logb man page
Prolog
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
Synopsis
#include <math.h> double logb(double x); float logbf(float x); long double logbl(long double x);
Description
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.12008 defers to the ISO C standard.
These functions shall compute the exponent of x, which is the integral part of log_{r} x, as a signed floatingpoint value, for nonzero 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.
Return Value
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.
Errors
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.
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 nonzero.
Rationale
None.
Future Directions
None.
See Also
feclearexcept(), fetestexcept(), ilogb(), scalbln()
The Base Definitions volume of POSIX.12008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>
Copyright
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 .