STRTOD(P) POSIX Programmer's Manual STRTOD(P)
strtod, strtof, strtold - convert a string to a double-precision number
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 dou-
ble, 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 iss-
2. A subject sequence interpreted as a floating-point constant or representing infinity
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
* 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:
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 sub-
ject 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 inter-
preted 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 str-
told(), then check errno.
Upon successful completion, these functions shall return the converted value. If no con-
version 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
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.
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 cur-
rent 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 dig-
its, 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 behav-
ior 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:
what_kind_of_number (char *s)
d = strtod(s, &endp);
if (s != endp && *endp == `\0')
printf("It's a float with value %g\n", d);
l = strtol(s, &endp, 0);
if (s != endp && *endp == `\0')
printf("It's an integer with value %ld\n", 1);
If the function is called with:
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.
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 STRTOD(P)