I have an array inside a struct like so:
typedef struct mystruct{
const char *myarr[30];
} mystruct;
I need to grow this array later in the program to 60 elements by creating a new array, duplicating the content, and then changing myarr to point to the new array.
I have tried the following:
const char newtable[n];
s->*myarr = newtable;
But gcc complains:
error: incompatible types in assignment
Any ideas as to the right way to accomplish this?
Assuming that you indeed want your array to contain char *s, not chars, you should define your structure like this:
typedef struct {
const char **myarr;
/* I assume you actually have more members here */
} mystruct;
and initialize it like this:
mystruct s;
s.myarr = (const char **) malloc(30 * sizeof(const char *));
if (!s.myarr) { /* handle out-of-memory condition somehow */ }
Then you can later extend it with realloc():
const char **tmp = (const char **) realloc(s.myarr, 60 * sizeof(const char *));
if (tmp) s.myarr = tmp;
else { /* handle out-of-memory condition somehow */ }
(Note that, if realloc() returns NULL, the original value of s.myarr will still be valid.)
foo->bar is just a shorthand for (*foo).bar. You should use . when you have the struct itself as a variable, and -> when you only have a pointer to the struct.Don't allocate it as an array inside the struct. Just leave it as a pointer:
const char *myarr;
Then this'll work:
const char newtable[n];
s->myarr = newtable;
You can still use it like an array, e.g.
char c = s->myarr[20];
You declare an array of pointer, not a pointer point to a array.
typedef struct { // no need mystruct here
const char *myarr;
} mystruct;
const char newtable[60];
s->myarr = newtable;
But const char *myarr is different from const char (*myarr)[30], and you didn't actually create a new array by const char newtable[60];, you maybe need malloc instead.
const char means readonly, so you should declare char newtable[60] or use char *newtable = malloc(60)So growing memory region at runtime should really involve dynamic memory allocation. I'd suggest something like this:
typedef mystruct {
char *data;
} mystruct;
[...]
char *ptr = realloc(s->data, 60); // that will copy your previous data over
if (ptr != NULL) {
s->data = ptr;
}
I don't think it has to be much more complicated than that really... especially you should avoid declaring a 60 elements array statically in the .data/.rodata section...
Hope it helps,
->*notation - but that is a C++ operator, not a C operator. Typo?