Defined in header <math.h>
#define isless(x, y) /* implementation defined */
(since C99)

Determines if the floating point number x is less than the floating-point number y, without setting floating-point exceptions.


x - floating point value
y - floating point value

Return value

Nonzero integral value if x < y, ​0​ otherwise.


The built-in operator< for floating-point numbers may raise FE_INVALID if one or both of the arguments is NaN. This function is a "quiet" version of operator<.


#include <stdio.h>
#include <math.h>
int main(void)
    printf("isless(2.0,1.0)      = %d\n", isless(2.0,1.0));
    printf("isless(1.0,2.0)      = %d\n", isless(1.0,2.0));
    printf("isless(INFINITY,1.0) = %d\n", isless(INFINITY,1.0));
    printf("isless(1.0,NAN)      = %d\n", isless(1.0,NAN));
    return 0;

Possible output:

isless(2.0,1.0)      = 0
isless(1.0,2.0)      = 1
isless(INFINITY,1.0) = 0
isless(1.0,NAN)      = 0


  • C11 standard (ISO/IEC 9899:2011):
    • The isless macro (p: 260)
    • F.10.11 Comparison macros (p: 531)
  • C99 standard (ISO/IEC 9899:1999):
    • The isless macro (p: 241)

See also

checks if the first floating-point argument is greater than the second
(function macro)
C++ documentation for isless

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