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LOG1P(3) Linux Programmer's Manual LOG1P(3)

NAME

     log1p, log1pf, log1pl -  logarithm of 1 plus argument

SYNOPSIS

     #include <math.h>

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

     Link with -lm.

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     log1p():
         _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
             || _XOPEN_SOURCE >= 500
             || /* Since glibc 2.19: */ _DEFAULT_SOURCE
             || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
     log1pf(), log1pl():
         _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
             || /* Since glibc 2.19: */ _DEFAULT_SOURCE
             || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

     These functions return a value equivalent to

         log (1 + x)

     The result is computed in a way that is accurate even if the value of x
     is near zero.

RETURN VALUE

     On success, these functions return the natural logarithm of (1 + x).

     If x is a NaN, a NaN is returned.

     If x is positive infinity, positive infinity is returned.

     If x is -1, a pole error occurs, and the  functions  return  -HUGE_VAL,
     -HUGE_VALF, or -HUGE_VALL, respectively.

     If  x  is  less  than  -1 (including negative infinity), a domain error
     occurs, and a NaN (not a number) is returned.

ERRORS

     See math_error(7) for information on how to determine whether an  error
     has occurred when calling these functions.

     The following errors can occur:

     Domain error: x is less than -1
            errno  is set to EDOM (but see BUGS).  An invalid floating-point
            exception (FE_INVALID) is raised.

     Pole error: x is -1
            errno is set to ERANGE (but see BUGS).  A divide-by-zero  float-
            ing-point exception (FE_DIVBYZERO) is raised.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     allbox; lbw27 lb lb  l  l  l.   Interface Attribute Value  T{  log1p(),
     log1pf(), log1pl() T}   Thread safety  MT-Safe

CONFORMING TO

     C99, POSIX.1-2001, POSIX.1-2008.

BUGS

     Before version 2.22, the glibc implementation did not set errno to EDOM
     when a domain error occurred.

     Before version 2.22, the glibc implementation  did  not  set  errno  to
     ERANGE when a range error occurred.