The Open Group Base Specifications Issue 7, 2018 edition

IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

Copyright © 2001-2018 IEEE and The Open Group

#### NAME

j0, j1, jn - Bessel functions of the first kind

#### SYNOPSIS

^{[XSI]} #include <math.h>

double j0(double *x*`);`

double j1(double *x*`);`

double jn(int *n*`, double` *x*`); `

#### DESCRIPTION

The *j0*(), *j1*(), and *jn*() functions shall compute Bessel functions of *x* of the first kind of orders
0, 1, and *n*, respectively.

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.

#### RETURN VALUE

Upon successful completion, these functions shall return the relevant Bessel value of *x* of the first kind.

If the *x* argument is too large in magnitude, or the correct result would cause underflow, 0 shall be returned and a range
error may occur.

^{[MXX]}
If *x* is NaN, a NaN shall be returned.

#### ERRORS

These functions may fail if:

- Range Error
- The value of
*x* was too large in magnitude, or an underflow occurred.
If the integer expression (*math_errhandling* & MATH_ERRNO) is non-zero, then *errno* shall be set to [ERANGE]. If
the integer expression (*math_errhandling* & MATH_ERREXCEPT) is non-zero, then the underflow floating-point exception
shall be raised.

No other errors shall occur.

*The following sections are informative.*

#### EXAMPLES

None.

#### APPLICATION USAGE

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.

#### RATIONALE

None.

#### FUTURE DIRECTIONS

None.

#### SEE ALSO

*feclearexcept*, *fetestexcept*, *isnan*, *y0*

XBD *Treatment of Error Conditions for Mathematical Functions*, *<math.h>*

#### CHANGE HISTORY

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 may fail [EDOM] error is removed for the case for NaN.

The RETURN VALUE and ERRORS sections are reworked for alignment of the error handling with the ISO/IEC 9899:1999
standard.

#### Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0350 [68] is applied.

*End of informative text.*

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