ILOGB(3)                   POSIX Programmer's Manual                  ILOGB(3)



PPRROOLLOOGG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NNAAMMEE
       ilogb, ilogbf, ilogbl -- return an unbiased exponent

SSYYNNOOPPSSIISS
       #include <math.h>

       int ilogb(double _x);
       int ilogbf(float _x);
       int ilogbl(long double _x);

DDEESSCCRRIIPPTTIIOONN
       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 return the exponent part of their argument _x.
       Formally, the return value is the integral part of log_r|_x| as a signed
       integral value, for non-zero _x, where _r is the radix of the machine's
       floating-point arithmetic, which is the value of FLT_RADIX defined in
       _<_f_l_o_a_t_._h_>.

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

RREETTUURRNN VVAALLUUEE
       Upon successful completion, these functions shall return the exponent
       part of _x as a signed integer value. They are equivalent to calling the
       corresponding _l_o_g_b() function and casting the returned value to type
       iinntt.

       If _x is 0, the value FP_ILOGB0 shall be returned.  On XSI-conformant
       systems, a domain error shall occur;
       otherwise, a domain error may occur.

       If _x is +-Inf, the value {INT_MAX} shall be returned.  On XSI-
       conformant systems, a domain error shall occur;
       otherwise, a domain error may occur.

       If _x is a NaN, the value FP_ILOGBNAN shall be returned.  On XSI-
       conformant systems, a domain error shall occur;
       otherwise, a domain error may occur.

       If the correct value is greater than {INT_MAX}, a domain error shall
       occur and an unspecified value shall be returned.  On XSI-conformant
       systems, a domain error shall occur and {INT_MAX} shall be returned.

       If the correct value is less than {INT_MIN}, a domain error shall occur
       and an unspecified value shall be returned.  On XSI-conformant systems,
       a domain error shall occur and {INT_MIN} shall be returned.

EERRRROORRSS
       These functions shall fail if:

       Domain Error
                   The correct value is not representable as an integer.

                   The _x argument is zero, NaN, or +-Inf.

                   If the integer expression (_m_a_t_h___e_r_r_h_a_n_d_l_i_n_g & MATH_ERRNO)
                   is non-zero, then _e_r_r_n_o shall be set to [[EEDDOOMM]].  If the
                   integer expression (_m_a_t_h___e_r_r_h_a_n_d_l_i_n_g & MATH_ERREXCEPT) is
                   non-zero, then the invalid floating-point exception shall
                   be raised.

       These functions may fail if:

       Domain Error
                   The _x argument is zero, NaN, or +-Inf.

                   If the integer expression (_m_a_t_h___e_r_r_h_a_n_d_l_i_n_g & MATH_ERRNO)
                   is non-zero, then _e_r_r_n_o shall be set to [[EEDDOOMM]].  If the
                   integer expression (_m_a_t_h___e_r_r_h_a_n_d_l_i_n_g & MATH_ERREXCEPT) is
                   non-zero, then the invalid floating-point exception shall
                   be raised.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

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

RRAATTIIOONNAALLEE
       The errors come from taking the expected floating-point value and
       converting it to iinntt, which is an invalid operation in
       IEEE Std 754-1985 (since overflow, infinity, and NaN are not
       representable in a type iinntt), so should be a domain error.

       There are no known implementations that overflow. For overflow to
       happen, {INT_MAX} must be less than LDBL_MAX_EXP*_l_o_g_2(FLT_RADIX) or
       {INT_MIN} must be greater than LDBL_MIN_EXP*_l_o_g_2(FLT_RADIX) if
       subnormals are not supported, or {INT_MIN} must be greater than
       (LDBL_MIN_EXP-LDBL_MANT_DIG)*_l_o_g_2(FLT_RADIX) if subnormals are
       supported.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _f_e_c_l_e_a_r_e_x_c_e_p_t(), _f_e_t_e_s_t_e_x_c_e_p_t(), _l_o_g_b(), _s_c_a_l_b_l_n()

       The Base Definitions volume of POSIX.1-2017, _S_e_c_t_i_o_n _4_._2_0, _T_r_e_a_t_m_e_n_t _o_f
       _E_r_r_o_r _C_o_n_d_i_t_i_o_n_s _f_o_r _M_a_t_h_e_m_a_t_i_c_a_l _F_u_n_c_t_i_o_n_s, <<ffllooaatt..hh>>, <<mmaatthh..hh>>

CCOOPPYYRRIIGGHHTT
       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 .



IEEE/The Open Group                  2017                             ILOGB(3)
