Nisam siguran da ovo radi ali evo pa probaj:

unsigned __int64 HexToInt( const char *hexstring )
{
unsigned __int64 veliki;

const char *kopija;
kopija = hexstring;

if ( *kopija == '\0' )
return 0;

veliki = 0;

while ( *kopija != '\0' )
{
if ( !isxdigit( (int)*kopija) )
return 0;

veliki <<= 4;

if ( *kopija <= '9' )
veliki += *kopija++ - '0';
else if ( *kopija >= 'a' )
veliki += *kopija++ - 'a' + 10;
else
veliki += *kopija++ - 'A' + 10;
}

return ( veliki );
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
ShowMessage( HexToInt( "fffffffffffffffff" ) );
}

void __fastcall TForm1::Button2Click(TObject *Sender)
{
__int64 i = 18446744073709551615;
char buffer[10];
_i64toa( i, buffer, 16 );

ShowMessage( buffer );
}

Zar ne postoji _atoi64? Mada ja sam uvek pre za sscanf varijantu


if ( sscanf( buffer, "%I64d", &amp;i ) == 1 )

{

// imas broj

}