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logb(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|ATTRIBUTES \|STANDARDS \|HISTORY \|SEE ALSO \|COLOPHON

logb(3)                  Library Functions Manuallogb(3)

NAME top

       logb, logbf, logbl - get exponent of a floating-point value

LIBRARY top

       Math library (libm,-lm)

SYNOPSIS top

#include <math.h>double logb(doublex);float logbf(floatx);long double logbl(long doublex);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):logb():           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L               || _XOPEN_SOURCE >= 500               || /* Since glibc 2.19: */ _DEFAULT_SOURCE               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCElogbf(),logbl():           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L               || /* Since glibc 2.19: */ _DEFAULT_SOURCE               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION top

       These functions extract the exponent from the internal floating-       point representation ofx and return it as a floating-point value.       The integer constantFLT_RADIX, defined in<float.h>, indicates       the radix used for the system's floating-point representation.  IfFLT_RADIXis 2,logb(x)is similar tofloor(log2(fabs(x))), except       that the latter may give an incorrect integer due to intermediate       rounding.       Ifx is subnormal,logb() returns the exponentx would have if it       were normalized.

RETURN VALUE top

       On success, these functions return the exponent ofx.       Ifx is a NaN, a NaN is returned.       Ifx is zero, then a pole error occurs, and the functions return       -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.       Ifx is negative infinity or positive infinity, then positive       infinity is returned.

ERRORS top

       Seemath_error(7) for information on how to determine whether an       error has occurred when calling these functions.       The following errors can occur:       Pole error:x is 0              A divide-by-zero floating-point exception (FE_DIVBYZERO) is              raised.       These functions do not seterrno.

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │logb(),logbf(),logbl()             │ Thread safety │ MT-Safe │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

       C11, POSIX.1-2008.

HISTORY top

       C99, POSIX.1-2001.logb() 4.3BSD (see IEEE.3 in the 4.3BSD manual).

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17logb(3)

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