atof, _atof_l, _wtof, _wtof_l

Статья
10/16/2008

Convert a string to double.

double atof(
   const char *str 
);
double _wtof(
   const wchar_t *str 
);

Параметры

str
String to be converted.
locale
Locale to use.

Возвращаемое значение

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 Visual C++ 2005, 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.

Заметки

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] [ {d | D | 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 (d, D, e, or E) and an optionally signed decimal integer.

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

Требования

Routine(s)	Required header
atof	<math.h> and <stdlib.h>
_atof_l	<math.h> and <stdlib.h>
_wtof, _wtof_l	<stdlib.h> or <wchar.h>

Пример

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

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

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

int main( void )
{
    char    *str = NULL;
    double  value = 0;

    // 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 'd' exponential formatting keyword.
    str = "3.1412764583d210";
    value = atof( str );
    printf( "Function: atof( \"%s\" ) = %e\n", str, value );

    // An example of the atof function
    // using the 'e' exponential formatting keyword.
    str = "  -2309.12E-15";
    value = atof( str );
    printf( "Function: atof( \"%s\" ) = %e\n", str, value );

}

Function: atof( "  3336402735171707160320 " ) = 3.336403e+021
Function: atof( "3.1412764583d210" ) = 3.141276e+210
Function: atof( "  -2309.12E-15" ) = -2.309120e-012