Thread: Two dimensional array as buffer for string list

> Is it safe to use a two dimensional array as buffer like that?
Yes, it looks OK.

> Some of the string lists are fixed:
> Other string lists are created within the function
You need to be careful about how you modify strings, if some are indeed fixed.

> Is listbuffer[n] guaranteed to point to listbuffer[n][0] ?
listbuffer[n] could have been written as &listbuffer[n][0], so yes.

If every string is close to MAXITEMLENGTH in length (the average is less than a pointer + malloc overhead), then the array is probably best.

But if you have many strings of widely differing lengths, then mallocing each one to the right length may be better.

But you also create the additional complication of perhaps having some pointers to allocated memory, and other pointers to string constants. This would complicate things when it comes to freeing memory.

The malloc overhead is the additional data needed by malloc to manage the memory pool. Each malloc call allocates the 'n' bytes you request.
But additionally, there may be things like:
- next/prev pointers
- the size of the block allocated
- padding at the end of the block to preserve memory alignment.

I mean if you do

itemlist[i] = "a string";

then later

itemlist[i] = malloc(20);

you may have a problem when it comes to freeing memory if you can't be sure whether itemlist[i] points at malloc'ed memory or not.

Ok, I see.

In this case, itemlist would be dedicated to the malloc'd memory.

It would be a separate pointer that the requested list is assigned to:

Code:

char *configlist[NCONFIGS] = {"Ser", "Par", "Coil a", "Coil b"};
int coilcapvalue[NCOILCVALS] = {50, 60, 70, 80, 90, 100, 110, 120, 130, 140};

int getitemlist(int paramtype, char ***list)
{
    switch(paramtype)
    {   case PT_COILCAP:
            for(int i = 0; i < NCOILCVALS; i++)
                sprintf(itemlist[i], "%2d pF", coilcapvalue[i]);

            *list = itemlist;
            return NCOILCVALS;

        case PT_CONFIG:
            *list = configlist;
            return NCONFIGS;
    }

    return 0;
}

Only two case examples shown - some cases will assign fixed string lists to the supplied pointer, and other cases will create the required strings in itemlist and assign itemlist to the pointer.

-