C : how is double number (e.g. 123.45) stored in a float variable or double variable or long double variable?

Question

#include 

int main () {
float a = 123.0; 

unsigned char ch, *cp;
ch = 0xff;

cp = (unsigned char *) &a;
printf (\"%02x\", ch&(*(cp+3)));
pri         


        

Answer 1

See Wikipedia: http://en.wikipedia.org/wiki/Single_precision_floating-point_format, which describes single precision floating point (a typical C float, but depends on the compiler) as a 1-bit sign, 8-bit biased exponent, and 24-bit mantissa (23-bits stored).

For your example:

123.0 = 42f60000_hex = 0 10000101 11101100000000000000000_bin
1-bit sign = 0 (indicating positive number)
8-bit biased exponent = 10000101_bin = 133_dec - 127_dec = 6_dec
23-bit mantissa = 11101100000000000000000_bin = 1.111011_bin (note implied leading 1)

Converting 1.111011_bin x 2⁶_dec = 1111011.0_bin = 123.0_dec