Header File 

math.h  

Category 

Math Routines 

Prototype 

double atan(double x); 

long double atanl(long double x); 

Description 

Calculates the arc tangent. 

atan calculates the arc tangent of the input value. 

atanl is the long double version; it takes a long double argument and returns a long double result. 

Return Value 

atan and atanl of a real argument return a value in the range -pi/2 to pi/2. Error handling for these functions can be modified through the functions _matherr and _matherrl. 

Example  

#include <stdio.h>
#include <math.h>
int main(void)
{
   double result;
   double x = 0.5;
   result = atan(x);
   printf("The arc tangent of %lf is %lf\n", x, result);
return(0);
}

Portability