[oclug] time for my weekly dumb question -> C and memory

[Taavi Burns]

On Sun, 1 Apr 2001, Greg Sarsons wrote:

> this may explain the difficulty I'm having.  So how do you then copy using 
> strcpy when you have a struct that contains  zero length strings like are in 
> my struct message example?
> 
> I'm trying to send a message but the behaviour is not stable. For example, 
> given the following: 
> 
> typedef struct{
> 	unsigned short url_lenght;
> 	char url[0];
> 	unsigned short lengthOfScopeList;
> 	char scope_list[0];
> } ServiceAdvert;  

Another similar problem can be caused by standard compiler optimisations:

char StringA[] = "Hello.";
char StringB[] = "Hello.";
StringA[0] = 'B';
puts(StringB); /* prints "Bello." */

Because the strings are statically allocated, some compilers will in some
situations save space by only keeping one copy of two identical strings
(but not always; depends on the compiler and optimisation settings).  I
_have_ seen it happen.

> char recvBuffer[100];  /*buffer used to send message*/
> 
> ServiceAdvert *aMessage;
> 
> aMessage = (ServiceAdvert *) recvBuffer; 
> 
> strcpy(aMessage->url, "www.somewhere.come");


Hmmm...you need to have a structure in memory that you can just dump to
output, eh?  Perhaps you could use char* in your structure, and then
aMessage->url = strdup("www.somewhere.com"); to set it.  Your output
procedures could then pull the string in before actually outputting.  It's
much easier to manipulate things in C when you use pointers (that's why
they exist!).

> 
> 
> I always get the correct length if I use strlen(aMessage->url);
> 
> However, as soon as I do a strcpy into the other zero length string, 
> scope_list, the url is messed up.
> 
> My intent is to then send this message.
> 
> Greg
> 
> On Sunday 01 April 2001 11:12, you wrote:
> > Greg Sarsons wrote:
> > > struct message {
> > >
> > >         char aString[0];
> > >
> > > }
> > >
> 
> >
> > Also,
> > 	strcpy("fred", string3) ;  /* is legal, works fine */
> > but any of
> > 	strcpy("fred", string1) ;
> > 	strcpy("fred", string2) ;
> > 	strcpy("susan", string3) ;
> > go off the end of the array with unpredictable results.
> 
> >
> > There's only one use I know of for
> >
> > struct message {
> > 	...
> > 	char aString[0];
> > }
> >
> > That's when the actual structure is variable-sized. In use there'll be
> > a non-zero length string after the message header and you can access it
> > as the aString element. When you're declaring the function, though, you
> > don't know its size so you pretend it is zero.
> >
> > I know this is moderately widely used, works fine with many compilers,
> > but I'm not certain if it is actually sanctified by ANSI, legal in the
> > C standard.
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taa

   Science is but a perversion of itself unless it has
   as its ultimate goal the betterment of humanity.
         --Nikola Tesla
/*eof*/