Syntax
#include <stdlib.h> double atof(const char *string);Description
atof converts 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 return type. The function stops reading the input string at the first character that it cannot recognize as part of a number; this character can be the null character that ends the string.
atof expects a string in the following form:
┌──────────────────────────────────────────────────────────────────────────────┐
│ │
│ >>──┬────────────┬──┬───┬──┬───────────────────────────┬──>
│
│ └─whitespace─┘ ├─+─┤ ├─digits──┬───┬──┬────────┬─┤ │
│ └─┴─┘ │ └─.─┘ └─digits─┘ │ │
│ └─.──digits─────────────────┘ │
│ │
│ >──┬──────────────────────┬──>< │
│ └─┬─e─┬──┬───┬──digits─┘ │
│ └─E─┘ ├─+─┤ │
│ └─┴─┘ │
│ │
└──────────────────────────────────────────────────────────────────────────────┘
The actual decimal point character (radix character) is determined by the LC_NUMERIC category of the current locale.
The white space consists of the same characters for which the isspace function is true, such as spaces and tabs. atof ignores leading white-space characters.
For atof, digits is one or more decimal digits; if no digits appear before the decimal point, at least one digit must appear after the decimal point. The decimal digits can precede an exponent, introduced by the letter e or E. The exponent is a decimal integer, which may be signed.
atof will not fail if a character other than a digit follows an E or if e is read in as an exponent. For example, 100elf will be converted to the floating-point value 100.0. The accuracy is up to 17 significant character digits. The string can also be "infinity", "inf", or "nan". These strings are case-insensitive, and can be preceded by a unary minus (-). They are converted to infinity and NaN values.
atof returns a double value produced by interpreting the input characters as a number. The return value is 0 if the function cannot convert the input to a value of that type. The return value is undefined in case of overflow.
This example shows how to convert numbers stored as strings to numerical values using the atof function.
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
double x;
char *s;
s = " -2309.12E-15";
x = atof(s); /* x = -2309.12E-15 */
printf("atof( %s ) = %G\n", s, x);
return 0;
/****************************************************************************
The output should be:
atof( -2309.12E-15 ) = -2.30912E-12
****************************************************************************/
}
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