atan2(), atan2f(), atan2l()

Updated: May 06, 2022

Compute the arctangent, determining the quadrant

Synopsis:

#include <math.h>

double atan2( double y, 
              double x );

float atan2f( float y, 
              float x );

long double atan2l( long double y, 
                    long double x );

Arguments:

y, x
The value (y/x) for which you want to find the angle.

Library:

libm
The general-purpose math library.
libm-sve
(QNX Neutrino 7.1 or later) A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

Note: Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

These functions compute the value of the arctangent (in radians) of y/x, using the signs of both arguments to determine the quadrant of the return value. No errors occur if both arguments are zero, or if y is zero.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Returns:

The arctangent of y/x, in the range [-π, π].

If y is: And x is: These functions return: Errors:
±0.0 Negative or -0.0 π, with the same sign as y (see below)
±0.0 Positive or +0.0 0.0 (see below)
±Inf Finite π/2, with the same sign as y
±Inf -Inf 3π/4, with the same sign as y
±Inf Inf π/4, with the same sign as y
Negative ±0.0 -π/2
Positive ±0.0 +π/2
Finite and positive -Inf +π
Finite and negative -Inf -π
Finite and positive Inf 0.0
Finite and negative Inf -0.0
Any value NaN x
NaN Any value y
Note: Although the library considers 0.0 to be equal to -0.0, atan2(0.0, 0.0) isn't equal to atan2(0.0, -0.0).

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Examples:

#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <stdlib.h>

int main( void )
{
    int except_flags;

    feclearexcept(FE_ALL_EXCEPT);

    printf( "%f\n", atan2( .5, 1. ) );

    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    return EXIT_SUCCESS;
}

produces the output:

0.463648

Classification:

C11, POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes