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.

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logb, logbf, logbl - radix-independent exponent

#include <math.h>

double logb(doublex);

float logbf(floatx);

long double logbl(long doublex);

These functions shall compute the exponent of

x, which is the integral part of log_r|x|, as a signed floating-point value, for non-zerox, whereris the radix of the machine’s floating-point arithmetic, which is the value of FLT_RADIX defined in the<float.h>header.If

xis subnormal it is treated as though it were normalized; thus for finite positivex:

1 <=x* FLT_RADIX**-logb(x) < FLT_RADIXAn application wishing to check for error situations should set

errnoto zero and callfeclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, iferrnois non-zero orfetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

Upon successful completion, these functions shall return the exponent of

x.If

xis ±0, a pole error shall occur andlogb(),logbf(), andlogbl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.If

xis NaN, a NaN shall be returned.If

xis ±Inf, +Inf shall be returned.

These functions shall fail if:

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then

Pole Error The value of xis ±0.errnoshall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point 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 non-zero.

None.

None.

feclearexcept() ,fetestexcept() ,ilogb() ,scalb() , the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions,<float.h>,<math.h>

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) 2001-2003 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 .

IEEE/The Open Group |
LOGB (P) | 2003 |