atan, atanf, atanl

 

C

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Concurrency support (C11)

Technical Specifications

Symbol index

 

Numerics

Common mathematical functions

Floating-point environment (C99)

Pseudo-random number generation

Complex number arithmetic (C99)

Type-generic math (C99)

 

Common mathematical functions

Types
div_tldiv_tlldiv_timaxdiv_t (C99)(C99)     float_tdouble_t (C99)(C99)     _Decimal32_t_Decimal64_t (C23)(C23)
Functions
Basic operations abslabsllabsimaxabs (C99)(C99) fabs divldivlldivimaxdiv (C99)(C99) fmod remainder (C99) remquo (C99) fma (C99) fdim (C99) nannanfnanlnandN (C99)(C99)(C99)(C23) Maximum/minimum operations fmax (C99) fmaximum (C23) fmaximum_mag (C23) fmaximum_num (C23) fmaximum_mag_num (C23)     fmin (C99) fminimum (C23) fminimum_mag (C23) fminimum_num (C23) fminimum_mag_num (C23) Exponential functions exp exp10 (C23) exp2 (C99) expm1 (C99) exp10m1 (C23) exp2m1 (C23) log log10 log2 (C99) log1plogp1 (C99)(C23) log10p1 (C23) log2p1 (C23) Power functions sqrt cbrt (C99) rootn (C23) rsqrt (C23) hypot (C99) compound (C23) pow pown (C23) powr (C23) Trigonometric and hyperbolic functions sin cos tan asin acos atan atan2 sinpi (C23) cospi (C23) tanpi (C23) asinpi (C23) acospi (C23) atanpi (C23) atan2pi (C23) 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) roundeven (C23) trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99) fromfpfromfpxufromfpufromfpx (C23)(C23)(C23)(C23) Floating-point manipulation functions ldexp frexp scalbnscalbln (C99)(C99) ilogbllogb (C99)(C23) logb (C99) modf nextafternexttoward (C99)(C99) nextupnextdown (C23)(C23) copysign (C99) canonicalize (C23) Narrowing operations fadd (C23) fsub (C23) fmul (C23) fdiv (C23) ffma (C23) fsqrt (C23) Quantum and quantum exponent functions quantizedN (C23) samequantumdN (C23) quantumdN (C23) llquantexpdN (C23) Decimal re-encoding functions encodedecdN (C23) decodedecdN (C23) encodebindN (C23) decodebindN (C23) Total order and payload functions totalorder (C23) getpayload (C23) setpayload (C23) setpayloadsig (C23) Classification fpclassify (C99) iscanonical (C23) isfinite (C99) isinf (C99) isnan (C99) isnormal (C99) signbit (C99) issubnormal (C23) iszero (C23) isgreater (C99) isgreaterequal (C99) isless (C99) islessequal (C99) islessgreater (C99) isunordered (C99) issignaling (C23) iseqsig (C23)
Macro constants
Special floating-point values HUGE_VALFHUGE_VALHUGE_VALLHUGE_VALDN (C99)(C99)(C23) INFINITYDEC_INFINITY (C99)(C23) NANDEC_NAN (C99)(C23) Arguments and return values FP_ILOGB0FP_ILOGBNAN (C99)(C99) FP_INT_UPWARDFP_INT_DOWNWARDFP_INT_TOWARDZEROFP_INT_TONEARESTFROMZEROFP_INT_TONEAREST (C23)(C23)(C23)(C23)(C23)     FP_LLOGB0FP_LLOGBNAN (C23)(C23) FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99)     Error handling MATH_ERRNOMATH_ERRNOEXCEPT (C99)(C99) math_errhandling (C99)	Fast operation indicators FP_FAST_FMAFFP_FAST_FMA (C99)(C99) FP_FAST_FADDFP_FAST_FADDLFP_FAST_DADDLFP_FAST_DMADDDN (C23)(C23)(C23)(C23)     FP_FAST_FMULFP_FAST_FMULLFP_FAST_DMULLFP_FAST_DMMULDN (C23)(C23)(C23)(C23) FP_FAST_FFMAFP_FAST_FFMALFP_FAST_DFMALFP_FAST_DMFMADN (C23)(C23)(C23)(C23) FP_FAST_FMALFP_FAST_FMADN (C99)(C23) FP_FAST_FSUBFP_FAST_FSUBLFP_FAST_DSUBLFP_FAST_DMSUBDN (C23)(C23)(C23)(C23) FP_FAST_FDIVFP_FAST_FDIVLFP_FAST_DDIVLFP_FAST_DMDIVDN (C23)(C23)(C23)(C23) FP_FAST_FSQRTFP_FAST_FSQRTLFP_FAST_DSQRTLFP_FAST_DMSQRTDN (C23)(C23)(C23)(C23)

 

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)
_Decimal32  atand32( _Decimal32 arg );	(4)	(since C23)
_Decimal64  atand64( _Decimal64 arg );	(5)	(since C23)
_Decimal128 atand128( _Decimal128 arg );	(6)	(since C23)
Defined in header <tgmath.h>
#define atan( arg )	(7)	(since C99)

1-6) Computes the principal value of the arc tangent of arg.

7) Type-generic macro: If the argument has type long double, (3) (atanl) is called. Otherwise, if the argument has integer type or the type double, (2) (atan) is called. Otherwise, (1) (atanf) is called. If the argument is complex, then the macro invokes the corresponding complex function (catanf, catan, catanl).

The functions (4-6) are declared if and only if the implementation predefines __STDC_IEC_60559_DFP__ (i.e. the implementation supports decimal floating-point numbers).

(since C23)

Parameters

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.

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 <math.h>
#include <stdio.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

• C23 standard (ISO/IEC 9899:2024):

• 7.12.4.3 The atan functions (p: TBD)

• 7.25 Type-generic math <tgmath.h> (p: TBD)

• F.10.1.3 The atan functions (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.12.4.3 The atan functions (p: 174)

• 7.25 Type-generic math <tgmath.h> (p: 272-273)

• F.10.1.3 The atan functions (p: 378)

• 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 (\({\small\arcsin{x} }\)arcsin(x)) (function)
acosacosfacosl (C99)(C99)	computes arc cosine (\({\small\arccos{x} }\)arccos(x)) (function)
tantanftanl (C99)(C99)	computes tangent (\({\small\tan{x} }\)tan(x)) (function)
catancatanfcatanl (C99)(C99)(C99)	computes the complex arc tangent (function)
C++ documentation for atan