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libc.a reference
modf
Syntax
 #include <math.h>
double modf(double x, double *pint);

Description
modf
breaks down x into its integer portion (which it
stores in *pint) and the remaining fractional portion, which it
returns. Both integer and fractional portions have the same sign as
x, except if x is a negative zero, in which case the integer
part is a positive zero.
Return Value
The fractional portion. If x is Inf
or NaN
, the
return value is zero, the integer portion stored in *pint is the
same as the value of x, and errno
is set to EDOM
.
Portability
ANSI/ISO C 
C89; C99

POSIX 
1003.21992; 1003.12001
