Konwersja string na double

#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

struct maybe_double {
  bool value_exists;
  double value;
};

struct maybe_double parse_float(const char *input) {
  struct maybe_double out = {.value_exists = false};
  if(strcmp(input,"0.0")==0)
    {
      out.value_exists=true;
      out.value=0.0;
  }
  else if(strtod(input,NULL)!=0)
    {
      out.value_exists=true;
      out.value=strtod(input,NULL);
    }
  else if(strtod(input,NULL)==0.0)
    out.value_exists=false;  
  return out;
}

#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

struct maybe_double {
  bool value_exists;
  double value;
};

struct maybe_double parse_float(const char *input) {
  struct maybe_double out = {.value_exists = false};
  char *endptr;
  if((out.value=strtod(input,&endptr)))
      if(*endptr=='\0')
        out.value_exists=true;
  else
    out.value_exists=false;  
  return out;
}

struct maybe_double parse_float(const char *input) {
  struct maybe_double out = {.value_exists = false};
  char *endptr = NULL;
  
  if (input != NULL) {
    out.value = strtod(input, &endptr);
    out.value_exists = (endptr != NULL && *endptr=='\0');
  }

  return out;
}

#include <stdbool.h>

struct maybe_double {
  bool value_exists;
  double value;
};

struct maybe_double parse_float(const char *input) {
  struct maybe_double out = {.value_exists = false};
  char *endptr;
  if(out.value=strtod(input,&endptr))
      if(*endptr=='\0')
        out.value_exists=true;
  if(out.value==0.0)
    if((*endptr=='\0')&&(strlen(input)>0))
      out.value_exists=true;
  else
    out.value_exists=false;  
  return out;
}

#include <stdbool.h>
#include <stdlib.h>
#include <string>
using namespace std;

enum parseResult { prOk,prHaveMore,prBad };

enum parseResult parseDouble(const char *str,double *value)
{
	char *out;
	*value=strtod(str,&out);
	if(out==str) return prBad;
	if(*out) return prHaveMore;
	return prOk;
}

// jeżeli poprawne liczymy tylko kiedy cały wiersz jest liczbą
bool isDouble(const char *str)
{
	char *out;
	strtod(str,&out);
	return (*str)&&(!*out);
}

// lub jeżeli potrzebna również liczba
bool isDouble(const char *str,double *value)
{
	char *out;
	*value=strtod(str,&out);
	return (*str)&&(!*out);
}

int main()
{
	double value;
	if(parseDouble("x",&value)==prBad) printf("Bad double value\n");
	if(parseDouble("7t",&value)==prHaveMore) printf("value=%lf; and more data in input\n",value);
	if(parseDouble("3.5.",&value)==prHaveMore) printf("value=%lf; and more data in input\n",value);
	if(parseDouble("666.13",&value)==prOk) printf("value=%lf;\n",value);
	if(parseDouble("1.34E1",&value)==prOk) printf("value=%lf;\n",value);
	return 0;
}