How can allocate multi dimensional array at run time?

Question

at design time I could have declare a variable like this:

char szDesignTimeArray[120][128];

The above declaration is 120 arrays of size 128. At run time I need to allocate the following:

char szRunTime[?][128];

I know the size of the arrays but I do not how many arrays I need to allocate. How can I declare this and allocate them when I know the number?

Thnaks all

C99 available? Then a variable length array may be the simplest. — Daniel Fischer, Sep 07 '12 at 12:06
http://stackoverflow.com/questions/455960/dynamic-allocating-array-of-arrays-in-c — tbsalling, Sep 07 '12 at 12:07

score 6 · Answer 1 · answered Sep 07 '12 at 12:06

6

I assume at run-time you know the Row_Size as well.

You can dynamically allocate a multidimensional array at run time, as follows:

#include <stdlib.h>

int **array;
array = malloc(nrows * sizeof(int *));
if(array == NULL)
    {
    fprintf(stderr, "out of memory\n");
    exit or return
    }
for(i = 0; i < nrows; i++)
    {
    array[i] = malloc(ncolumns * sizeof(int));
    if(array[i] == NULL)
        {
        fprintf(stderr, "out of memory\n");
        exit or return
        }
    }

Reference:

http://www.eskimo.com/~scs/cclass/int/sx9b.html

answered Sep 07 '12 at 12:06

Sandeep Singh

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3

Remember that it's important to free the sub-arrays as well, and not only do `free(array);`. – Some programmer dude Sep 07 '12 at 12:08
Wouldn't `calloc` be used instead of `malloc` here? – krzysz00 Sep 07 '12 at 12:11
And how would you memcpy this array to another location? How would you use bsearch, qsort etc? Oops. Don't use pointer-to-pointer notation, read up on array pointers instead. – Lundin Sep 07 '12 at 12:52

score 3 · Answer 2 · answered Sep 07 '12 at 12:18

3

With the length of the rows statically know, you could also allocate

char (*szRunTime)[128];
// obtain row count
szRunTime = malloc(rowCount * sizeof *szRunTime);

memory to a pointer to char[128]. That way, you get a contiguous block of memory, which may give better locality, and you need only free one pointer.

If the number of rows is not too large, using a variable length array,

rowCount = whatever;
char szRunTime[rowCount][128];

may however be the best option if C99 or later is supported.

answered Sep 07 '12 at 12:18

Daniel Fischer

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This is the best solution by far. If you use the misguided (but common) pointer-to-pointer notation, you end up with the array declared in segments all over the heap. Such an array will not be compatible with various C library functions (mempcy, bsearch, qsort etc). – Lundin Sep 07 '12 at 12:48

score 1 · Accepted Answer · answered Sep 07 '12 at 12:08

1

use this ,, where Variable is the how many array you want :

char **szRunTime = malloc(sizeof(char *)*Variable);
int i;
for(i=0 ; i<Variable ; i++)
    szRunTime[i] = malloc(sizeof(char)*128);

answered Sep 07 '12 at 12:08

Rami Jarrar

4,523
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And how would you memcpy this array to another location? How would you use bsearch, qsort etc? Oops. Don't use pointer-to-pointer notation, read up on array pointers instead. – Lundin Sep 07 '12 at 12:53

How can allocate multi dimensional array at run time?

3 Answers3