Originální popis anglicky:
strtod, strtof, strtold - convert a string to a double-precision number
Návod, kniha: POSIX Programmer's Manual
#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);
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 null byte 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 plus or minus sign,
then one of the following:
- *
- A non-empty sequence of decimal digits optionally
containing a radix character, then an optional exponent part
- *
- A 0x or 0X, then a non-empty sequence of hexadecimal digits
optionally containing a radix character, then an optional binary exponent
part
- *
- One of INF or INFINITY, ignoring case
- *
- One of NAN or NAN(n-char-sequence_opt), 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-sequence_opt) 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 program's 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
str is stored in the object
pointed to by
endptr, provided that
endptr is not a null
pointer.
The
strtod() function 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.
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].
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.
None.
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 the base
documents. 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.
None.
None.
isspace() ,
localeconv() ,
scanf() ,
setlocale() ,
strtol() , the Base Definitions volume of
IEEE Std 1003.1-2001, Chapter 7, Locale,
<float.h>,
<stdlib.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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
.
