malloc, realloc, calloc

This is a discussion on malloc, realloc, calloc within the C Programming forums, part of the General Programming Boards category; Hi All; I am having serious problems getitng the above to work for strings in C, can do it no ...

  1. #1
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    malloc, realloc, calloc

    Hi All;

    I am having serious problems getitng the above to work for strings in C, can do it no problem for int*

    here is so some code

    Code:
    #define SIZE 7
    #define STR_SIZE 256
    int main(void)
    {
    	char *array[SIZE];
    	int i;
    
    	for(i=0;i<7;i++)
    	{
    		array[i]=malloc(STR_SIZE);
    		printf("Enter a string:");
    		scanf("%s", array[i]);
    	}
    	
    	for(i=0;i<7;i++)
    		printf("%s\n", array[i]);
            return 0;
    
    	free(array);
    }
    This code is ok, but how would i go about making it more dynamic, saying increasing the array size, i need to use realloc but cant get it working, can some show us?

    Also If i wasnt to use char* array[SIZE] but use char* string how would i do it.


    Please note this is not a homework question but something i am doing to get better at C and cant this working at all.

    Thanks

  2. #2
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    If you want the size to be dynamic, then use a double pointer. Declare your array like:
    Code:
    char **array;
    array = malloc(array_size * sizeof(*array));
    for(i = 0; i < array_size; i++)
    {
        array[i] = malloc(STR_SIZE);
        ...
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  3. #3
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    Code:
    int main(void)
    {
    	char *array[7];
    	int i;
    
    	for(i=0;i<7;i++)
    	{
    		array[i]= (char*)malloc(sizeof(array[i])); //cast the malloc to char*
    		printf("Enter a string:");
    		scanf("%s", array[i]);
    	}
    	
    	for(i=0;i<7;i++)
    		printf("%s\n", array[i]);
            return 0;
    
    	free(array);
    }
    Last edited by valthyx; 08-13-2009 at 11:59 AM.

  4. #4
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    valthyx: All you added was a cast to malloc() which is a bad practice.
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  5. #5
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    hmm... valthyx, your example seems no better, if not worse: it changes the use of named constants to magic numbers and leaves the call to free() after the return. Casting the retun value of malloc is unnecessary anyway.
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    Ok thats good, but the main thing i want to do is jsut use a char *ptr to get dynamically increase when it takes in a string. I have somehting like this:

    Code:
    nt main()
    {
    	int i;
    	char *pdata;
    	int n;
    	printf("Enter number of segments we want");
    	scanf("%d", &i);
    	pdata = calloc(i, sizeof(256));
    	if(pdata==NULL)
    		printf("err");
    
    	for(n=0; n<i;n++)
    	{
    		printf("Enter a name");
    		scanf("%s", &pdata[i]);
    	}
    
    	
    	for(n=0;n<i;n++)
    	{
    		printf("%s\n",pdata[n]);
                    }
    	free(pdata);
    }
    But the print prints out null.
    Trying to create a dynamic pointer of char* to strings, is it possible?


    Thanks

  7. #7
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    sizeof(256) doesn't return what you think it does. sizeof(256) is probably 4.

    I don't see how my original example fails to suit your needs.
    Code:
    nt main()
    {
    	int i;
    	char **pdata;
    	int n;
    	printf("Enter number of segments we want");
    	scanf("%d", &i);
    	pdata = malloc(i * sizeof(*pdata));
    	if(pdata==NULL)
    		printf("err");
    
    	for(n=0; n<i;n++)
    	{
                    pdata[n] = malloc(256);
    		printf("Enter a name");
    		scanf("%s", pdata[n]);
    	}
    
    	
    	for(n=0;n<i;n++)
    	{
    		printf("%s\n",pdata[n]);
                    free(pdata[n]);
            }
    	free(pdata);
    }
    bit∙hub [bit-huhb] n. A source and destination for information.

  8. #8
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    HI;

    Working with your example now, but only one thing else i need to know and maybe you can explain it to me.

    How would i use calloc and realloc in these situations, as i would like to be able increase the **array as i go along. Also after i use realloc or calloc to i need to malloc the array[i] again?

    Thanks

  9. #9
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    You can realloc the array if you need to resize it to something else. If you realloc to make the array bigger, don't forget to call malloc() for each of the new indexes that you have created.
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  10. #10
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    So something like this

    pdata=realloc(*pdata, sizeof(*pdata));

    which will give one extra onto my array and if i wanted more if be:

    pdata=realloc(*pdata, 4*sizeof(*pdata)); ?


    You totally explain the dynamic stuff to me now, be messing with it for awhile and got half way only.
    Thanks

  11. #11
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    No, that's not right. Your code will shrink pdata down to 1 index and 4 indexes respectively. You would need something like this:

    Code:
    size_t array_size = 10;
    char** pdata;
    
    pdata = malloc(array_size * sizeof(*pdata));
    
    ...
    
    // Now we want to resize pdata from 10 to 15
    array_size = 15;
    pdata = realloc(pdata, array_size * sizeof(*pdata));
    bit∙hub [bit-huhb] n. A source and destination for information.

  12. #12
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    Why does it shrink? Can you explain that?

    Thanks for showing us how to do it.


    Thanks

  13. #13
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    If you call realloc() with a size that is smaller than the size it currently is, then it will shrink. In other words:
    Code:
    int* i = malloc(10 * sizeof(int));
    i = realloc(i, 5 * sizeof(int)); // shrink i from 10 indexes down to 5
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  14. #14
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    If you want to be really careful, assign the result of realloc() to another pointer instead of pdata. If the return value of realloc is not a null pointer, then assign that auxiliary pointer's value to pdata. This ensures that if realloc was not able to allocate space, you still have a pointer to the already allocated array's first element.
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  15. #15
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    Hi Guys;

    Thanks a mill, totally misunderstood how realloc worked and it was killing me!

    I thought it added on to the current load!

    Here is my code now:
    Code:
    int main()
    {
    	int i;
    	char **pdata;
    	//pdata = malloc(1 * sizeof(*pdata));
            pdata=calloc(4, sizeof(*pdata));
    
    	for(i=0;i<4;i++)
    	{
    		printf("Enter a name");
    	        pdata[i] = malloc(STR_SIZE);
    		scanf("%s", pdata[i]);
    	}
    
    	pdata=realloc(pdata, 5*sizeof(*pdata));
    
    	pdata[4]=malloc(STR_SIZE);
    	printf("Enter a string");
    	scanf("%s", pdata[4]);
    
    	for(i=0;i<5;i++)
    	{
    	     printf("\n%s", pdata[i]);
    	     free(pdata[i]);
    	}
    }


    Thanks again.

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