segmentation fault with negative index

This code fragment gives segfault on the line with ->, please note n=3

real_t _b[n+1];
real_t * b = 开发者_开发问答_b+1;
std::fill( b, b + n , (real_t)0.0 );
for ( unsigned c = 0; c < n; c ++ )
  {
->   b[c-1] = 0; b[c] = 1;
     Lsolve( xtmp, lu, b, n ); 

I'm told this is because I'm in 64-bit (Linux amd64, gcc 4.6, debug flag -O0)

anyone could tell me more?

It's to do with the two's complement value of the index being added to the address, it works fine in 32 bit but not 64

Detailed here: http://www.devx.com/tips/Tip/41349