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.1‐2008 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 minus-sign, 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 locales, other implementation-defined
       subject sequences 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.1‐2008, 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, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                          STRTOD(3P)