logb (3p)  Linux Manuals
logb: radixindependent exponent
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.NAME
logb, logbf, logbl  radixindependent exponent
SYNOPSIS
#include <math.h>
double logb(double x);
float logbf(float x);
long double logbl(long double x);
DESCRIPTION
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, a pole error shall occur and logb(), logbf(), and logbl() shall return HUGE_VAL, HUGE_VALF, and HUGE_VALL, respectively.
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.
The following sections are informative.
EXAMPLES
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
FUTURE DIRECTIONS
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology  Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 20012003 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 http://www.opengroup.org/unix/online.html .
SEE ALSO
feclearexcept(), fetestexcept(), ilogb(), scalb(), the Base Definitions volume of IEEE Std 1003.12001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>