'\" et
.TH LOG "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
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.

.SH NAME
log,
logf,
logl
\(em natural logarithm function
.SH SYNOPSIS
.LP
.nf
#include <math.h>
.P
double log(double \fIx\fP);
float logf(float \fIx\fP);
long double logl(long double \fIx\fP);
.fi
.SH DESCRIPTION
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2008 defers to the ISO\ C standard.
.P
These functions shall compute the natural logarithm of their argument
.IR x ,
log\d\s-3\fIe\fR\s+3\u(\fIx\fR).
.P
An application wishing to check for error situations should set
.IR errno
to zero and call
.IR feclearexcept (FE_ALL_EXCEPT)
before calling these functions. On return, if
.IR errno
is non-zero or \fIfetestexcept\fR(FE_INVALID | FE_DIVBYZERO |
FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.
.SH "RETURN VALUE"
Upon successful completion, these functions shall return the natural
logarithm of
.IR x .
.P
If
.IR x
is \(+-0, a pole error shall occur and
\fIlog\fR(),
\fIlogf\fR(),
and
\fIlogl\fR()
shall return \(miHUGE_VAL, \(miHUGE_VALF, and \(miHUGE_VALL,
respectively.
.P
For finite values of
.IR x
that are less than 0,
or if
.IR x
is \(miInf,
a domain error shall occur, and
either a NaN (if supported), or
an implementation-defined value shall be returned.
.P
If
.IR x
is NaN, a NaN shall be returned.
.P
If
.IR x
is 1, +0 shall be returned.
.P
If
.IR x
is +Inf,
.IR x
shall be returned.
.SH ERRORS
These functions shall fail if:
.IP "Domain\ Error" 12
The finite value of
.IR x
is negative,
or
.IR x
is \(miInf.
.RS 12 
.P
If the integer expression (\fImath_errhandling\fR & MATH_ERRNO) is
non-zero, then
.IR errno
shall be set to
.BR [EDOM] .
If the integer expression (\fImath_errhandling\fR & MATH_ERREXCEPT) is
non-zero, then the invalid floating-point exception shall be raised.
.RE
.IP "Pole\ Error" 12
The value of
.IR x
is zero.
.RS 12 
.P
If the integer expression (\fImath_errhandling\fR & MATH_ERRNO) is
non-zero, then
.IR errno
shall be set to
.BR [ERANGE] .
If the integer expression (\fImath_errhandling\fR & MATH_ERREXCEPT) is
non-zero, then the divide-by-zero floating-point exception shall be
raised.
.RE
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
On error, the expressions (\fImath_errhandling\fR & MATH_ERRNO) and
(\fImath_errhandling\fR & MATH_ERREXCEPT) are independent of each
other, but at least one of them must be non-zero.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIexp\fR\^(\|)",
.IR "\fIfeclearexcept\fR\^(\|)",
.IR "\fIfetestexcept\fR\^(\|)",
.IR "\fIisnan\fR\^(\|)",
.IR "\fIlog10\fR\^(\|)",
.IR "\fIlog1p\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 4.19" ", " "Treatment of Error Conditions for Mathematical Functions",
.IR "\fB<math.h>\fP"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

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