'\" t .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) .\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk .\" .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" References consulted: .\" Linux libc source code .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) .\" 386BSD man pages .\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu) .\" Modified 1995-08-14 by Arnt Gulbrandsen .\" Modified 2002-07-27 by Walter Harms .\" (walter.harms@informatik.uni-oldenburg.de) .\" .TH log 3 2024-05-02 "Linux man-pages 6.8" .SH NAME log, logf, logl \- natural logarithmic function .SH LIBRARY Math library .RI ( libm ", " \-lm ) .SH SYNOPSIS .nf .B #include .P .BI "double log(double " x ); .BI "float logf(float " x ); .BI "long double logl(long double " x ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .P .BR logf (), .BR logl (): .nf _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE .fi .SH DESCRIPTION These functions return the natural logarithm of .IR x . .SH RETURN VALUE On success, these functions return the natural logarithm of .IR x . .P If .I x is a NaN, a NaN is returned. .P If .I x is 1, the result is +0. .P If .I x is positive infinity, positive infinity is returned. .P If .I x is zero, then a pole error occurs, and the functions return .RB \- HUGE_VAL , .RB \- HUGE_VALF , or .RB \- HUGE_VALL , respectively. .P If .I x is negative (including negative infinity), then a domain error occurs, and a NaN (not a number) is returned. .SH ERRORS See .BR math_error (7) for information on how to determine whether an error has occurred when calling these functions. .P The following errors can occur: .TP Domain error: \fIx\fP is negative .I errno is set to .BR EDOM . An invalid floating-point exception .RB ( FE_INVALID ) is raised. .TP Pole error: \fIx\fP is zero .I errno is set to .BR ERANGE . A divide-by-zero floating-point exception .RB ( FE_DIVBYZERO ) is raised. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR log (), .BR logf (), .BR logl () T} Thread safety MT-Safe .TE .SH STANDARDS C11, POSIX.1-2008. .SH HISTORY C99, POSIX.1-2001. .P The variant returning .I double also conforms to SVr4, 4.3BSD, C89. .SH BUGS In glibc 2.5 and earlier, taking the .BR log () of a NaN produces a bogus invalid floating-point .RB ( FE_INVALID ) exception. .SH SEE ALSO .BR cbrt (3), .BR clog (3), .BR log10 (3), .BR log1p (3), .BR log2 (3), .BR sqrt (3)