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?

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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