B = (double **)malloc(m*sizeof(double *));
if ( B == NULL ) return NULL;
B[0] = (double *)malloc(m*n*sizeof(double));
if ( B[0] == NULL ) { free(B); return NULL; }
/* now set the other pointers */
for ( i = 1; i < m; i++ )
  B[i] = B[0] + i*n;