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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?

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  • 2
    You can't just start off with the array size at 60? Commented Oct 10, 2011 at 3:43
  • No, the 30 and 60 were just simple examples. In reality, the program is like a hashtable. Commented Oct 10, 2011 at 3:46
  • It's a bit unclear what you're trying to do: const char* arr[30] is an array of 30 char* but then you try to change it to point to an array of n char. Commented Oct 10, 2011 at 3:52
  • You have ->* notation - but that is a C++ operator, not a C operator. Typo? Commented Oct 10, 2011 at 4:33
  • Ah, then I am using the wrong operator. I'm trying to have an array of strings that holds 30 elements, and then change it to larger later. Commented Oct 10, 2011 at 5:16

4 Answers 4

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3

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.)

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2 Comments

I'm having a little trouble using this solution: Compiler is complaining: error: request for member ‘myarr’ in something not a structure or union. NOTE: I am trying to do typedef struct stringtable{} instead of just typedef struct{} because I need to use -> syntax rather than . syntax. Struct vs Object?
In C, 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.
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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];
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1 Comment

Thanks for responding Graham, please take a look at my response to lostyzd
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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.

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5 Comments

Thanks for the response, I'm attempting this right now but I'm getting an error when trying to add elements to the array like so: s->myarr[arrindex] = str. Error: assignment of read-only location
@user987042 My bad, I forget to tell you that const char means readonly, so you should declare char newtable[60] or use char *newtable = malloc(60)
Thank you sir, this has solved my array sizing issues. Unfortunately, this change broke the following line: if(strcmp(s->myarr[arrindex], "") == 0). This line results in a segmentation fault now. Is this because strcmp takes a const variable, and I am no longer declaring it as const?
Actually, I'm unable to do even the following: printf("%s\n", s->myarr[10]); Any ideas?
@user987042 It seems that u r using an "char **" as the answer u marked, so please update your question with more details, I don't understand what you are trying to do.
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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,

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