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
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.
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 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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
isspace() , localeconv() , scanf() , setlocale() , strtol() , the Base
Definitions volume of IEEE Std 1003.1-2001, Chapter 7, Locale,
<float.h>, <stdlib.h>
COPYRIGHT
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 .