The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


cos, cosf, cosl - cosine function


#include <math.h>

double cos(double
float cosf(float
long double cosl(long double


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

These functions shall compute the cosine of their argument x, measured in radians.

An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.


Upon successful completion, these functions shall return the cosine of x.

[MX] [Option Start] If x is NaN, a NaN shall be returned.

If x is ±0, the value 1.0 shall be returned.

If x is ±Inf, a domain error shall occur, and either a NaN (if supported), or an implementation-defined value shall be returned. [Option End]


These functions shall fail if:

Domain Error
[MX] [Option Start] The x argument is ±Inf.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised. [Option End]

The following sections are informative.


Taking the Cosine of a 45-Degree Angle
#include <math.h>
double radians = 45 * M_PI / 180;
double result;
result = cos(radians);


These functions may lose accuracy when their argument is near an odd multiple of /2 or is far from 0.

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.






acos(), feclearexcept(), fetestexcept(), isnan(), sin(), tan(), the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <math.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated to indicate how an application should check for an error. This text was previously published in the APPLICATION USAGE section.

Issue 6

The cosf() and cosl() functions are added for alignment with the ISO/IEC 9899:1999 standard.

The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.

End of informative text.

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