число точности (precision number)
Пролог (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.
Имя (Name)
strtod, strtof, strtold — convert a string to a double-precision
number
Синопсис (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);
Описание (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.
Возвращаемое значение (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].
Ошибки (Error)
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.
Примеры (Examples)
None.
Использование в приложениях (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.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
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)
