Section: POSIX Programmer's Manual (3P)
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
--- test if x is less than y
int isless(real-floating x, real-floating y);
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here and the
ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.
macro shall determine whether its first argument is less than its
second argument. The value of
shall be equal to (x) < (y); however, unlike
(x) < (y),
shall not raise the invalid floating-point exception when
Upon successful completion, the
macro shall return the value of (x
) < (y
is NaN, 0 shall be returned.
No errors are defined.
The following sections are informative.
The relational and equality operators support the usual mathematical
relationships between numeric values. For any ordered pair of numeric
values, exactly one of the relationships (less, greater, and equal) is
true. Relational operators may raise the invalid floating-point
exception when argument values are NaNs. For a NaN and a numeric value,
or for two NaNs, just the unordered relationship is true. This macro
is a quiet (non-floating-point exception raising) version of a
relational operator. It facilitates writing efficient code that
accounts for NaNs without suffering the invalid floating-point
exception. In the SYNOPSIS section,
indicates that the argument shall be an expression of
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
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