Question 6.19

How do I write functions which accept two-dimensional arrays when the ``width'' is not known at compile time?

It's not easy. One way is to pass in a pointer to the [0][0] element, along with the two dimensions, and simulate array subscripting ``by hand:''

	f2(aryp, nrows, ncolumns)
	int *aryp;
	int nrows, ncolumns;
	{ ... array[i][j] is accessed as aryp[i * ncolumns + j] ... }

This function could be called with the array from question 6.18 as

	f2(&array[0][0], NROWS, NCOLUMNS);

It must be noted, however, that a program which performs multidimensional array subscripting ``by hand'' in this way is not in strict conformance with the ANSI C Standard; according to an official interpretation, the behavior of accessing (&array[0][0])[x] is not defined for x >= NCOLUMNS.

gcc allows local arrays to be declared having sizes which are specified by a function's arguments, but this is a nonstandard extension.

When you want to be able to use a function on multidimensional arrays of various sizes, one solution is to simulate all the arrays dynamically, as in question 6.16.

See also questions 6.18, 6.20, and 6.15.

References: ANSI Sec. 3.3.6
ISO Sec. 6.3.6

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