# log1p

## 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.
    


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