dtoa.h File Reference

#include <wireshark.h>

Go to the source code of this file.

Functions
WS_DLL_PUBLIC char *	dtoa_g_fmt (char *buf, double value)
	Convert a double to its shortest exact decimal representation.

Detailed Description

David M. Gay dtoa (double to ASCII string) implementation header file

Wireshark - Network traffic analyzer By Gerald Combs geral.nosp@m.d@wi.nosp@m.resha.nosp@m.rk.o.nosp@m.rg Copyright 1998 Gerald Combs

SPDX-License-Identifier: dtoa

Function Documentation

dtoa_g_fmt()

WS_DLL_PUBLIC char * dtoa_g_fmt	(	char *	buf,
		double	value
	)

Convert a double to its shortest exact decimal representation.

Stores the closest decimal approximation to value in buf; it suffices to declare buf char buf[32];

Specifically, this finds the shortest possible string that when converted back to a double will be equal to the original value. There is no single value that can be passed to snprintf("%.*g") that will work for all cases.

E.g., for the IEEE 754 double closest to 1/7th (0x1.2492492492492p-3) 17 (DBL_DECIMAL_DIG) digits are required; neither "0.1428571428571428" nor "0.1428571428571429" suffice, converting to 0x1.249249249249p-3 and 0x1.2492492492494p-3, respectively. However, for the double closest to 0.2 (0x1.999999999999ap-3), the closest string with 17 significant digits is "0.20000000000000001", not "0.2", even though both convert to the same double and would test as equal. So DBL_DECIMAL_DIG is sufficient for serialization but not necessary in all cases and can look particularly worse in formats where trailing zeros are removed.

Note C++17 provides std::to_chars to provide the same result, though the difficulty in implementation caused this to be one of the last widely supported features across C++ standard libraries. It is not part of the C standard library functions.

Parameters

buf	Output buffer to store the decimal string (minimum 32 bytes).
value	Input double value to convert.

Returns

Pointer to buf containing the resulting string.