Other Alias
strtod, strtoldSYNOPSIS
#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 whitespace characters (as specified by isspace())
 2.
 A subject sequence interpreted as a floatingpoint 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 floatingpoint 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 nonempty sequence of decimal digits optionally containing a radix character, then an optional exponent part
 *
 A 0x or 0X, then a nonempty 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(ncharsequence_opt), ignoring case in the
NAN part, where:

ncharsequence: digit nondigit ncharsequence digit ncharsequence nondigit

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first nonwhitespace 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 floatingpoint number, the sequence of characters starting with the first digit or the decimalpoint 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 floatingpoint number, or if a binary exponent part does not appear in a hexadecimal floatingpoint 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(ncharsequence_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 nchar sequences is implementationdefined. 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 implementationdefined 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 wellformed 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 standardcompliant 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 floatingpoint numbers in hexadecimal notation without requiring that either a decimal point or the binary exponent be present.
RATIONALE
None.
FUTURE DIRECTIONS
None.
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) 20012003 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 .