modf man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

modf, modff, modfl — decompose a floating-point number


#include <math.h>

double modf(double x, double *iptr);
float modff(float value, float *iptr);
long double modfl(long double value, long double *iptr);


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2008 defers to the ISO C standard.

These functions shall break the argument x into integral and fractional parts, each of which has the same sign as the argument. It stores the integral part as a double (for the modf() function), a float (for the modff() function), or a long double (for the modfl() function), in the object pointed to by iptr.

Return Value

Upon successful completion, these functions shall return the signed fractional part of x.

If x is NaN, a NaN shall be returned, and *iptr shall be set to a NaN.

If x is ±Inf, ±0 shall be returned, and *iptr shall be set to ±Inf.


No errors are defined.

The following sections are informative.

Application Usage

The modf() function computes the function result and *iptr such that:

a = modf(x, iptr) ;
x == a+*iptr ;

allowing for the usual floating-point inaccuracies.

See Also

frexp(), isnan(), ldexp()

The Base Definitions volume of POSIX.1‐2008, <math.h>