Decimal Float (Using the GNU Compiler Collection (GCC))

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6.1.6 Decimal Floating Types ¶

As an extension, GNU C supports decimal floating types as defined in the N1312 draft of ISO/IEC WDTR24732. GCC does not yet implement the later specification of decimal floating point in the C23 standard, primarily due to problems with library support. The N1312 draft support is available in all dialects of C, but not all targets support decimal floating types.

The decimal floating types are _Decimal32, _Decimal64, and _Decimal128. They use a radix of ten, unlike the floating types float, double, and long double whose radix is not specified by the C standard but is usually two.

Support for decimal floating types includes the arithmetic operators add, subtract, multiply, divide; unary arithmetic operators; relational operators; equality operators; and conversions to and from integer and other floating types. Use a suffix ‘df’ or ‘DF’ in a literal constant of type _Decimal32, ‘dd’ or ‘DD’ for _Decimal64, and ‘dl’ or ‘DL’ for _Decimal128. The draft TR also defines the suffixes ‘d’ and ‘D’ for type double.

GCC support of decimal float as specified by the draft technical report is incomplete:

When the value of a decimal floating type cannot be represented in the integer type to which it is being converted, the result is undefined rather than the result value specified by the draft technical report.
GCC does not provide the C library functionality associated with math.h, fenv.h, stdio.h, stdlib.h, and wchar.h, which must come from a separate C library implementation. Because of this the GNU C compiler does not define macro __STDC_DEC_FP__ to indicate that the implementation conforms to the technical report.

Types _Decimal32, _Decimal64, and _Decimal128 are supported by the DWARF debug information format.