Defined in header <math.h>
float       fdimf( float x, float y );
(1) (since C99)
double      fdim( double x, double y );
(2) (since C99)
long double fdiml( long double x, long double y );
(3) (since C99)
Defined in header <tgmath.h>
#define fdim( x, y )
(4) (since C99)
1-3) Returns the positive difference between x and y, that is, if x>y, returns x-y, otherwise (if x≤y), returns +0.
4) Type-generic macro: If any argument has type long double, fdiml is called. Otherwise, if any argument has integer type or has type double, fdim is called. Otherwise, fdimf is called.


x, y - floating point value

Return value

If successful, returns the positive difference between x and y.

If a range error due to overflow occurs, +HUGE_VAL, +HUGE_VALF, or +HUGE_VALL is returned.

If a range error due to underflow occurs, the correct value (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

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

  • If either argument is NaN, NaN is returned


Equivalent to fmax(x-y, 0) except for the NaN handling requirements.


#include <stdio.h>
#include <math.h>
#include <errno.h>
#include <fenv.h>
int main(void)
    printf("fdim(4, 1) = %f, fdim(1, 4)=%f\n", fdim(4,1), fdim(1,4));
    printf("fdim(4,-1) = %f, fdim(1,-4)=%f\n", fdim(4,-1), fdim(1,-4));
    //error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("fdim(1e308, -1e308) = %f\n", fdim(1e308, -1e308));
    if(errno == ERANGE) perror("    errno == ERANGE");
    if(fetestexcept(FE_OVERFLOW)) puts("    FE_OVERFLOW raised");

Possible output:

fdim(4, 1) = 3.000000, fdim(1, 4)=0.000000
fdim(4,-1) = 5.000000, fdim(1,-4)=5.000000
fdim(1e308, -1e308) = inf
    errno == ERANGE: Numerical result out of range
    FE_OVERFLOW raised


  • C11 standard (ISO/IEC 9899:2011):
    • The fdim functions (p: 257)
    • 7.25 Type-generic math <tgmath.h> (p: 373-375)
    • F.10.9.1 The fdim functions (p: 530)
  • C99 standard (ISO/IEC 9899:1999):
    • The fdim functions (p: 238)
    • 7.22 Type-generic math <tgmath.h> (p: 335-337)
    • F.9.9.1 The fdim functions (p: 466)

See also

computes absolute value of an integral value (|x|)
determines larger of two floating-point values

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