[ILUG] C string concat question revisited

[kevin lyda]

On Tue, Jul 03, 2001 at 09:34:43AM +0000, Conor Daly wrote:
> > then the answer is "it depends."  if that element of the struct is already
> > pointing at a string, you could use strncpy (if you know the length of
> > the data pointed to by s).  or you could free it and use strdup2.
> 
> "if you know the length of the data pointed to by s"
> 
> Therin lies the rub.  I'm trying to be general here so I don't know the
> length of the destination char *array* 's'.  I can find out the length of the
> string *within* 's' using strlen() but I don't know of a function that will
> return the amount of memory *allocated* to 's'.  
>
> > or you could free it and use strdup2.
> This looks likely.  so I use 
> 
> foo.s = mystrdup2(foo.s, s2);

but you didn't free it first?

> where
> 
> char * mystrdup2(char *s1, char *s2) {
> 
> /* create a tmp string to hold 's1' while we free that */
> 	char *tmp;
> 
> /* take a copy of s1 */
> 	if( (tmp = strdup(s1)) == NULL) return(s1); /* we failed to make a copy so bail out */
> 
> /* then free it */
> 	free(s1);
> 
> /* now use strdup2() to concat the two strings */
> 	if( (s1 = strdup2(tmp, s2)) == NULL) {
> /* the copy failed so we want to return the original string unchanged.
>  * Maybe we should think about some way for the caller to be told that
>  * we couldn't do the requested job but if we return NULL, we've 
>  * destroyed the original string which might upset the user!
>  */
> 		return(tmp);
> 	} else {
> /* free the tmp string */
> 		free(tmp);
> /* and return the new pointer to s1 */
> 		return(s1);
> 	}
> 
> }

what a bizzare way to accomplish that.  i get the impression that you
think this function will be have this way:

    /*
     * string1 = mem location 100; mem loc 100 contains "abc\0"
     * string2 = mem location 200; mem loc 200 contains "xyz\0"
     */
    string12 = mystrdup(string1, string2)
    /*
     * string1 = mem loc 100; the contents of 100 are unpredictable
     * string2 = mem location 200; mem loc 200 contains "xyz\0"
     * string3 = mem location 300; mem loc 200 contains "abcxyz\0"
     */

note that string1 is now in an undefined state.

> All this depends on being able to do malloc and free stuff with bits of
> structs.  Like:
> 
> struct foo {
> 	char *s;
> } foozle;  /* to plageri[sz]e a term of Kevin's)
> 
> foozle.s = (char *) malloc( <number of chars required> * sizeof(char));
> 
> and 
> 
> free(foozle.s);

yes, of course.  you have to be able to manipulate them.  foozle.s is
a char * and can be used like that wherever.

kevin