strtod(3p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

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

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       fscanf(3p), isspace(3p), localeconv(3p), setlocale(3p),
       strtol(3p)

       The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale,
       float.h(0p), stdlib.h(0p)

COPYRIGHT         top

       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)

Pages that refer to this page: stdlib.h(0p)awk(1p)printf(1p)atof(3p)fscanf(3p)localeconv(3p)nan(3p)setlocale(3p)strtol(3p)strtold(3p)strtoul(3p)