[ILUG] C string concat question revisited

[kevin lyda]

On Tue, Jul 03, 2001 at 08:38:50AM +0000, Conor Daly wrote:
> On Fri, May 04, 2001 at 01:11:07PM +0100 or thereabouts, kevin lyda wrote:
> > #define STRSIZE 80
> > char *
> > strdup2(char *s1, char *s2)
> > {
> >     char *s;
> > 
> >     if ((s=(char *)malloc(STRSIZE + 1)) == NULL) {
> > 	return NULL;
> >     }
> >     /* the following initialises the string.  s[0]=0;s[STRSIZE]=0; *
> >      * would work too.                                             */
> >     memset(s, 0, STRSIZE + 1);
> >     /* from here on in we don't let str* routines near s[STRSIZE]  *
> >      * so we know it will stay 0.                                  */
> >     strncpy(s, s1, STRSIZE);
> >     strncat(s, s2, STRSIZE - strlen(s));
> >     return s;
> > }
> > 
> > strlen works by counting chars until it reaches '\0'.  therefore don't use
> >     it if you're not sure the string is null terminated.
> > sizeof(char) is not required.

well, i left some thoughts out.  all the str* routines assume strings are
nul (not NULL) terminated.  a more dynamic version would malloc a char *
of strlen(s1) + strlen(s2) + 1;

> Some while back, Kevin posted the above which allocates *new* memory for a
> string s which contains s1 and s2 (useage: s = strdup2(s1, s2); ).  
> Now, I have a string struct.s which, as the name suggests, is part of an 
> already allocated struct.  If I want to set *that* to contain s1 and s2, can 
> I use the above ( struct.s = strdup2(s1, s2); ) or do I just use strncpy() 
> and strncat()?

so how is it defined?  if it's

    struct foo {
	char s[123];
    };

then no, the memory is already allocated.  in fact using strdup2 would
be an error.  otoh, if it is defined as:

    struct foo {
	char *s;
    };

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.

kevin