atof, _atof_l, _wtof, _wtof_l

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06/05/2017

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The latest version of this topic can be found at atof, _atof_l, _wtof, _wtof_l.

Convert a string to double.

Syntax

double atof(  
   const char *str   
);  
double _atof_l(  
   const char *str,  
   _locale_t locale  
);  
double _wtof(  
   const wchar_t *str   
);  
double _wtof_l(  
   const wchar_t *str,  
   _locale_t locale  
);

Parameters

str
String to be converted.

locale
Locale to use.

Return Value

Each function returns the double value produced by interpreting the input characters as a number. The return value is 0.0 if the input cannot be converted to a value of that type.

In all out-of-range cases, errno is set to ERANGE. If the parameter passed in is NULL, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, these functions set errno to EINVAL and return 0.

Remarks

These functions convert a character string to a double-precision, floating-point value.

The input string is a sequence of characters that can be interpreted as a numerical value of the specified type. The function stops reading the input string at the first character that it cannot recognize as part of a number. This character may be the null character ('\0' or L'\0') terminating the string.

The str argument to atof and _wtof has the following form:

[whitespace] [sign] [digits] [.digits] [ {e | E }[sign]digits]

A whitespace consists of space or tab characters, which are ignored; sign is either plus (+) or minus (–); and digits are one or more decimal digits. If no digits appear before the decimal point, at least one must appear after the decimal point. The decimal digits may be followed by an exponent, which consists of an introductory letter (e, or E) and an optionally signed decimal integer.

The UCRT versions of these functions do not support conversion of Fortran-style (d or D) exponent letters. This non-standard extension was supported by earlier versions of the CRT, and may be a breaking change for your code.

The versions of these functions with the _l suffix are identical except that they use the locale parameter passed in instead of the current locale.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _MBCS not defined	_MBCS defined	_UNICODE defined
`_tstof`	`atof`	`atof`	`_wtof`
`_ttof`	`atof`	`atof`	`_wtof`

Requirements

Routine(s)	Required header
`atof`, `_atof_l`	C: <math.h> or <stdlib.h> C++: <cstdlib>, <stdlib.h>, <cmath> or <math.h>
`_wtof`, `_wtof_l`	C: <stdlib.h> or <wchar.h> C++: <cstdlib>, <stdlib.h> or <wchar.h>

Example

This program shows how numbers stored as strings can be converted to numeric values using the atof and _atof_l functions.

// crt_atof.c  
//  
// This program shows how numbers stored as   
// strings can be converted to numeric  
// values using the atof and _atof_l functions.  

#include <stdlib.h>  
#include <stdio.h>  
#include <locale.h>  

int main(void)
{
    char    *str = NULL;
    double value = 0;
    _locale_t fr = _create_locale(LC_NUMERIC, "fr-FR");

    // An example of the atof function  
    // using leading and training spaces.  
    str = "  3336402735171707160320 ";
    value = atof(str);
    printf("Function: atof(\"%s\") = %e\n", str, value);

    // Another example of the atof function  
    // using the 'E' exponential formatting keyword.  
    str = "3.1412764583E210";
    value = atof(str);
    printf("Function: atof(\"%s\") = %e\n", str, value);

    // An example of the atof and _atof_l functions  
    // using the 'e' exponential formatting keyword  
    // and showing different decimal point interpretations.  
    str = "  -2,309e-25";
    value = atof(str);
    printf("Function: atof(\"%s\") = %e\n", str, value);
    value = _atof_l(str, fr);
    printf("Function: _atof_l(\"%s\", fr)) = %e\n", str, value);
}

Function: atof
("  3336402735171707160320 ") = 3.336403e+21
Function: atof
("3.1412764583E210") = 3.141276e+210
Function: atof
("  -2,309e-25") = -2.000000e+00
Function: _atof_l("  -2,309e-25", fr)) = -2.309000e-25

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