atan, atanf, atanl

C

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Error handling

Dynamic memory management

Date and time utilities

Strings library

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Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

Numerics

Common mathematical functions

Floating-point environment

Complex number arithmetics

Random number generation

Type-generic math

Common mathematical functions

Functions
Basic operations
abslabsllabsimaxabs (C99) fabs divldivlldivimaxdiv (C99) fmod	remainder (C99) remquo (C99) fma (C99) fmax (C99) fmin (C99) fdim (C99) nannanfnanl (C99)(C99)(C99)
Exponential functions
exp exp2 (C99) expm1 (C99)	log log10 log1p (C99) log2 (C99)
Power functions
sqrt cbrt (C99)	hypot (C99) pow
Trigonometric and hyperbolic functions
sin cos tan asin acos atan atan2	sinh cosh tanh asinh (C99) acosh (C99) atanh (C99)
Error and gamma functions
erf (C99) erfc (C99)	lgamma (C99) tgamma (C99)
Nearest integer floating point operations
ceil floor roundlroundllround (C99)(C99)(C99)	trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99)
Floating point manipulation functions
ldexp scalbnscalbln (C99)(C99) ilogb (C99) logb (C99)	frexp modf nextafternexttoward (C99)(C99) copysign (C99)
Classification
fpclassify (C99) isfinite (C99) isinf (C99) isnan (C99) isnormal (C99) signbit (C99)	isgreater (C99) isgreaterequal (C99) isless (C99) islessequal (C99) islessgreater (C99) isunordered (C99)
Types
div_tldiv_tlldiv_timaxdiv_t (C99)(C99)	float_tdouble_t (C99)(C99)
Macro constants
HUGE_VALFHUGE_VALHUGE_VALL (C99)(C99) FP_FAST_FMAFFP_FAST_FMAFP_FAST_FMAL (C99)(C99)(C99) math_errhandlingMATH_ERRNOMATH_ERRNOEXCEPT (C99)(C99)(C99)	INFINITY (C99) NAN (C99) FP_ILOGB0FP_ILOGBNAN (C99)(C99) FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99)

Defined in header <math.h>
float atanf( float arg );	(1)	(since C99)
double atan( double arg );	(2)
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).

Parameters

arg

-

floating point value

Return value

If no errors occur, the arc tangent of arg (arctan(arg)) in the range [-

2

; +

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 +, +/2 is returned
• If the argument is -, -/2 is returned
• if the argument is NaN, NaN is returned

Notes

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.

Example

Run this code

#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));
}

Output:

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

References

• C11 standard (ISO/IEC 9899:2011):

• 7.12.4.3 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):

• 7.12.4.3 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):

• 4.5.2.3 The atan function

See also

atan2atan2fatan2l (C99)(C99)	computes arc tangent, using signs to determine quadrants (function)
asinasinfasinl (C99)(C99)	computes arc sine (arcsin(x)) (function)
acosacosfacosl (C99)(C99)	computes arc cosine (arccos(x)) (function)
tantanftanl (C99)(C99)	computes tangent (tan(x)) (function)
catancatanfcatanl (C99)(C99)(C99)	computes the complex arc tangent (function)