History

The long double type was present in the original 1989 C standard,^[1] 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., 0.3333333333333333333333333333333333L or 3.1415926535897932384626433832795029L for quadruple precision. 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 (generally 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.^[2] 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.^[3]

Compilers may also use long double for the IEEE 754 quadruple-precision binary floating-point format (binary128). This is the case on HP-UX,^[4] Solaris/SPARC,^[5] MIPS with the 64-bit or n32 ABI,^[6] 64-bit ARM (AArch64)^[7] (on operating systems using the standard AAPCS calling conventions, such as Linux), and z/OS with FLOAT(IEEE)^[8][9][10]. Most implementations are in software, but some processors have hardware support.

On some PowerPC systems,^[11] 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), e.g. on 32-bit ARM,^[12] 64-bit ARM (AArch64) (on Windows^[13] and macOS^[14]) and on 32-bit MIPS^[15] (old ABI, a.k.a. o32).

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),^[16] On some other architectures, long double can be double-double (e.g. on PowerPC^[17][18][19]) or 128-bit quadruple precision (e.g. on SPARC^[20]). As of gcc 4.3, a quadruple precision is also supported on x86, but as the nonstandard type __float128 rather than long double.^[21]

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.^[22] (NetBSD 7.0 and later, however, defaults to 80-bit extended precision ^[23]). However, it is possible to override this within an individual program via the FLDCW "floating-point load control-word" instruction.^[22] 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++^[24]). The Intel C++ Compiler for x86, on the other hand, enables extended-precision mode by default.^[25] On IA-32 OS X, long double is 80-bit extended precision.^[26]