In C and related programming languages, long double refers to a floating-point data type that is often more precise than double-precision. As with C's other floating-point types, it may not necessarily map to an IEEE format.
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History
The long double type was present in the original 1989 C standard, but support was improved by the 1999 revision of the C standard, or C99, which extended the standard library to include functions operating on long double such as sinl() and strtold().
Long double constants are floating-point constants suffixed with "L" or "l" (lower-case L), e.g., 0.333333333333333333L. Without a suffix, the evaluation depends on FLT_EVAL_METHOD.
Implementations
On the x86 architecture, most C compilers implement long double as the 80-bit extended precision type supported by x86 hardware (sometimes stored as 12 or 16 bytes to maintain data structure alignment), as specified in the C99 / C11 standards (IEC 60559 floating-point arithmetic (Annex F)). An exception is Microsoft Visual C++ for x86, which makes long double a synonym for double. The Intel C++ compiler on Microsoft Windows supports extended precision, but requires the /Qlong‑double switch for long double to correspond to the hardware's extended precision format.
Compilers may also use long double for a 128-bit quadruple precision format. This is the case on HP-UX and on Solaris/SPARC machines. This format is currently implemented in software due to lack of hardware support.
On some PowerPC and SPARCv9 machines, long double is implemented as a double-double arithmetic, where a long double value is regarded as the exact sum of two double-precision values, giving at least a 106-bit precision; with such a format, the long double type does not conform to the IEEE floating-point standard. Otherwise, long double is simply a synonym for double (double precision).
With the GNU C Compiler, long double is 80-bit extended precision on x86 processors regardless of the physical storage used for the type (which can be either 96 or 128 bits), On some other architectures, long double can be double-double (e.g. on PowerPC) or 128-bit quadruple precision (e.g. on SPARC). As of gcc 4.3, a quadruple precision is also supported on x86, but as the nonstandard type __float128 rather than long double.
Although the x86 architecture, and specifically the x87 floating-point instructions on x86, supports 80-bit extended-precision operations, it is possible to configure the processor to automatically round operations to double (or even single) precision. Conversely, in extended-precision mode, extended precision may be used for intermediate compiler-generated calculations even when the final results are stored at a lower precision (i.e. FLT_EVAL_METHOD == 2). With gcc on Linux, 80-bit extended precision is the default; on several BSD operating systems (FreeBSD and OpenBSD), double-precision mode is the default, and long double operations are effectively reduced to double precision. (NetBSD 7.0 and later, however, defaults to 80-bit extended precision ). However, it is possible to override this within an individual program via the FLDCW "floating-point load control-word" instruction. On x86_64 the BSDs default to 80-bit extended precision. Microsoft Windows with Visual C++ also sets the processor in double-precision mode by default, but this can again be overridden within an individual program (e.g. by the _controlfp_s function in Visual C++). The Intel C++ Compiler for x86, on the other hand, enables extended-precision mode by default. On OS X, long double is 80-bit extended precision .
Other specifications
In CORBA (from specification of 3.0, which uses "ANSI/IEEE Standard 754-1985" as its reference), "the long double data type represents an IEEE double-extended floating-point number, which has an exponent of at least 15 bits in length and a signed fraction of at least 64 bits", with GIOP/IIOP CDR, whose floating-point types "exactly follow the IEEE standard formats for floating point numbers", marshalling this as what seems to be IEEE 754-2008 binary128 a.k.a. quadruple precision without using that name.