Format
#include <stdlib.h>
double atof(const char *string);Language Level: ANSI
Threadsafe: Yes.
Locale Sensitive: The behavior of this function might be affected by the LC_CTYPE and LC_NUMERIC categories of the current locale. For more information, see Understanding CCSIDs and Locales.
Description
The atof() function 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 numeric 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.
The atof() function expects a string in the following form:
>>-+------------+--+-----+--+-digits--+---+--+--------+-+-------> '-whitespace-' +- + -+ | '-.-' '-digits-' | '- – -' '-.--digits-----------------' >--+------------------------+---------------------------------->< '-+-e-+--+-----+--digits-' '-E-' +- + -+ '- – -'
The white space consists of the same characters for which the isspace()function is true, such as spaces and tabs. The atof() function ignores leading white-space characters.
For the atof() function, 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 might be signed.
The atof() function 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.
Return Value
The atof() function returns a double value that is 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. In case of overflow, the function sets errno to ERANGE and returns the value -HUGE_VAL or +HUGE_VAL.
Example that uses atof()
This example shows how to convert numbers that are stored as strings to numeric values.
#include <stdlib.h>
#include <stdio.h>
int main(void)
{
double x;
char *s;
s = " -2309.12E-15";
x = atof(s); /* x = -2309.12E-15 */
printf("x = %.4e\n",x);
}
/******************* Output should be similar to: ***************
x = -2.3091e-12
*/Related Information