# strtod (3p)

# PROLOG

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.# NAME

strtod, strtof, strtold — convert a string to a double-precision number# SYNOPSIS

#include <stdlib.h>

double strtod(const char *restrictnptr, char **restrictendptr); float strtof(const char *restrictnptr, char **restrictendptr); long double strtold(const char *restrictnptr, char **restrictendptr);

# DESCRIPTION

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

**'+'**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:digitnondigitn-char-sequence digitn-char-sequence nondigit

*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*.

# 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.

**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 this standard. One such example would be:

intwhat_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; }

what_kind_of_number ("0x10")

It's an integer with value 16

It's a float with value 16

# RATIONALE

None.# FUTURE DIRECTIONS

None.# SEE ALSO

*,*

*fscanf*()*,*

*isspace*()*,*

*localeconv*()*,*

*setlocale*()*The Base Definitions volume of POSIX.1‐2008,*

*strtol*()*Chapter 7*,

*Locale*,

*,*

**<float.h>**

**<stdlib.h>**