strtod

STRTOD(3P)                  POSIX Programmer's Manual                 STRTOD(3P)



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
       strtod, strtof, strtold — convert a string to a double-precision number

SYNOPSIS
       #include <stdlib.h>

       double strtod(const char *restrict nptr, char **restrict endptr);
       float strtof(const char *restrict nptr, char **restrict endptr);
       long double strtold(const char *restrict nptr, char **restrict endptr);

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 convert the initial portion of the string pointed
       to by nptr to double, float, and long double representation,
       respectively. First, they decompose the input string into three parts:

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

        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
           'e' or the character 'E', 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 'p' or the character 'P',
           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-char-sequenceopt), 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-char-sequenceopt) 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 endptr, provided that endptr
       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
       LC_NUMERIC).  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 nptr is stored in the object
       pointed to by endptr, provided that endptr is not a null pointer.

       These functions shall not change the setting of errno 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 errno to 0,
       then call strtod(), strtof(), or strtold(), then check errno.

RETURN VALUE
       Upon successful completion, these functions shall return the converted
       value. If no conversion could be performed, 0 shall be returned, and
       errno may be set to [EINVAL].

       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 errno shall be set to [ERANGE].

       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 errno set to [ERANGE].

ERRORS
       These functions shall fail if:

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

       These functions may fail if:

       EINVAL No conversion could be performed.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       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 <float.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 strtod() 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.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       fscanf(), isspace(), localeconv(), setlocale(), strtol()

       The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale,
       <float.h>, <stdlib.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 .



IEEE/The Open Group                   2017                            STRTOD(3P)