Re: Seg Fault: Debugging Beginner

[Don Dudley <ddudley mybearcreek com>]

On Sat, 2003-12-20 at 12:18, Michael Torrie wrote:
> On Sun, 2003-12-21 at 05:19, Sukrit K Mehra wrote:
> > Thanks all for helping me throughout the last two weeks. 
> > I have a code snippet that compiles prefectly but the app then gives a 
> > segmentation fault on running. Same code snippet elsewhere in the application 
> > works alright.
> > How can I correct it? What do seg fault generally mean?
>
> A seg fault occurs when you access memory that you don't have permission
> to access.  This is a common C problem, as C requires that you allocate
> and manage your own memory.
>
> To find the place in your code where it is segfaulting, compile your
> program with a "-g" (both compile and link stage) parameter.  Then use a
> debugger like ddd or kdbg (comes with KDE), or even commandline gdb and
> you can see the exact line of code where the problem is occurring.  You
> can also use a debugger to step through your code to watch variables and
> make sure they are right.  Debuggers seem to be lost arts these days,
> but I assure you learning a powerful debugging interface like ddd is
> well worth your time.
>
> To fix a seg fault, you need to make sure you're not accessing memory
> beyond the end of an array, or that your pointer isn't NULL.  Also you
> need to make sure that your pointer is pointing into valid memory.
>
> Michael
>
>
> > The code is given below.
> >
> > void display_db(void) {
> >     int i, j;
> >     gint row_status = -1;
> >     gchar *text[3];
> >     PGresult *res;
> >     PGconn *conn;
> >     conn = PQconnectdb(CONSTRING);
> >     if(PQstatus(conn) == CONNECTION_BAD){
> >             fprintf(stderr, "Connection to database failed.\n");
> >         fprintf(stderr, "%s", PQerrorMessage(conn));
> >         PQfinish(conn);
> >         exit(1);
> >         }
> >
> >     gtk_clist_clear(GTK_CLIST(lookup_widget (GTK_WIDGET(window1), "clist1")));
> >     gtk_clist_freeze(GTK_CLIST(lookup_widget (GTK_WIDGET(window1), 
> > "clist1")));
> >     for(i = 0; i < PQntuples(res); i++) {
> >             for(j=0; j <PQnfields(res); j++) {
> >             text[j] = PQgetvalue(res, i, j);
> >             }
> >         row_status = gtk_clist_append(GTK_CLIST(lookup_widget 
> > (GTK_WIDGET(window1), "clist1")), text);
> >     }
> >     g_free(text);
> >     PQclear(res);    
> >     PQfinish(conn);
> >     gtk_clist_thaw(GTK_CLIST(lookup_widget (GTK_WIDGET(window1), "clist1")));
> > }

Michael,
If you haven't fixed this yet, you might look at your inner for loop,
where I'm sure you're hoping PQnfields(res) doesn't ever exceed 3, the
size of your text array.  You could change it to 
... j < PQnfields(res) && j <3; j++);
just to be safe.

Don