modf, modff, modfl

< c‎ | numeric‎ | math
Common mathematical functions
Basic operations
Exponential functions
Power functions
Trigonometric and hyperbolic functions
Error and gamma functions
Nearest integer floating point operations
Floating point manipulation functions
Macro constants
Defined in header <math.h>
float       modff( float arg, float* iptr );
(1) (since C99)
double      modf( double arg, double* iptr );
long double modfl( long double arg, long double* iptr );
(3) (since C99)
1-3) Decomposes given floating point value arg into integral and fractional parts, each having the same type and sign as arg. The integral part (in floating-point format) is stored in the object pointed to by iptr.


arg - floating point value
iptr - pointer to floating point value to store the integral part to

Return value

If no errors occur, returns the fractional part of x with the same sign as x. The integral part is put into the value pointed to by iptr.

The sum of the returned value and the value stored in *iptr gives arg (allowing for rounding).

Error handling

This function is not subject to any errors specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • If arg is ±0, ±0 is returned, and ±0 is stored in *iptr.
  • If arg is ±∞, ±0 is returned, and ±∞ is stored in *iptr.
  • If arg is NaN, NaN is returned, and NaN is stored in *iptr.
  • The returned value is exact, the current rounding mode is ignored


This function behaves as if implemented as follows:

double modf(double value, double *iptr)
    int save_round = fegetround();
    *iptr = std::nearbyint(value);
    return copysign(isinf(value) ? 0.0 : value - (*iptr), value);


#include <stdio.h>
#include <math.h>
#include <float.h>
int main(void)
    double f = 123.45;
    printf("Given the number %.2f or %a in hex,\n", f, f);
    double f3;
    double f2 = modf(f, &f3);
    printf("modf() makes %.2f + %.2f\n", f3, f2);
    int i;
    f2 = frexp(f, &i);
    printf("frexp() makes %f * 2^%d\n", f2, i);
    i = ilogb(f);
    printf("logb()/ilogb() make %f * %d^%d\n", f/scalbn(1.0, i), FLT_RADIX, i);
    // special values
    f2 = modf(-0.0, &f3);
    printf("modf(-0) makes %.2f + %.2f\n", f3, f2);
    f2 = modf(-INFINITY, &f3);
    printf("modf(-Inf) makes %.2f + %.2f\n", f3, f2);

Possible output:

Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
modf() makes 123.00 + 0.45
frexp() makes 0.964453 * 2^7
logb()/ilogb() make 1.92891 * 2^6
modf(-0) makes -0.00 + -0.00
modf(-Inf) makes -INF + -0.00


  • C11 standard (ISO/IEC 9899:2011):
  • The modf functions (p: 246-247)
  • F.10.3.12 The modf functions (p: 523)
  • C99 standard (ISO/IEC 9899:1999):
  • The modf functions (p: 227)
  • F.9.3.12 The modf functions (p: 460)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • The modf function

See also

rounds to nearest integer not greater in magnitude than the given value