2d arrays

[Anders Andersson]

In <1065 at nih-csl.nih.gov> pfeifer at alw.nih.gov (John Pfeifer) writes:

>In article <1991Mar13.183807.23254 at ux1.cso.uiuc.edu>, sasg0244 at uxa.cso.uiuc.edu (Steven Arthur Sivier) writes:
>|> 
>|> i've always tried to avoid 2d arrays, but because of a library
>|> function i need to call i need to use 2d arrays. the problem
>|> is i have to malloc the 2d array. can i just malloc the array
>|> to a char pointer and then use it as a 2d array? that is, if
>|> the array has dimensions x by y can i use:
>|> 	char *array;
>|> 	array = malloc(x * y);
>|> 	array[0][0] = 'a'

>If I understand you correctly, no.  char[][] is an array of arrays, ie. **char  Try:
>	char **array;
>	array = malloc(x*sizeof(*char));
>	for(i=0;i<x;i++)
>		array[i] = malloc(y*sizeof(char));
> 
>|> also, inasmuch as i need to pass this array to a library function,
>|> can i just send the pointer even though the function expects a
>|> 2d array?

>Yes.
>See section A8.6.2 in K&R (2nd ed.)

This is not so. Allocating with a loop does not give a 2d array, it gives
an array of (pointers to) arrays. It's easy to confuse them, since C uses
exactly the same syntax for both 2d arrays and arrays of arrays. Normally
the distinction is not crucial, but in this case, when a library module that
(presumably) is precompiled is involved the difference is important, since
the two approaces generate different memory lay-outs. If the library routine
really wants a 2d array, you must not use more than one call to malloc().
However, this is awkward, so it is a fair chance that the library routine
uses an array of arrays. If it does use a 2d array, the simplest way is to 
allocate a ordinary 1d array and use it as such, doing index calculation 
yourself, like:
       	char *array;
	array = malloc(x * y);
	array[0] = 'a'; 
	array[i * y + j] = 'b'; /* instead of array[i][j] */
where y is the width of the array (number of columns in a row). If y is known
in advance (e.g. 10 columns but unknown number of rows) this simplyfies to:
	typedef char row[y]; /* for clarity - a row of chars */
	row *array;
	array = (row *)malloc(sizeof(row) * x);
	array[0][0] = 'a';
	array[i][j] = 'b';

-Anders Andersson (andand at cia.docs.uu.se - mail just *might* get through, but
				           I wouldn't bet on it)