C: strtof wrong conversion by a unit

Question

I'm having problems with strtof conversions, some values it works well but another ones it returns +1 or -1 of the value. My program reads the value from a file but for simplicity I have tried putting directly the value to strtof and it has the same behavior.

My simple code:

#include <stdlib.h>

int main(int argc, char **argv)
{
    printf("%f", strtof("17309217",NULL));
}

It returns

17309216.000000

The same value that it gives if I convert "17309216" or "17309215".

I have been doing some more tests and it seems a "pattern", "17309214" works well but "17309213", "17309212" and "17309211" all return "17309212". After this "17309210" works well but "17309209", "17309208" and "17309207" return "17309208", and so on...

What is the explanation for this behaviour? Now I convert the string with other methods, I don't need a method to convert it. I only want to know why strtof is doing this, so I can decide where I can use it and where can't.

I have been reading some other questions asked related to wrong conversions but all are related to the decimal rounding, not an entire unit.

Thanks in advance,

Perhaps you meant to use **strtod** and use **double**s – Antti Haapala -- Слава Україні Aug 17 '17 at 08:34 — Antti Haapala -- Слава Україні, Aug 17 '17 at 08:34

score 0 · Accepted Answer · answered Aug 17 '17 at 08:18

0

You're converting to a 32-bit floating point which may not have the precision to handle that many digits.

answered Aug 17 '17 at 08:18

Mateen Ulhaq

24,552
19
101
135

Thanks, I know I was missing something. My fault. – Salva Savall Aug 17 '17 at 09:16

C: strtof wrong conversion by a unit

1 Answers1