Re: OT: Problems with strings

[Hongli Lai <h lai chello nl>]

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 */