C++ logb()

C++ logb() returns logarithm of absolute value of given argument. FLT_RADIX is used as base for the logarithm calculation.

FLT_RADIX is 2 or greater, but generally 2. Please check this value before using logb() function.


The syntax of C++ logb() is



xA double, float, long double, or any integral type value.


The return value depends on the type of value passed for parameter x.

The return value of logb(x) is

  • double if x is double or integral type.
  • float if x is float.
  • long double if x is long double.

The synopsis of logb() function is

double logb(double x);
float logb(float x);
long double logb(long double x);
double logb(T x); // for integral type argument values

logb() is a function of cmath library. Include cmath library at the start of program, if using logb() function.


In this example, we read a value from user into variable x, and find the logarithm value of |x| using logb() function.

C++ Program

#include <iostream>
using namespace std;

int main() {
    double x;
    cout << "Enter a number : ";
    cin >> x;
    double result = logb(x);
    cout << "logb(" << x << ") : " << result << endl;


Enter a number : 16
logb(16) : 4
Program ended with exit code: 0
Enter a number : -16
logb(-16) : 4
Program ended with exit code: 0
Enter a number : 0
logb(0) : -inf
Program ended with exit code: 0


In this C++ Tutorial, we learned the syntax of C++ logb(), and how to use this function to find the logarithm of absolute value of given number, with base value of FLT_RADIX, with the help of examples.