Section: POSIX Programmer's Manual (3P)
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
--- arc tangent function
double atan(double x);
float atanf(float x);
long double atanl(long double x);
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 POSIX.1-2017 defers to the ISO C standard.
These functions shall compute the principal value of the arc tangent of
An application wishing to check for error situations should set
to zero and call
before calling these functions. On return, if
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 arc
in the range [-π/2,π/2] radians.
is NaN, a NaN shall be returned.
shall be returned.
is ±Inf, ±π/2 shall be returned.
is subnormal, a range error may occur
should be returned.
is not returned,
shall return an implementation-defined value no greater in magnitude
than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.
These functions may fail if:
- Range Error
The value of
If the integer expression (math_errhandling & MATH_ERRNO) is
shall be set to
If the integer expression (math_errhandling & MATH_ERREXCEPT) is
non-zero, then the underflow floating-point exception shall be raised.
The following sections are informative.
On error, the expressions (math_errhandling
& MATH_ERRNO) and
& MATH_ERREXCEPT) are independent of each
other, but at least one of them must be non-zero.
The Base Definitions volume of POSIX.1-2017,
Section 4.20, Treatment of Error Conditions for Mathematical Functions,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
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