[Techtalk] C Programming Question

[Julie]

Malcolm Tredinnick wrote:
> 
> On Wed, May 29, 2002 at 12:18:25PM +0800, Sujita Purushothaman wrote:
> > Hi,
> >     I'm having a problem with pointer-to-array in C.
> > I have to use the structure char  *somename[ ] . I also
> > need to store/assign data to the strings halfway thro the
> > program. ( I know I can initialise it as :
> > char  *somename[ ] = {"data1","data2"}; etc.
> > At some point in the program I need to change "data1"
> > to "somethingelse1" . How do I do that?
> 
> This does what you want:
> 
>         /* Reserve the space */
>         int length = strlen("somethingselse1");
>         somename[0] = (char *) malloc(length);
>         strncpy(somename[0], "somethingelse1", length);
> 
> Note that if the initial value ("data1") was not allocated as above, you
> may need to free its memory (however, in the above case, this is not
> required).

That's pretty barfy code.  Try something like

	somename[0] = strdup ("somethingelse1");

It completely avoids the off-by-one error that you (and a lot of
other programmers) made.  If you really, really want to know the
length of "somethingelse1", and "somethingelse1" is a fixed string,
consider

	int length = sizeof "somethingelse1";

	somename[0] = strcpy (malloc (length), "somethingelse1");

If you're curious about the use of "sizeof" above, try this
program --

main ()
{
	int length = sizeof "five";

	printf ("length = %d\n", length);
}

The advantage of using "sizeof" instead of strlen() is you
can do static allocations using sizeof, but not with strlen().
So you can do clever things like

char *temp_file_name = "/tmp/proggyXXXXXXXX";
char *temp_file_pid = temp_file_name + sizeof "/tmp/proggy";

	...
	sprintf (temp_file_pid, "%08.8d", getpid ());
	...

and avoid run-time dorking with addresses and what-not.  Or
perhaps even more bizarre

char	temp_file_name[sizeof "/tmp/proggy" + 3 * sizeof (pid_t) + 1];

	...
	sprintf (temp_file_name, "/tmp/proggy%d", getpid ());
	...

Handy for writing code on machines with PIDs that are 16, 32
or 64 bits long.
-- 
Julianne Frances Haugh             Life is either a daring adventure
jockgrrl at austin.rr.com                 or nothing at all.
					    -- Helen Keller