PROLOG  NAME  SYNOPSIS  DESCRIPTION  RETURN VALUE  ERRORS  EXAMPLES  APPLICATION USAGE  RATIONALE  FUTURE DIRECTIONS  SEE ALSO  COPYRIGHT 

LOG2(3P) POSIX Programmer's Manual LOG2(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.
log2, log2f, log2l — compute base 2 logarithm functions
#include <math.h> double log2(double x); float log2f(float x); long double log2l(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.1‐2008 defers to the ISO C standard. These functions shall compute the base 2 logarithm of their argument x, log2(x). 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 nonzero or fetestexcept(FE_INVALID  FE_DIVBYZERO  FE_OVERFLOW  FE_UNDERFLOW) is nonzero, an error has occurred.
Upon successful completion, these functions shall return the base 2 logarithm of x. If x is ±0, a pole error shall occur and log2(), log2f(), and log2l() shall return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively. For finite values of x that are less than 0, or if x is −Inf, a domain error shall occur, and either a NaN (if supported), or an implementationdefined value shall be returned. If x is NaN, a NaN shall be returned. If x is 1, +0 shall be returned. If x is +Inf, x shall be returned.
These functions shall fail if: Domain Error The finite value of x is less than zero, or x is −Inf. If the integer expression (math_errhandling & MATH_ERRNO) is nonzero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is nonzero, then the invalid floatingpoint exception shall be raised. Pole Error The value of x is zero. If the integer expression (math_errhandling & MATH_ERRNO) is nonzero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is nonzero, then the dividebyzero floatingpoint exception shall be raised. The following sections are informative.
None.
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 nonzero.
None.
None.
feclearexcept(3p), fetestexcept(3p), log(3p) The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, math.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology  Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.12008 with the 2013 Technical Corrigendum 1
applied.) 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 http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/manpages/reporting_bugs.html .
IEEE/The Open Group 2013 LOG2(3P)