Re: OT: Problems with strings

[Stefan Kost <ensonic hora-obscura de>]

Hi,

as a final note, what for are you doing the memset() in the last line.
The buffer variable point to the memory location returned by
  const gchar* gtk_entry_get_text(GtkEntry *entry);
The 'const gchar*' indicats that you should not modify nor free the contents.
So if you just want to print out the contaent you can do:
  g_print("entry contains '%s'\n",gtk_entry_get_text(GTK_ENTRY(entry_widget)));
but if you want to keep it, do this instead
  gchar *post;
  post=g_strdup(gtk_entry_get_text(GTK_ENTRY(entry_widget)));
  ...
  g_free(post);

Ciao
  Stefan
> Magnus Wirström wrote:
>
> > Hi.
> >
> > I'm a newbie so please don't fry me :)
> > I have a strange problem (at least i think it's strange ;) ). In my 
> > app i set up a gchar like this
> >
> >     gchar *post;
> >
> >     entry_widget = lookup_widget(GTK_WIDGET(button), "text_bolag");
> >     buffer = gtk_entry_get_text(GTK_ENTRY(entry_widget));
> >     strcpy(post,buffer);
> >     memset(buffer,0,100);
> >
> > When i run it I get segmentation fault and the debugger tell me this 
> > error.
> > post        (gchar *)0x12 <Address 0x12 out of bounds>
> >
> > What could generate a such runtime error ? I have no idea why i get 
> > this and it have been working before. I'm thankful for any suggestion.
> >
> > If you wanna more info please email me.
>
>
> It seems you are a newbie to C. I suggest you to borrow a few books 
> about C from your local library.
>
> Let us take a look at this piece of code:
>  >     strcpy(post,buffer);
>
> The function strcpy copies the content of 'buffer' into the memory block 
> pointed to by 'post'. However, 'post' is uninitialized, so it doesn't 
> point to any memory block! (or actually, it points to random, 
> uninitialized garbage memory blocks that you cannot use, because it's 
> uninitialized).
>
> You must first allocate memory:
> post = malloc(100);
> strcpy(post, buffer);
>
> However, there are problems in this example! What if 'buffer' is larger 
> than 100 bytes? You will get memory corruption. (in this case, it's a 
> buffer overflow, so it's a security bug!) There are two ways to solve 
> the problem.
> 1. Find out how much memory you need, and allocate that much.
>    gchar *post;
>    int len;
>    ...
>    len = strlen(buffer);
>    /* We allocate len+1 bytes, because strlen() returns the length of 
> the string, excluding the trailing NUL */
>    post = malloc(len + 1);
>    strcpy(post, buffer);
>
> 2. Use strncpy(), which puts a limit into how many bytes it will copy at 
> most.
>    post = malloc(100);
>    /* Fill the entire memory block full of zeroes. If 'buffer' is longer 
> than 99 bytes then this ensures that 'post' is correctly NUL-terminated. */
>    memset(post, 0, 100);
>    strncpy(post, buffer, 99);
>
>
> I hope you've learned from what I wrote.
>
> But, there is a much simpler solution to your problem. You just want to 
> duplicate a string. In that case, use strdup() (or the glib equivalent: 
> g_strdup()):
>
> gchar *post;
>
> entry_widget = lookup_widget(GTK_WIDGET(button), "text_bolag");
> buffer = gtk_entry_get_text(GTK_ENTRY(entry_widget));
> post = g_strdup(buffer);
> memset(buffer,0,100);
> /* Don't forget to free 'post' when you don't need it anymore */
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