228
228             sprintf(pt_->buffer() + offset,
Current language:  auto; currently c++
(gdb)






	
		
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Re: segmentation fault

[Raúl Huertas]

Daniel Jacobowitz escribió:
> On Sat, Nov 11, 2006 at 09:29:39PM +0100, Raúl Huertas wrote:
>   
>> No. ;)
>> Maybe I'm wrong, but sptr is a pointer, so it uses 0x804b028 and 0x804b029.
>> The memory pointed by this pointer is the one that has size 16.
>>     
>
> You're incorrect, in fact.  The original poster's interpretation is
> reasonable, assuming that GDB has printed out the correct values of
> variables.
>
>   
Yes, you are right, and I was incorrect:

struct servent
{
   char memory[16];
};

int main()
{
    servent * sptr1 = new servent;
    servent * sptr2 = new servent;
    servent * sptr3 = sptr1;
}

----------------
(gdb) p sptr1
$1 = (servent *) 0x804a008
(gdb) p sptr2
$2 = (servent *) 0x804a020
(gdb) p sptr3
$3 = (servent *) 0x804a008

gdb shows the content of the pointer, not the position of the pointer. 
My idiot error... :*)

Re: segmentation fault

[Daniel Jacobowitz]

On Sat, Nov 11, 2006 at 09:29:39PM +0100, Raúl Huertas wrote:
> No. ;)
> Maybe I'm wrong, but sptr is a pointer, so it uses 0x804b028 and 0x804b029.
> The memory pointed by this pointer is the one that has size 16.

You're incorrect, in fact.  The original poster's interpretation is
reasonable, assuming that GDB has printed out the correct values of
variables.

-- 
Daniel Jacobowitz
CodeSourcery

Re: segmentation fault

[Raúl Huertas]

Oliver Block escribió:
> Hello,
> ...
> (gdb) p sizeof(struct servent)
> $1 = 16
>
> When I extract
>
> from #2:
> sptr = (struct servent *) 0x804b028
>
> from #4:
> r = 0x804b030 
>
> as servent has a size of 16 bytes
>
> 804b028 + 16 = 0x804b038
>
> These two variables seem to overlap. Do you agree? 
>
>   
No. ;)
Maybe I'm wrong, but sptr is a pointer, so it uses 0x804b028 and 0x804b029.
The memory pointed by this pointer is the one that has size 16.

Regards,
Raul.

segmentation fault

[Oliver Block]

Hello,

a program causes a segmentation fault and I'd like to know if I interpret the 
following gdb output correctly (text continues after output):
************************************************************************************
#1  0x4012eb3f in gethostbyname () from /lib/tls/libc.so.6
No symbol table info available.
#2  0x08049312 in http_send_recv (hostname=0x804a340 "www.oliver-block.eu",
    request=0x804b030 "GET /checkip.php HTTP/1.0\r\nHost: 
www.oliver-block.eu\r\n\r\n") at ddnscli.c:168
        hptr = (struct hostent *) 0x804b068
        sptr = (struct servent *) 0x804b028
        sin = {sin_family = 2, sin_port = 0, sin_addr = {s_addr = 0},
  sin_zero = "\000\000\000\000\000\000\000"}
        s = 134514323
        retval = 45
        n = 0
        buflen = 1024
#3  0x080498ee in get_ip (qry=0x804b008) at ddnscli.c:323
        tmp = 0x0
        r = 0x804b030 "GET /checkip.php HTTP/1.0\r\nHost: 
www.oliver-block.eu\r\n\r\n"
#4  0x080499fa in main (argc=3, argv=0xbffff1a4) at ddnscli.c:398
        option = 8 '\b'
        q = (struct query *) 0x804b008
(gdb) p sizeof(struct servent)
$1 = 16
****************************************************************************
When I extract

from #2:
sptr = (struct servent *) 0x804b028

from #4:
r = 0x804b030 "GET /checkip.php HTTP/1.0\r\nHost: www.oliver-block.eu\r\n\r\n"

as servent has a size of 16 bytes

804b028 + 16 = 0x804b038

These two variables seem to overlap. Do you agree? 

Regards,

Oliver
-- 
Leben ist mehr als ... 
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segmentation fault

[Oliver Block]

Hello,

a program causes a segmentation fault and I'd like to know if I interpret the 
following gdb output correctly (text continues after output):
************************************************************************************
#1  0x4012eb3f in gethostbyname () from /lib/tls/libc.so.6
No symbol table info available.
#2  0x08049312 in http_send_recv (hostname=0x804a340 "www.oliver-block.eu",
    request=0x804b030 "GET /checkip.php HTTP/1.0\r\nHost: 
www.oliver-block.eu\r\n\r\n") at ddnscli.c:168
        hptr = (struct hostent *) 0x804b068
        sptr = (struct servent *) 0x804b028
        sin = {sin_family = 2, sin_port = 0, sin_addr = {s_addr = 0},
  sin_zero = "\000\000\000\000\000\000\000"}
        s = 134514323
        retval = 45
        n = 0
        buflen = 1024
#3  0x080498ee in get_ip (qry=0x804b008) at ddnscli.c:323
        tmp = 0x0
        r = 0x804b030 "GET /checkip.php HTTP/1.0\r\nHost: 
www.oliver-block.eu\r\n\r\n"
#4  0x080499fa in main (argc=3, argv=0xbffff1a4) at ddnscli.c:398
        option = 8 '\b'
        q = (struct query *) 0x804b008
(gdb) p sizeof(struct servent)
$1 = 16
****************************************************************************
When I extract

from #2:
sptr = (struct servent *) 0x804b028

from #4:
r = 0x804b030 "GET /checkip.php HTTP/1.0\r\nHost: www.oliver-block.eu\r\n\r\n"

as servent has a size of 16 bytes

804b028 + 16 = 0x804b038

These two variables seem to overlap. Do you agree? 

Regards,

Oliver