# strtod man page

strtod, strtof, strtold — convert ASCII string to floating-point number

## Synopsis

**#include <stdlib.h>**

**double strtod(const char ****nptr***, char *****endptr***);****float strtof(const char ****nptr***, char *****endptr***);****long double strtold(const char ****nptr***, char *****endptr***);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

**strtof**(), **strtold**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

## Description

The **strtod**(), **strtof**(), and **strtold**() functions convert the initial portion of the string pointed to by *nptr* to *double*, *float*, and *long double* representation, respectively.

The expected form of the (initial portion of the) string is optional leading white space as recognized by isspace(3), an optional plus ('+') or minus sign ('-') and then either (i) a decimal number, or (ii) a hexadecimal number, or (iii) an infinity, or (iv) a NAN (not-a-number).

A *decimal number* consists of a nonempty sequence of decimal digits possibly containing a radix character (decimal point, locale-dependent, usually '.'), optionally followed by a decimal exponent. A decimal exponent consists of an 'E' or 'e', followed by an optional plus or minus sign, followed by a nonempty sequence of decimal digits, and indicates multiplication by a power of 10.

A *hexadecimal number* consists of a "0x" or "0X" followed by a nonempty sequence of hexadecimal digits possibly containing a radix character, optionally followed by a binary exponent. A binary exponent consists of a 'P' or 'p', followed by an optional plus or minus sign, followed by a nonempty sequence of decimal digits, and indicates multiplication by a power of 2. At least one of radix character and binary exponent must be present.

An *infinity* is either "INF" or "INFINITY", disregarding case.

A *NAN* is "NAN" (disregarding case) optionally followed by a string, *(n-char-sequence)*, where *n-char-sequence* specifies in an implementation-dependent way the type of NAN (see Notes).

## Return Value

These functions return the converted value, if any.

If *endptr* is not NULL, a pointer to the character after the last character used in the conversion is stored in the location referenced by *endptr*.

If no conversion is performed, zero is returned and (unless *endptr* is null) the value of *nptr* is stored in the location referenced by *endptr*.

If the correct value would cause overflow, plus or minus **HUGE_VAL** (**HUGE_VALF**, **HUGE_VALL**) is returned (according to the sign of the value), and **ERANGE** is stored in *errno*. If the correct value would cause underflow, zero is returned and **ERANGE** is stored in *errno*.

## Errors

**ERANGE**Overflow or underflow occurred.

## Attributes

For an explanation of the terms used in this section, see attributes(7).

Interface | Attribute | Value |
---|---|---|

strtod(), strtof(), strtold() | Thread safety | MT-Safe locale |

## Conforming to

POSIX.1-2001, POSIX.1-2008, C99.

**strtod**() was also described in C89.

## Notes

Since 0 can legitimately be returned on both success and failure, the calling program should set *errno* to 0 before the call, and then determine if an error occurred by checking whether *errno* has a nonzero value after the call.

In the glibc implementation, the *n-char-sequence* that optionally follows "NAN" is interpreted as an integer number (with an optional '0' or '0x' prefix to select base 8 or 16) that is to be placed in the mantissa component of the returned value.

## Example

See the example on the strtol(3) manual page; the use of the functions described in this manual page is similar.

## See Also

atof(3), atoi(3), atol(3), nan(3), nanf(3), nanl(3), strfromd(3), strtol(3), strtoul(3)

## Colophon

This page is part of release 4.13 of the Linux *man-pages* project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

## Referenced By

atof(3), atoi(3), collectd.conf(5), dirfile-format(5), explain(1), explain(3), explain_lca2010(1), explain_strtod(3), explain_strtod_or_die(3), explain_strtof(3), explain_strtof_or_die(3), explain_strtold(3), explain_strtold_or_die(3), gawk(1), locale(7), nan(3), PCPIntro(1), pmstore(1), scanf(3), strfromd(3), strtol(3), strtonum(3), strtoul(3).

The man pages strtof(3) and strtold(3) are aliases of strtod(3).