# Qlong-double

Changes the default size of the long double data type. This content is specific to C++; it does not apply to DPC++.

## Syntax

Linux: | None

Windows: | /Qlong-double

## Arguments

None

## Default

OFF | The default size of the long double data type is 64 bits.

## Description

This option changes the default size of the long double data type to 80 bits.

However, the alignment requirement of the data type is 16 bytes, and its size must be a multiple of its alignment, so the size of a long double on Windows* is also 16 bytes. Only the lower 10 bytes (80 bits) of the 16 byte space will have valid data stored in it.

Note that the Microsoft* compiler and Microsoft*-provided library routines (such as printf or `long double` math functions) do not provide support for 80-bit floating-point values. As a result, this option should only be used when referencing symbols within parts of your application built with this option or symbols in libraries that were built with this option.

## IDE Equivalent

None

## Alternate Options

None

Parent topic: Data Options