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

LOGB(3P) POSIX Programmer's Manual LOGB(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.
logb, logbf, logbl — radixindependent exponent
#include <math.h> double logb(double x); float logbf(float x); long double logbl(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 exponent of x, which is the integral part of logr x, as a signed floatingpoint value, for non zero x, where r is the radix of the machine's floatingpoint arithmetic, which is the value of FLT_RADIX defined in the <float.h> header. If x is subnormal it is treated as though it were normalized; thus for finite positive x: 1 <= x * FLT_RADIX−logb(x) < FLT_RADIX 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 exponent of x. If x is ±0, logb(), logbf(), and logbl() shall return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively. On systems that support the IEC 60559 FloatingPoint option, a pole error shall occur; otherwise, a pole error may occur. If x is NaN, a NaN shall be returned. If x is ±Inf, +Inf shall be returned.
These functions shall fail if: Pole Error The value of x is ±0. 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. These functions may fail if: Pole Error The value of x is 0. 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), ilogb(3p), scalbln(3p) The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, float.h(0p), 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 LOGB(3P)