INFINITYSection: Linux Programmer's Manual (3)
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NAMEINFINITY, NAN, HUGE_VAL, HUGE_VALF, HUGE_VALL - floating point constants
#include <math.h> INFINITY NAN HUGE_VAL
DESCRIPTIONThe 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 signalling NaN. See IEC 60559:1989.
AVAILABILITYOn 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.
SEE ALSOfpclassify(3), feature_test_macros(7)