Header File 

stdlib.h  

Category 

Conversion Routines, Math Routines 

Prototype 

double strtod(const char *s, char **endptr); 

double wcstod(const wchar_t *s, wchar_t **endptr); 

long double _strtold(const char *s, char **endptr); 

long double _wcstold(const wchar_t *s, wchar_t **endptr); 

Description 

Convert a string to a double or long double value. 

strtod converts a character string, s, to a double value. s is a sequence of characters that can be interpreted as a double value; the characters must match this generic format: 

[ws] [sn] [ddd] [.] [ddd] [fmt[sn]ddd] 

where:

strtod also recognizes +INF and -INF for plus and minus infinity, and +NAN and -NAN for not-a-number. 

For example, here are some character strings that strtod can convert to double: 

+ 1231.1981 e-1 

502.85E2 

+ 2010.952 

strtod stops reading the string at the first character that cannot be interpreted as an appropriate part of a double value. 

If endptr is not null, strtod sets *endptr to point to the character that stopped the scan (*endptr = &stopper). endptr is useful for error detection. 

_strtold is the long double version; it converts a string to a long double value. 

Return Value 

These functions return the value of s as a double (strtod) or a long double (_strtold). In case of overflow, they return plus or minus HUGE_VAL (strtod) or _LHUGE_VAL (_strtold). 

Example  

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
   char input[80], *endptr;
   double value;
   printf("Enter a floating point number:");
   gets(input);
   value = strtod(input, &endptr);
   printf("The string is %s the number is %lf\n", input, value);
   return 0;
}

Portability