Long double

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In C and related programming languages, long double refers to a floating point data type that may, and usually does, have greater than double precision.

On the x86 architecture, most compilers implement long double as the 80-bit extended precision type supported by that hardware (sometimes stored as 96 bits, or 12 bytes to maintain 32-bit alignment). On some other architectures, compilers may use long double for a 128-bit quadruple precision type, which may be implemented in hardware or purely in software. Otherwise, long double is simply a synonym for double (double precision). The long double type may or may not conform to the IEEE floating-point standard.

The long double type was standardized in the 1999 revision of the C standard, or C99, which defined its interactions with other floating-point types and extended the standard library to include mathematical functions operating on long double.