INFINITY man page
INFINITY, NAN, HUGE_VAL, HUGE_VALF, HUGE_VALL — floating-point constants
#define _ISOC99_SOURCE /* See feature_test_macros(7) */ #include <math.h> INFINITY NAN HUGE_VAL HUGE_VALF HUGE_VALL
The macro INFINITY expands to a float constant representing positive infinity.
The macro NAN expands to a float constant representing a quiet NaN (when supported). A quiet NaN is a NaN ("not-a-number") that does not raise exceptions when it is used in arithmetic. The opposite is a signaling NaN. See IEC 60559:1989.
The macros HUGE_VAL, HUGE_VALF, HUGE_VALL expand to constants of types double, float and long double, respectively, that represent a large positive value, possibly positive infinity.
On a glibc system, the macro HUGE_VAL is always available. Availability of the NAN macro can be tested using #ifdef NAN, and similarly for INFINITY, HUGE_VALF, HUGE_VALL. They will be defined by <math.h> if _ISOC99_SOURCE or _GNU_SOURCE is defined, or __STDC_VERSION__ is defined and has a value not less than 199901L.
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HUGE_VAL(3), HUGE_VALF(3), HUGE_VALL(3) and NAN(3) are aliases of INFINITY(3).