#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);

double atof(const char *str);

Then they 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:

n-char-sequence:
 digit
 nondigit
 n-char-sequence digit
 n-char-sequence nondigit

In default mode for strtod(), only decimal, INF/INFINITY, and NAN/NAN(n-char-sequence) forms are recognized. In C99/SUSv3 mode, hexadecimal strings are also recognized.

In default mode for strtod(), the n-char-sequence in the NAN(n-char-sequence) form can contain any character except ')' (right parenthesis) or '\e0' (null). In C99/SUSv3 mode, the n-char-sequence can contain only upper and lower case letters, digits, and '_' (underscore).

The strtof() and strtold() functions always function in C99/SUSv3-conformant mode.

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) is interpreted as a floating constant of the C language, except that the radix character is 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 is interpreted as negated. A character sequence INF or INFINITY is interpreted as an infinity. A character sequence NAN or NAN(n-char-sequence(opt)) is interpreted as a quiet NaN. 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 either the decimal or hexadecimal form, the value resulting from the conversion is rounded correctly according to the prevailing floating point rounding direction mode. The conversion also raises floating point inexact, underflow, or overflow exceptions as appropriate.

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 defaults to a period ('.').

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

The strtod() function does not change the setting of errno if successful.

The atof(str) function call is equivalent to strtod(nptr, (char **)NULL).

If the correct value is outside the range of representable values, \(+-HUGE_VAL, \(+-HUGE_VALF, or \(+-HUGE_VALL is returned (according to the sign of the value), a floating point overflow exception is raised, and errno is set to ERANGE.

If the correct value would cause an underflow, the correctly rounded result (which may be normal, subnormal, or zero) is returned, a floating point underflow exception is raised, and errno is set to ERANGE.

These functions may fail if: EINVAL

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 IEEE Std 1003.1-200x. 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 == '\e0')
 printf("It's a float with value %g\en", d);
 else
 {
 l = strtol(s, &endp, 0);
 if (s != endp && *endp == '\e0')
 printf("It's an integer with value %ld\en", 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.

The DESCRIPTION and RETURN VALUES sections above are very similar to the wording used by the Single UNIX Specification version 2 (SUSv2) and the 1989 C Standard to describe the behavior of the strtod() function. Since some users have reported that they find the description confusing, the following notes might be helpful.