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

       log1p, log1pf, log1pl — compute a natural logarithm

SYNOPSIS

       #include <math.h>

       double log1p(double x);
       float log1pf(float x);
       long double log1pl(long double x);

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 loge(1.0 + 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 1.0 + x.

       If x is -1, a pole error shall occur and log1p(),  log1pf(),  and  log1pl()  shall  return
       -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

       For finite values of x that are less than -1, 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 ±0, or +Inf, x shall be returned.

       If x is subnormal, a range error may occur
       and x should be returned.

       If x is not returned, log1p(), log1pf(), and  log1pl()  shall  return  an  implementation-
       defined value no greater in magnitude than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.

ERRORS

       These functions shall fail if:

       Domain Error
                   The finite value of x is less than -1, 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 -1.

                   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.

       These functions may fail if:

       Range Error The value of x is subnormal.

                   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 underflow 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

       feclearexcept(), fetestexcept(), log()

       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 .