Dynamic array (or something like that) in C

Hello,

I have a structure like this:

Code:

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typedef struct
{
        char *name;
        char keys[20][6];
} row;

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However, I don't want to specify how many keys I can have in the array.
Can I replace it with something like **keys and then reallocate memory when I add data ? Any suggestions how to do it ?

Best wishes,
D5

You can have either a pointer to a fixed array:

Code:

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char (*keys)[6]

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or a pointer to pointer and allocate two arrays.

Code:

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char **keys;

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In a 32-bit machine, if your key is always 6 chars, you loose 4 bytes per key if you go with the second approach.

--
Mats

You can do it like this.

Code:

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#include <stdio.h>
#include <string.h>

typedef struct
{
        char *name;
        char **keys;
} row;

int main()
{
    row temp;
    int i;

    temp.name = malloc(50 * sizeof(char));
    strcpy(temp.name, "Banana Man");

    temp.keys = malloc(5 * sizeof(char*));

    for ( i = 0; i < 5; ++i )
    {
        temp.keys[i] = malloc(3 * sizeof(char));
        memcpy(temp.keys[i], "1234567890" + i, 3);
    }

    puts(temp.name);

    for ( i = 0; i < 5; ++i )
    {
        int j;

        for ( j = 0; j < 3; ++j )
        {
            putchar(temp.keys[i][j]);
        }

        putchar('\n');
    }

    return 0;
}

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The program never frees the memory. Bad.

Quote:

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Originally Posted by matsp

In a 32-bit machine, if your key is always 6 chars, you loose 4 bytes per key if you go with the second approach.

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Forgive my ignorance, but how are you losing 4 bytes?

Quote:

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Originally Posted by cs32

Forgive my ignorance, but how are you losing 4 bytes?

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Because you'll have an extra pointer for every array element, that you won't need in this particular case since you have fixed-width keys.

Quote:

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The program never frees the memory. Bad.

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I'm sure you have my best interests at heart and weren't just trying to be a know it all, so thanks for the nitpick. Anyway, my OS will free memory when the program ends and there's no point being anal about that kind of thing when it's just a little example for this thread. You might as well blast me for using // comments in C, or using magic numbers, or mixing tabs and spaces for indentation while you're at it. ;)

I could blame for all of it, but not freeing memory is not a good example for others, sooo... :)
I know the OS frees up resources at the end of a program, but I find it very bad practice to rely on that. There was a discussion about it before, and most agreed that it might be an "optimization," but certainly not a good idea just to be sloppy.
It's going to burn you someday, especially in C++.

Quote:

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Anyway, my OS will free memory when the program ends and there's no point being anal about that kind of thing when it's just a little example for this thread.

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I raised this point before, and while the opinion was divided, a general consensus is that we should teach best practices here. A simple solution is to show a code snippet instead, with the expectation that the reader should either know the rest, or better find out for himself.

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You might as well blast me for using // comments in C

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Those are standard in C99.

Quote:

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or mixing tabs and spaces for indentation while you're at it.

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Elysia is well known for doing that, I do it myself when I find the indentation unacceptable for reading.

Quote:

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most agreed that it might be an "optimization," but certainly not a good idea just to be sloppy.

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Because I relied on an "optimization" it means I was being sloppy? :rolleyes:

Quote:

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It's going to burn you someday, especially in C++.

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I doubt it, but you can believe whatever you want to.

Quote:

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Originally Posted by Banana Man

Because I relied on an "optimization" it means I was being sloppy? :rolleyes:

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Yes. It's sloppy not to free memory. There was no need for such an optimization here.

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I doubt it, but you can believe whatever you want to.

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Really? How well do you actually know C++?

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a general consensus is that we should teach best practices here

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Yeah, but who dictates what best practices are?

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Those are standard in C99.

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I thought "best practice" was not to use C99. :rolleyes:

Quote:

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Originally Posted by Banana Man

Yeah, but who dictates what best practices are?

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Describe to me how not freeing memory is good practice.

Quote:

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Yeah, but who dictates what best practices are?

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Common sense: if you do not free what you allocate, you end up with a memory leak. That the OS may act like a garbage collector is irrelevant.

I do not entirely agree with this stand myself (it sounds a little too harsh), but I have to agree that since we are concentrating on standard C, and standard C is not concerned with whether the OS does such a reclamation of memory, we should teach as if this is not the case, and indeed it might not be the case.

You can read the discussion for yourself: Concerning delete/delete[] at program exit

Note that it is in the C++ programming forum, not here.

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I thought "best practice" was not to use C99.

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That's Prelude's opinion, but since that version of the C standard exists, to say that // is not part of C would be factually incorrect.

Quote:

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Describe to me how not freeing memory is good practice.

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Sorry, but I don't want to take over this thread until desmond5 says the original question is answered. I also think that you're not going to take any of my opinions seriously, so explaining them to you is pointless.