Rounding modes: std::from_chars versus strtod/strtof

A recent C++ standard (C++17) introduced new functions to parse floating-point numbers std::from_chars, from strings (e.g., ASCII text) to binary numbers. How should such a function parse values that cannot be represented exactly? The specification states that the resulting value rounded to nearest. This means that 1.0000000000000000001 and 0.999999999999999999 become exactly 1.0.

The C language has its own functions (strtod/strtof). I could not find a reference in the standard as to how it should round, but the source code suggests that the functions round according to the current floating-point rounding mode, as determined by the fegetround() function. One can round toward zero, up, down or to nearest. I wrote a small command-line utility to test it out. And indeed, I get the following results under LLVM and GCC:

1.0000000000000000001 1.00001 1.0 1.0
0.999999999999999999 1.0 0.999999 0.999999

Thus you cannot assume that, in general, std::from_chars will agree with strtod/strtof even for just boring strings such as 0.999999999999999999.

Thankfully, you can check the rounding mode efficiently.

Published by

Daniel Lemire

A computer science professor at the University of Quebec (TELUQ).

Leave a Reply

Your email address will not be published. The comment form expects plain text. If you need to format your text, you can use HTML elements such strong, blockquote, cite, code and em. For formatting code as HTML automatically, I recommend

You may subscribe to this blog by email.