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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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log(3) Library Functions Manual log(3)
log, logf, logl - natural logarithmic function
Math library (libm, -lm)
#include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
logf(), logl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
These functions return the natural logarithm of x.
On success, these functions return the natural logarithm of x.
If x is a NaN, a NaN is returned.
If x is 1, the result is +0.
If x is positive infinity, positive infinity 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 (including negative infinity), then a domain
error occurs, and a NaN (not a number) 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:
Domain error: x is negative
errno is set to EDOM. An invalid floating-point exception
(FE_INVALID) is raised.
Pole error: x is zero
errno is set to ERANGE. A divide-by-zero floating-point
exception (FE_DIVBYZERO) is raised.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ log(), logf(), logl() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
C99, POSIX.1-2001.
The variant returning double also conforms to SVr4, 4.3BSD, C89.
In glibc 2.5 and earlier, taking the log() of a NaN produces a
bogus invalid floating-point (FE_INVALID) exception.
cbrt(3), clog(3), log10(3), log1p(3), log2(3), sqrt(3)
This page is part of the man-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 is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 log(3)
Pages that refer to this page: expm1(3), ilogb(3), log10(3), log1p(3), log2(3), logb(3), matherr(3), __pmconnectlogger(3), math_error(7)
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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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