|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
STRTOD(3P) POSIX Programmer's Manual STRTOD(3P)
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.
strtod, strtof, strtold — convert a string to a double-precision
number
#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);
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.
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 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.
None.
None.
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)
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)
Pages that refer to this page: stdlib.h(0p), awk(1p), printf(1p), atof(3p), fscanf(3p), localeconv(3p), nan(3p), setlocale(3p), strtol(3p), strtold(3p), strtoul(3p)