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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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logb(3) Library Functions Manual logb(3)
logb, logbf, logbl - get exponent of a floating-point value
Math library (libm, -lm)
#include <math.h>
double logb(double x);
float logbf(float x);
long double logbl(long double x);
Feature Test Macro Requirements for glibc (see
feature_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_SOURCE
logbf(), logbl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
These functions extract the exponent from the internal floating-
point representation of x and return it as a floating-point value.
The integer constant FLT_RADIX, defined in <float.h>, indicates
the radix used for the system's floating-point representation. If
FLT_RADIX is 2, logb(x) is similar to floor(log2(fabs(x))), except
that the latter may give an incorrect integer due to intermediate
rounding.
If x is subnormal, logb() returns the exponent x would have if it
were normalized.
On success, these functions return the exponent of x.
If x is a NaN, a NaN is returned.
If x is zero, then a pole error occurs, and the functions return
-HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
If x is negative infinity or positive infinity, then positive
infinity is returned.
See math_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 set errno.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ logb(), logbf(), logbl() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
C99, POSIX.1-2001.
logb() 4.3BSD (see IEEE.3 in the 4.3BSD manual).
ilogb(3), log(3)
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user-space interface documentation) project. Information about
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Linux man-pages 6.15 2025-05-17 logb(3)
Pages that refer to this page: ilogb(3), remquo(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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