Long double - Revision history

Alex: /* Properties */

2014-08-20T22:22:56Z

‎Properties

Alex: Added useful info

2014-08-20T22:22:35Z

Added useful info

Brian: Created page with "{{DISPLAYTITLE:long double}} `long double` is a primitive floating-point data type in C (and subsequently C++) that is required by the standard to be at least as long ..."

2012-02-13T04:05:31Z

Created page with "{{DISPLAYTITLE:long double}} <code>long double</code> is a primitive floating-point data type in C (and subsequently C++) that is required by the standard to be at least as long ..."

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{{DISPLAYTITLE:long double}}
<code>long double</code> is a primitive floating-point data type in C (and subsequently C++) that is required by the standard to be at least as long as a <code>double</code>. That is, the values it can represent must be a superset of the values that a <code>double</code> can represent.

On many architectures, <code>long double</code> is identical to <code>double</code>, since <code>double</code> is usually a [[IEEE 754|binary64]] and many architectures do not support a longer floating-point data type. On Intel x86 machines, however, <code>long double</code> is often a 80-bit ''extended precision'' format that originated with the Intel 8087 numeric coprocessor. This format is almost exactly analogous to the [[IEEE 754]] formats, except that its significand does not carry an implicit leading 1. It consists of a sign bit, a 15-bit biased exponent field, and a 64-bit significand field. Because the x86 processors are little-endian, when this kind of number is stored in memory, the lowest-address 8 bytes are used for the significand, the next byte is used for the least significant 8 bits of the exponent, and the last byte contains the sign bit as the most significant bit, followed by the most significant 7 bits of the exponent.

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 The range of a <code>long double</code> can be checked in C using <code><float.h></code> and C++ using <code>std::numeric_limits</code>. This 80-bit format typically has a range of roughly 1.9E-4932 ... 1.1E+4932 and holds 19-20 significant digits.
-Even though the <code>long double</code> may be stored using 96 or even 128 bits (you can check the number of bits using <code>4*sizeof(long double)</code>), it still only has 80 bits of precision. The extra padding bits are to help modern architectures align to 8 or 16 bit boundaries. In GCC, the compiler switches <code>-m96bit-long-double</code> and <code>-m128bit-long-double</code> may be used to control the storage size of a <code>long double</code>, without of course, changing its precision in any way.
+Even though the <code>long double</code> may be stored using 96 or even 128 bits (you can check the number of bits using <code>8*sizeof(long double)</code>), it still only has 80 bits of precision. The extra padding bits are to help modern architectures align to 8 or 16 bit boundaries. In GCC, the compiler switches <code>-m96bit-long-double</code> and <code>-m128bit-long-double</code> may be used to control the storage size of a <code>long double</code>, without of course, changing its precision in any way.
 ==Usage==

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	On many architectures, <code>long double</code> is identical to <code>double</code>, since <code>double</code> is usually a [[IEEE 754\|binary64]] and many architectures do not support a longer floating-point data type. On Intel x86 machines, however, <code>long double</code> is often a 80-bit ''extended precision'' format that originated with the Intel 8087 numeric coprocessor. This format is almost exactly analogous to the [[IEEE 754]] formats, except that its significand does not carry an implicit leading 1. It consists of a sign bit, a 15-bit biased exponent field, and a 64-bit significand field. Because the x86 processors are little-endian, when this kind of number is stored in memory, the lowest-address 8 bytes are used for the significand, the next byte is used for the least significant 8 bits of the exponent, and the last byte contains the sign bit as the most significant bit, followed by the most significant 7 bits of the exponent.		On many architectures, <code>long double</code> is identical to <code>double</code>, since <code>double</code> is usually a [[IEEE 754\|binary64]] and many architectures do not support a longer floating-point data type. On Intel x86 machines, however, <code>long double</code> is often a 80-bit ''extended precision'' format that originated with the Intel 8087 numeric coprocessor. This format is almost exactly analogous to the [[IEEE 754]] formats, except that its significand does not carry an implicit leading 1. It consists of a sign bit, a 15-bit biased exponent field, and a 64-bit significand field. Because the x86 processors are little-endian, when this kind of number is stored in memory, the lowest-address 8 bytes are used for the significand, the next byte is used for the least significant 8 bits of the exponent, and the last byte contains the sign bit as the most significant bit, followed by the most significant 7 bits of the exponent.
		+
		+	In Pascal, the corresponding data types are <code>Extended</code> (Free/Borland/GNU Pascal) and also <code>LongReal</code> (GNU Pascal), which also hold 80 bits of information.
		+
		+	==Properties==
		+	The range of a <code>long double</code> can be checked in C using <code><float.h></code> and C++ using <code>std::numeric_limits</code>. This 80-bit format typically has a range of roughly 1.9E-4932 ... 1.1E+4932 and holds 19-20 significant digits.
		+
		+	Even though the <code>long double</code> may be stored using 96 or even 128 bits (you can check the number of bits using <code>4*sizeof(long double)</code>), it still only has 80 bits of precision. The extra padding bits are to help modern architectures align to 8 or 16 bit boundaries. In GCC, the compiler switches <code>-m96bit-long-double</code> and <code>-m128bit-long-double</code> may be used to control the storage size of a <code>long double</code>, without of course, changing its precision in any way.
		+
		+	==Usage==
		+	To <code>scanf</code> or <code>printf</code> a <code>long double</code>, one should use the format specifier <code>%Lf</code> (<code>%llf</code> also works sometimes, although it is nonstandard).
		+
		+	A well-known bug on Windows is that MinGW uses the Microsoft C run-time libraries which do not have good support for the <code>long double</code> type. Attempts to print it will result in weird behaviour. As a workaround, a <code>long double</code> may be cast to <code>double</code> before being printed.

Long double - Revision history

Alex: /* Properties */

Alex: Added useful info

Brian: Created page with "{{DISPLAYTITLE:long double}} long double is a primitive floating-point data type in C (and subsequently C++) that is required by the standard to be at least as long ..."

Brian: Created page with "{{DISPLAYTITLE:long double}} `long double` is a primitive floating-point data type in C (and subsequently C++) that is required by the standard to be at least as long ..."