sizeof long double and precision not matching?

Question

Consider the following C code:

#include 
int main(int argc, char* argv[]) 
{
    const long double ld = 0.12345678901234567890123456789012345L         


        

Answer 1

The long double format in your C implementation uses an Intel format with a one-bit sign, a 15-bit exponent, and a 64-bit significand (ten bytes total). The compiler allocates 16 bytes for it, which is wasteful but useful for some things such as alignment. However, the 64 bits provide only log₁₀(2⁶⁴) digits of significance, which is about 20 digits.