atof(3)atof(3)NAME
atof, strtod, strtof, strtold - Converts a character string to a dou‐
ble-precision floating-point value
SYNOPSIS
#include <stdlib.h>
double atof(
const char *nptr ); double strtod(
const char *nptr,
char **endptr ); float strtof(
const char *nptr,
char **endptr ); long double strtold(
const char *nptr,
char **endptr );
LIBRARY
Standard C Library (libc)
PARAMETERS
Points to the character string to convert. Specifies either a null
value, a pointer to the character that ended the scan, or a pointer to
a null value.
DESCRIPTION
The atof() function converts, to a double floating-point value, the
string pointed to by the nptr parameter - up to the first character
that is inconsistent with the format of a floating-point number. Lead‐
ing space characters are ignored. A call to this function is equivalent
to a call to strtod(nptr, (char **) NULL), except for error handling.
When the value cannot be represented, the result is undefined.
The strtod(), strtof(), and strtold() functions convert the initial
portion of the string pointed to by the nptr parameter to double,
float, and long double representation, respectively. First, the input
string is decomposed into the following three parts: An initial, possi‐
bly empty, sequence of space characters (as specified by the isspace()
function). A subject sequence interpreted as a floating-point con‐
stant. A final string of one or more unrecognized characters, includ‐
ing the terminating null character of the input string.
After decomposition of the string, the subject sequence is converted to
a floating-point number and the resulting value is returned. A subject
sequence is defined as the longest initial subsequence of the input
string, starting with the first non-space character, that is of the
expected form. The expected form and order of the subject sequence is:
An optional plus (+) or minus (-) sign. A sequence of digits option‐
ally containing a radix character. An optional exponent part. An expo‐
nent part consists of e or E, followed by an optional sign, which is
followed by one or more decimal digits.
The subject sequence contains no characters when the input string is
empty or consists entirely of space characters, or when the first non-
space character is other than a sign, a digit, or a radix character.
For the strtod(), strtof(), and strtold() functions, when the value of
the endptr parameter is not (char**) NULL, a pointer to the character
that terminated the scan is stored at *endptr.
When a floating-point value cannot be formed, *endptr is set to nptr.
The strings NaN ("not a number"), Inf, and Infinity (the case of the
characters does not matter) are recognized as valid only when the pro‐
gram is compiled with the -ieee option.
NOTES
The setlocale() function may affect the radix character used in the
conversion result. Full use
RETURN VALUES
When the string is empty or begins with an unrecognized character, +0.0
is returned as the floating-point value.
If the calling routine is compiled with IEEE floating point enabled
(-ieee option), errno will be set to ERANGE if the conversion under‐
flows to zero. Similarly, if the value overflows, ERANGE will be set
and a properly signed infinity will be returned.
If the calling routine is not compiled with IEEE floating point
enabled, any underflow will cause errno to be set to ERANGE and a prop‐
erly signed zero to be returned. An overflow will cause errno to be set
to ERANGE and will return a properly signed DBL_MAX, FLOAT_MAX, or
LDBL_MAX.
Upon successful completion, all of the functions return the converted
floating-point value.
ERRORS
If the atof(), strtod(), strtof(), or strtold() function fails, errno
may be set to the following value: The input string is out of range
(that is, the subject sequence cannot be converted to a floating-point
value without causing underflow or overflow).
SEE ALSO
Functions: atoi(3), scanf(3)atof(3)
