float.h - Linux


The float.h header file defines the floating-point environment, which includes constants and functions that allow you to control and access the floating-point unit (FPU) of your system. The FPU performs floating-point calculations, and float.h provides the means to configure and manipulate these calculations.


#include <float.h>


float.h provides various constants that represent the characteristics of the floating-point environment:

  • FLT_ROUNDS: Indicates the rounding mode used by the FPU. Possible values are:
    • FE_TONEAREST: Rounds to the nearest value.
    • FE_UPWARD: Rounds towards positive infinity.
    • FE_DOWNWARD: Rounds towards negative infinity.
    • FE_TOWARDZERO: Rounds towards zero.
  • FLT_RADIX: Specifies the radix (base) used for floating-point calculations. Typically, this is 2 (binary).
  • FLT_MANT_DIG: Indicates the number of bits in the mantissa (significant digits) of the floating-point representation.
  • FLT_DIG: Specifies the maximum number of decimal digits that can be represented without loss of precision.
  • FLT_EPSILON: Represents the smallest positive number that can be added to 1.0 without causing a change in its value.


To print the rounding mode and number of significant digits in the floating-point representation:

#include <float.h>
#include <stdio.h>

int main() {
  printf("Rounding mode: ");
  switch (FLT_ROUNDS) {
    case FE_TONEAREST: printf("FE_TONEAREST\n"); break;
    case FE_UPWARD: printf("FE_UPWARD\n"); break;
    case FE_DOWNWARD: printf("FE_DOWNWARD\n"); break;
    case FE_TOWARDZERO: printf("FE_TOWARDZERO\n"); break;
  printf("Significant digits: %d\n", FLT_MANT_DIG);
  return 0;

Common Issues

One common issue is accidentally mixing floating-point and integer arithmetic, which can lead to unexpected results. Always be aware of the data types you are using and cast them appropriately.


float.h is essential for controlling the behavior of floating-point calculations in your C programs. It allows you to configure the rounding mode, exception handling, and other settings. You can use float.h to optimize floating-point performance, improve accuracy, and ensure consistent behavior across different systems.

Related Commands

  • math.h: Provides mathematical functions, including floating-point operations.
  • ctype.h: Provides functions for character type classification.
  • limits.h: Provides information about the limits of various data types.