erfc (3)
Leading comments
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NAME
erfc, erfcf, erfcl  complementary error functionSYNOPSIS
#include <math.h> double erfc(double x); float erfcf(float x); long double erfcl(long double x);Link with lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
erfc():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
erfcf(), erfcl():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 600 _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
DESCRIPTION
These functions return the complementary error function of x, that is, 1.0  erf(x).RETURN VALUE
On success, these functions return the complementary error function of x, a value in the range [0,2].If x is a NaN, a NaN is returned.
If x is +0 or 0, 1 is returned.
If x is positive infinity, +0 is returned.
If x is negative infinity, +2 is returned.
If the function result underflows and produces an unrepresentable value, the return value is 0.0.
If the function result underflows but produces a representable (i.e., subnormal) value, that value is returned, and a range error occurs.
ERRORS
See math_error(7) for information on how to determine whether an error has occurred when calling these functions.The following errors can occur:
 Range error: result underflow (result is subnormal)
 An underflow floatingpoint exception (FE_UNDERFLOW) is raised.
These functions do not set errno.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
erfc(), erfcf(), erfcl()  Thread safety  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.The variant returning double also conforms to SVr4, 4.3BSD.