# strtod

## 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.
    


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