STRTOD(3)                  POSIX Programmer's Manual                 STRTOD(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
       strtod, strtof, strtold -- convert a string to a double-precision
       number

SSYYNNOOPPSSIISS
       #include <stdlib.h>

       double strtod(const char *restrict _n_p_t_r, char **restrict _e_n_d_p_t_r);
       float strtof(const char *restrict _n_p_t_r, char **restrict _e_n_d_p_t_r);
       long double strtold(const char *restrict _n_p_t_r, char **restrict _e_n_d_p_t_r);

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 convert the initial portion of the string pointed
       to by _n_p_t_r to ddoouubbllee, ffllooaatt, and lloonngg ddoouubbllee representation,
       respectively. First, they decompose the input string into three parts:

        1. An initial, possibly empty, sequence of white-space characters (as
           specified by _i_s_s_p_a_c_e())

        2. A subject sequence interpreted as a floating-point constant or
           representing infinity or NaN

        3. A final string of one or more unrecognized characters, including
           the terminating NUL character of the input string

       Then they shall attempt to convert the subject sequence to a floating-
       point number, and return the result.

       The expected form of the subject sequence is an optional ''++'' or ''--''
       sign, then one of the following:

        *  A non-empty sequence of decimal digits optionally containing a
           radix character; then an optional exponent part consisting of the
           character ''ee'' or the character ''EE'', optionally followed by a ''++'' or
           ''--'' character, and then followed by one or more decimal digits

        *  A 0x or 0X, then a non-empty sequence of hexadecimal digits
           optionally containing a radix character; then an optional binary
           exponent part consisting of the character ''pp'' or the character ''PP'',
           optionally followed by a ''++'' or ''--'' character, and then followed by
           one or more decimal digits

        *  One of INF or INFINITY, ignoring case

        *  One of NAN or NAN(_n_-_c_h_a_r_-_s_e_q_u_e_n_c_e_o_p_t), ignoring case in the NAN
           part, where:


               n-char-sequence:
                   digit
                   nondigit
                   n-char-sequence digit
                   n-char-sequence nondigit

       The subject sequence is defined as the longest initial subsequence of
       the input string, starting with the first non-white-space character,
       that is of the expected form. The subject sequence contains no
       characters if the input string is not of the expected form.

       If the subject sequence has the expected form for a floating-point
       number, the sequence of characters starting with the first digit or the
       decimal-point character (whichever occurs first) shall be interpreted
       as a floating constant of the C language, except that the radix
       character shall be used in place of a period, and that if neither an
       exponent part nor a radix character appears in a decimal floating-point
       number, or if a binary exponent part does not appear in a hexadecimal
       floating-point number, an exponent part of the appropriate type with
       value zero is assumed to follow the last digit in the string. If the
       subject sequence begins with a <hyphen-minus>, the sequence shall be
       interpreted as negated. A character sequence INF or INFINITY shall be
       interpreted as an infinity, if representable in the return type, else
       as if it were a floating constant that is too large for the range of
       the return type. A character sequence NAN or NAN(_n_-_c_h_a_r_-_s_e_q_u_e_n_c_e_o_p_t)
       shall be interpreted as a quiet NaN, if supported in the return type,
       else as if it were a subject sequence part that does not have the
       expected form; the meaning of the _n-char sequences is implementation-
       defined. A pointer to the final string is stored in the object pointed
       to by _e_n_d_p_t_r, provided that _e_n_d_p_t_r is not a null pointer.

       If the subject sequence has the hexadecimal form and FLT_RADIX is a
       power of 2, the value resulting from the conversion is correctly
       rounded.

       The radix character is defined in the current locale (category
       _L_C___N_U_M_E_R_I_C).  In the POSIX locale, or in a locale where the radix
       character is not defined, the radix character shall default to a
       <period> (''..'').

       In other than the C or POSIX locale, additional locale-specific subject
       sequence forms may be accepted.

       If the subject sequence is empty or does not have the expected form, no
       conversion shall be performed; the value of _n_p_t_r is stored in the
       object pointed to by _e_n_d_p_t_r, provided that _e_n_d_p_t_r is not a null
       pointer.

       These functions shall not change the setting of _e_r_r_n_o if successful.

       Since 0 is returned on error and is also a valid return on success, an
       application wishing to check for error situations should set _e_r_r_n_o to
       0, then call _s_t_r_t_o_d(), _s_t_r_t_o_f(), or _s_t_r_t_o_l_d(), then check _e_r_r_n_o.

RREETTUURRNN VVAALLUUEE
       Upon successful completion, these functions shall return the converted
       value. If no conversion could be performed, 0 shall be returned, and
       _e_r_r_n_o may be set to [[EEIINNVVAALL]].

       If the correct value is outside the range of representable values,
       +-HUGE_VAL, +-HUGE_VALF, or +-HUGE_VALL shall be returned (according to
       the sign of the value), and _e_r_r_n_o shall be set to [[EERRAANNGGEE]].

       If the correct value would cause an underflow, a value whose magnitude
       is no greater than the smallest normalized positive number in the
       return type shall be returned and _e_r_r_n_o set to [[EERRAANNGGEE]].

EERRRROORRSS
       These functions shall fail if:

       EERRAANNGGEE The value to be returned would cause overflow or underflow.

       These functions may fail if:

       EEIINNVVAALL No conversion could be performed.

       _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
       If the subject sequence has the hexadecimal form and FLT_RADIX is not a
       power of 2, and the result is not exactly representable, the result
       should be one of the two numbers in the appropriate internal format
       that are adjacent to the hexadecimal floating source value, with the
       extra stipulation that the error should have a correct sign for the
       current rounding direction.

       If the subject sequence has the decimal form and at most DECIMAL_DIG
       (defined in _<_f_l_o_a_t_._h_>) significant digits, the result should be
       correctly rounded. If the subject sequence _D has the decimal form and
       more than DECIMAL_DIG significant digits, consider the two bounding,
       adjacent decimal strings _L and _U, both having DECIMAL_DIG significant
       digits, such that the values of _L, _D, and _U satisfy _L <= _D <= _U.  The
       result should be one of the (equal or adjacent) values that would be
       obtained by correctly rounding _L and _U according to the current
       rounding direction, with the extra stipulation that the error with
       respect to _D should have a correct sign for the current rounding
       direction.

       The changes to _s_t_r_t_o_d() introduced by the ISO/IEC 9899:1999 standard
       can alter the behavior of well-formed applications complying with the
       ISO/IEC 9899:1990 standard and thus earlier versions of this standard.
       One such example would be:


           int
           what_kind_of_number (char *s)
           {
               char *endp;
               double d;
               long l;

               d = strtod(s, &endp);
               if (s != endp && *endp == `\0')
                   printf("It's a float with value %g\n", d);
               else
               {
                   l = strtol(s, &endp, 0);
                   if (s != endp && *endp == `\0')
                       printf("It's an integer with value %ld\n", 1);
                   else
                       return 1;
               }
               return 0;
           }

       If the function is called with:


           what_kind_of_number ("0x10")

       an ISO/IEC 9899:1990 standard-compliant library will result in the
       function printing:


           It's an integer with value 16

       With the ISO/IEC 9899:1999 standard, the result is:


           It's a float with value 16

       The change in behavior is due to the inclusion of floating-point
       numbers in hexadecimal notation without requiring that either a decimal
       point or the binary exponent be present.

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _f_s_c_a_n_f(), _i_s_s_p_a_c_e(), _l_o_c_a_l_e_c_o_n_v(), _s_e_t_l_o_c_a_l_e(), _s_t_r_t_o_l()

       The Base Definitions volume of POSIX.1-2017, _C_h_a_p_t_e_r _7, _L_o_c_a_l_e,
       <<ffllooaatt..hh>>, <<ssttddlliibb..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                            STRTOD(3)
