Converting aligned array uint8[8] to double - ios

Question

I am facing a bit of a challenge trying to convert an aligned array uint8[8] to a double.
It was particularly easy to convert uint8[4] to long with bit-operations, but i understand that the double can become messy in terms of a sign bit?
In Java i simply use ByteBuffer.wrap(bytes).getDouble() but i assume its not that easy in C.
I tried to implement this code, but the last command gives the error Expression is not assignable and Shift count >= width of type
long tempHigh = 0;
long tempLow = 0;
double sum = 0;
tempHigh |= buffer[0] & 0xFF;
tempHigh <<= 8;
tempHigh |= buffer[1] & 0xFF;
tempHigh <<= 8;
tempHigh |= buffer[2] & 0xFF;
tempHigh <<= 8;
tempHigh |= buffer[3] & 0xFF;
tempLow |= buffer[4] & 0xFF;
tempLow <<= 8;
tempLow |= buffer[5] & 0xFF;
tempLow <<= 8;
tempLow |= buffer[6] & 0xFF;
tempLow <<= 8;
tempLow |= buffer[7] & 0xFF;
sum |= ((tempHigh & 0xFFFF) <<= 32) + (tempLow & 0xFFFF);
How can this procedure be done correctly or just resolve the error i have made?
Thanks in advance.

Answer 1

double is a floating-point type; it doesn't support bitwise operations such as |.
You could do something like:
double sum;
memcpy(&sum, buffer, sizeof(sum));
But be aware of endianness issues.

Answer 2

The portable way to do it is to read out the sign, exponent, and mantissa values into integer variables with bitwise arithmetic, then call ldexp to apply the exponent.
OK, here's some code. Beware it might have mismatched parentheses or off-by-one errors.
unsigned char x[8]; // your input; code assumes little endian
long mantissa = ((((((x[6]%16)*256 + x[5])*256 + x[4])*256 + x[3])*256 + x[2])*256 + x[1])*256 + x[0];
int exp = x[7]%128*16 + x[6]/16 - 1023;
int sign = 1-x[7]/128*2;
double y = sign*ldexp(0x1p53 + mantissa, exp-53);

Answer 3

How about a union? Write to the long part as you have, then the double is automagically correct. Something like this:
union
{
double sum;
struct
{
long tempHigh;
long tempLow;
}v;
}u;
u.v.tempHigh = 0;
u.v.tempHigh |= buffer[0] & 0xFF;
u.v.tempHigh <<= 8;
u.v.tempHigh |= buffer[1] & 0xFF;
u.v.tempHigh <<= 8;
u.v.tempHigh |= buffer[2] & 0xFF;
u.v.tempHigh <<= 8;
u.v.tempHigh |= buffer[3] & 0xFF;
u.v.tempLow |= buffer[4] & 0xFF;
u.v.tempLow <<= 8;
u.v.tempLow |= buffer[5] & 0xFF;
u.v.tempLow <<= 8;
u.v.tempLow |= buffer[6] & 0xFF;
u.v.tempLow <<= 8;
u.v.tempLow |= buffer[7] & 0xFF;
printf("%f", u.sum);