Pointer to pointer

[slippy]

Suppose a function f takes pointer to pointer to double A. B is a variable of the same type (declared in main() ). How do you (i) allocate space to A in f() and (ii) modify the contents of B via f()? I keep getting segmentation faults when trying to access elements of A in f() with things like *A[i] = (something). i is a loop variable.

void f(double **A) {

int i = 0;
/* ... */

}

int main(int argc, char *argv[]) {

        double **B=0;

        f(B);

}

[tabstop]

(i) How do you ever allocate space for anything? Now the thing to worry about is that it is not possible, as you have it to change B in main, since B is passed by value not by pointer.
(ii) If you haven't done (i), you can't do (ii).

[Sebastiani]

Just remember this: a pointer should always point to a valid address in memory. So if you have a double**, then both the double* that it points to and the double that that points to must be a valid address, if that makes sense. Here's an example using both real and dynamically allocated variables:

#include <stdio.h>
#include <stdlib.h>

int main( void )
{
	double
		d1 = 1024, 
		* dp1 = &d1, 
		** dpp1 = &dp1, 
		** dpp2;
	dpp2 = malloc( sizeof( double* ) );
	*dpp2 = malloc( sizeof( double ) );
	**dpp2 = **dpp1;
	printf( "Value: %f\n", **dpp2 );
	free( *dpp2 );
	free( dpp2 );
	return 0;
}