# atof

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< c‎ | string‎ | byte

Defined in header ``<stdlib.h>`` |  |
double atof( const char* str ); |  |

Interprets a floating-point value in a byte string pointed to by `str`.

Function discards any whitespace characters (as determined by isspace) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating-point representation and converts them to a floating-point value. The valid floating-point value can be one of the following:

- decimal floating-point expression. It consists of the following parts:

  - (optional) plus or minus sign
  - nonempty sequence of decimal digits optionally containing decimal-point character (as determined by the current C locale) (defines significand)
  - (optional) `e` or `E` followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 10)

- hexadecimal floating-point expression. It consists of the following parts:

  - (optional) plus or minus sign
  - `0x` or `0X`
  - nonempty sequence of hexadecimal digits optionally containing a decimal-point character (as determined by the current C locale) (defines significand)
  - (optional) `p` or `P` followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 2)

- infinity expression. It consists of the following parts:

  - (optional) plus or minus sign
  - `INF` or `INFINITY` ignoring case

- not-a-number expression. It consists of the following parts:

  - (optional) plus or minus sign
  - `NAN` or `NAN(`char_sequence`)` ignoring case of the `NAN` part. char_sequence can only contain digits, Latin letters, and underscores. The result is a quiet NaN floating-point value.

(since C99)

- any other expression that may be accepted by the currently installed C locale

### Parameters

str | - | pointer to the null-terminated byte string to be interpreted

### Return value

double value corresponding to the contents of `str` on success. If the converted value falls out of range of the return type, the return value is undefined. If no conversion can be performed, 0.0 is returned.

### Notes

The name stands for "ASCII to float".

### Example

Run this code

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

    int main(void)
    {
        printf("%g\n", atof("  -0.0000000123junk"));
        printf("%g\n", atof("0.012"));
        printf("%g\n", atof("15e16"));
        printf("%g\n", atof("-0x1afp-2"));
        printf("%g\n", atof("inF"));
        printf("%g\n", atof("Nan"));
        printf("%g\n", atof("1.0e+309"));   // UB: out of range of double
        printf("%g\n", atof("0.0"));
        printf("%g\n", atof("junk"));       // no conversion can be performed
    }

Possible output:

    -1.23e-08
    0.012
    1.5e+17
    -107.75
    inf
    nan
    inf
    0
    0

### References

- C11 standard (ISO/IEC 9899:2011):

  - 7.22.1.1 The atof function (p: 341)

- C99 standard (ISO/IEC 9899:1999):

  - 7.20.1.1 The atof function (p: 307)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.10.1.1 The atof function

### See also

strtofstrtodstrtold (C99)(C99) | converts a byte string to a floating-point value (function)
C++ documentation for atof

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