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table of contents

LOG(3P) | POSIX Programmer's Manual | LOG(3P) |

# 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.

# NAME¶

log, logf, logl — natural logarithm function

# SYNOPSIS¶

#include <math.h>

double log(doublex); float logf(floatx); long double logl(long doublex);

# 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.1‐2017 defers to the ISO C standard.

These functions shall compute the natural logarithm of their
argument *x*, log*e*(*x*).

An application wishing to check for error situations should set
*errno* to zero and call *feclearexcept*(FE_ALL_EXCEPT) before
calling these functions. On return, if *errno* is non-zero or
*fetestexcept*(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, an error has occurred.

# RETURN VALUE¶

Upon successful completion, these functions shall return the
natural logarithm of *x*.

If *x* is ±0, a pole error shall occur and
*log*(), *logf*(), and *logl*() shall return -HUGE_VAL,
-HUGE_VALF, and -HUGE_VALL, respectively.

For finite values of *x* that are less than 0, or if *x*
is -Inf, a domain error shall occur, and either a NaN (if supported), or an
implementation-defined value shall be returned.

If *x* is NaN, a NaN shall be returned.

If *x* is 1, +0 shall be returned.

If *x* is +Inf, *x* shall be returned.

# ERRORS¶

These functions shall fail if:

- Domain Error
- The finite value of
*x*is negative, or*x*is -Inf.

If the integer expression (*math_errhandling* &
MATH_ERRNO) is non-zero, then *errno* shall be set to **[EDOM]**. If
the integer expression (*math_errhandling* & MATH_ERREXCEPT) is
non-zero, then the invalid floating-point exception shall be raised.

- Pole Error
- The value of
*x*is zero.

If the integer expression (*math_errhandling* &
MATH_ERRNO) is non-zero, then *errno* shall be set to **[ERANGE]**.
If the integer expression (*math_errhandling* & MATH_ERREXCEPT) is
non-zero, then the divide-by-zero floating-point exception shall be
raised.

*The following sections are informative.*

# EXAMPLES¶

None.

# APPLICATION USAGE¶

On error, the expressions (*math_errhandling* &
MATH_ERRNO) and (*math_errhandling* & MATH_ERREXCEPT) are
independent of each other, but at least one of them must be non-zero.

# RATIONALE¶

None.

# FUTURE DIRECTIONS¶

None.

# SEE ALSO¶

* exp()*,

*,*

*feclearexcept*()*,*

*fetestexcept*()*,*

*isnan*()*,*

*log10*()

*log1p*()The Base Definitions volume of POSIX.1‐2017, *Section
4.20*, *Treatment of Error Conditions for Mathematical Functions*,
**<math.h>**

# COPYRIGHT¶

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

2017 | IEEE/The Open Group |