atan, atanf, atanl

Defined in header <math.h>
float       atanf( float arg );
(1) (since C99)
double      atan( double arg );
long double atanl( long double arg );
(3) (since C99)
Defined in header <tgmath.h>
#define atan( arg )
(4) (since C99)
1-3) Computes the principal value of the arc tangent of arg.
4) Type-generic macro: If the argument has type long double, atanl is called. Otherwise, if the argument has integer type or the type double, atan is called. Otherwise, atanf is called. If the argument is complex, then the macro invokes the corresponding complex function (catanf, catan, catanl).


arg - floating point value

Return value

If no errors occur, the arc tangent of arg (arctan(arg)) in the range [- π/2 ; +π/2] radians, is returned.

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

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

  • If the argument is ±0, it is returned unmodified
  • If the argument is +∞, +π/2 is returned
  • If the argument is -∞, -π/2 is returned
  • if the argument is NaN, NaN is returned


POSIX specifies that in case of underflow, arg is returned unmodified, and if that is not supported, an implementation-defined value no greater than DBL_MIN, FLT_MIN, and LDBL_MIN is returned.


#include <stdio.h>
#include <math.h>
int main(void)
    printf("atan(1) = %f, 4*atan(1)=%f\n", atan(1), 4*atan(1));
    // special values
    printf("atan(Inf) = %f, 2*atan(Inf) = %f\n", atan(INFINITY), 2*atan(INFINITY));
    printf("atan(-0.0) = %+f, atan(+0.0) = %+f\n", atan(-0.0), atan(0));


atan(1) = 0.785398, 4*atan(1)=3.141593
atan(Inf) = 1.570796, 2*atan(Inf) = 3.141593
atan(-0.0) = -0.000000, atan(+0.0) = +0.000000


  • C11 standard (ISO/IEC 9899:2011):
    • The atan functions (p: 238-239)
    • 7.25 Type-generic math <tgmath.h> (p: 373-375)
    • F.10.1.3 The atan functions (p: 519)
  • C99 standard (ISO/IEC 9899:1999):
    • The atan functions (p: 219)
    • 7.22 Type-generic math <tgmath.h> (p: 335-337)
    • F.9.1.3 The atan functions (p: 456)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The atan function

See also

computes arc tangent, using signs to determine quadrants
computes arc sine (\({\small\arcsin{x} }\)arcsin(x))
computes arc cosine (\({\small\arccos{x} }\)arccos(x))
computes tangent (\({\small\tan{x} }\)tan(x))
computes the complex arc tangent
C++ documentation for atan

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