What is wrong with usage of atof function?

Question

int main()
{
    char str[10]="3.5";
    printf("%lf",atof(str));
    return 0;
}

This is a simple code I am testing at ideone.com. I am getting the output as

-0.371627

[Possible duplicate](http://stackoverflow.com/questions/4264127/correct-format-specifier-for-double-in-printf). — Alexey Frunze, Feb 17 '13 at 05:21
@AlexeyFrunze: Not at all. `%lf` is perfectly fine for printing a `double`. — Kerrek SB, May 12 '14 at 09:50

score 17 · Accepted Answer · answered Feb 17 '13 at 05:15

17

You have not included stdlib.h. Add proper includes:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char str[10]="3.5";
    printf("%lf",atof(str));
    return 0;
}

Without including stdlib.h, atof() is declare implicitly and the compiler assumes it returns an int.

answered Feb 17 '13 at 05:15

FatalError

OTOH, it looks like `%lf` is only proper in C99's `printf()` and not in C89's. – Alexey Frunze Feb 17 '13 at 05:20
Thanks, this was the error. Though I never heard of this before. – Naved Alam Feb 17 '13 at 05:25
1

Of what? Of the need to include headers or enable all warnings to see potential problems that the compiler can find? – Alexey Frunze Feb 17 '13 at 05:27
so this is the correct answer... but can someone explain why C is allowing me to call a function that does not exist? – bravosierra99 Jan 05 '21 at 01:05

score 0 · Answer 2 · edited May 23 '17 at 11:45

0

edited May 23 '17 at 11:45

Community

answered Feb 17 '13 at 05:13

austin

no, `%lf` is the correct specifier for double, there's no issue with that. It's just that [*"If the converted value falls out of range of the return type, the return value is undefined"*](https://en.cppreference.com/w/c/string/byte/atof) which is the same reason why [`atoi()` shouldn't be used](https://stackoverflow.com/q/17710018/995714) – phuclv Dec 22 '20 at 00:50

2 Answers2