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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 dif-
       fer (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(double x);
       float logf(float x);
       long double logl(long double x);


DESCRIPTION
       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 feclearex-
       cept(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  excep-
       tion 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 indepen-
       dent 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
       IEEE Std 1003.1-2001, Section 4.18, 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, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (C) 2001-2003 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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                              LOG(3P)