trunc, truncf, truncl

Defined in header <math.h>
float       truncf( float arg );
(1) (since C99)
double      trunc( double arg );
(2) (since C99)
long double truncl( long double arg );
(3) (since C99)
Defined in header <tgmath.h>
#define trunc( arg )
(4) (since C99)
1-3) Computes the nearest integer not greater in magnitude than arg.
4) Type-generic macro: If arg has type long double, truncl is called. Otherwise, if arg has integer type or the type double, trunc is called. Otherwise, truncf is called.


arg - floating point value

Return value

If no errors occur, the nearest integer value not greater in magnitude than arg (in other words, arg rounded towards zero), is returned.

Return value

Error handling

Errors are reported as specified in math_errhandling.

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

  • The current rounding mode has no effect.
  • If arg is ±∞, it is returned, unmodified
  • If arg is ±0, it is returned, unmodified
  • If arg is NaN, NaN is returned


FE_INEXACT may be (but isn't required to be) raised when truncating a non-integer finite value.

The largest representable floating-point values are exact integers in all standard floating-point formats, so this function never overflows on its own; however the result may overflow any integer type (including intmax_t), when stored in an integer variable.

The implicit conversion from floating-point to integral types also rounds towards zero, but is limited to the values that can be represented by the target type.


#include <math.h>
#include <stdio.h>
int main(void)
    printf("trunc(+2.7) = %+.1f\n", trunc(2.7));
    printf("trunc(-2.7) = %+.1f\n", trunc(-2.7));
    printf("trunc(-0.0) = %+.1f\n", trunc(-0.0));
    printf("trunc(-Inf) = %+f\n",   trunc(-INFINITY));

Possible output:

trunc(+2.7) = +2.0
trunc(-2.7) = -2.0
trunc(-0.0) = -0.0
trunc(-Inf) = -inf


  • C11 standard (ISO/IEC 9899:2011):
    • The trunc functions (p: 253-254)
    • 7.25 Type-generic math <tgmath.h> (p: 373-375)
    • F.10.6.8 The trunc functions (p: 528)
  • C99 standard (ISO/IEC 9899:1999):
    • The trunc functions (p: 234)
    • 7.22 Type-generic math <tgmath.h> (p: 335-337)
    • F.9.6.8 The trunc functions (p: 464)

See also

computes largest integer not greater than the given value
computes smallest integer not less than the given value
rounds to nearest integer, rounding away from zero in halfway cases
C++ documentation for trunc

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